The Annual and Diurnal Variations of Cloud Amounts and Cloud Types at Six Arizona Cities
Item Type text; Report
Authors Sellers, William D.
Publisher Institute of Atmospheric Physics, University of Arizona (Tucson, AZ)
Rights Public Domain: This material has been identified as being free of known restrictions under U.S. copyright law, including all related and neighboring rights.
Download date 10/10/2021 00:07:56
Item License http://creativecommons.org/publicdomain/mark/1.0/
Link to Item http://hdl.handle.net/10150/634453 UNIVERSITY OF ARIZONA
INSTITUTE OF ATMOSPHERIC PHYSICS
SCIENTIFIC REPORT NO. 8
THE ANNUAL AND DIURNAL VARIATIONS OF CLOUD
AMOUNTS AND CLOUD TYPES AT SIX ARIZONA CITIES
William D. Sellers
This study was partially supported by the Associated Universities for Research in Astronomy.
September 1, 195 8 TABLE OF CONTENTS
Page
ABSTRACT.. • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 1• • • •
I. IN'l:RODUCTION • ••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 1 II. TOPOGRAPHY OF THE SIX ARIZONA CITIES •••••••••••••••••••••••••••••••••• 3
A. Phoenix . ..••....•...••...... ••...... 3
B. Flagstaff • ...•...... 3 c. Winslow • •.•••.••.••.••.•••.••••••••••••••••••••••••••••...•••••••• 4 D. Prescott ...... •...... •..••...... •...•.•...... 4
E. Tucson • ..••...••.••.•...... •...•.•••..•.••••.••.••...••..•••.• 4
F. Y1.lIIla • ••••••••••• • •••••• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •5
III. NATlJRE OF THE DMA•••••••••••••••••••••••••••••••••••••••••••••••••••• 5
A. Elements . •.••....••••.••...... •.•.•.•..••.•.•.•...... •. 5
1. Cloud antou.nt s .•.•••.•....••....••.•.•••••.••••....•••...•.••.. 5
2. Cloud types • ••..•..•..•.•••••.•.•.•.•••.•..•.••....•....•••.•• 6 3. Thun.derstorms ••••••••••••••••••••••••••••••••••••••••••••••••• 10
B. Period of record•••······•··•··•••·••••··••••··•·•···••·••••····•· 10
1. Cloud aDlO'llnt s •••.••••••••..•••.•••••••••••••••.••••••.•••••••• 10
2. Cloud types .•.•.••.••...... •.•.•••.•.••..•.•••..••.••...•...• 12 3. Thunderstorms • ••••••..•••••.•..•••••.••••••••••••••••••••••••• 12 c. Homogeneity of the data ••.••••••••••••••••••••••••.••••••••••••••• 12
1. Changes in observing methods •••••••••••••••••••••••••••••••••• 12 2. Different periods of record for cloud amount and cloud type
statistics •••••••••••••••••••••••••••••••••••••••••••••••••••• 12 3. The smaller number of observations available for Flagstaff than for the other stations ••••••••••••••••••••••••••••••••••• 16 4. Differences in observing procedures ••••••••••••••••••••••••••• 16 TABLE OF CONTENTS (Cont'd) Page D. Nighttime observations ••••••••••••••••••••••••••••••••••••••••••• 20
DJ. ANNUAL V.ARIATION OF CLOUD .AMOUNTS, CLOUD TYPES, AND THUNDERSTOBMS •••• 21
A. Cloud amollnt s •..•••••...•••..•.••••.••••••.••••.•••••.•.••••..•.• 21
1. Average cloud cover •••••••••••••••••••••••••••••••••••••••••• 21
2. Frequency of occurrence •••••••••••••••••••••••••••••••••••••• 22 B. Cloud types and thunderstorms •••••••••••••••••••••••••••••••••••• 23 1. Introduction • ...... •...... •..•...... •...... 23 2. Stratocum.ulus . ...••.•.••..•..•.••..•.••.••.•..•.•.•...•..••.. 24 3. Ctunulus • ••••••••••••••••••••••••••••••••••••••••••••••••••••• 24 4. Cumulonimbus and thunderstorms ••••••••••••••••••••••••••••••• 24 a. Cumulonimbus •••..•...••.•...•.•••••••••.••....••••••••••• 24
b. Thunderstorms ...••••••••••••••••••••••••••••.•••... , ••••• 25 5. Altostr atus . ....•.•.•.....••..•.••.••••...... 25 6. Altoctnnulus .•...•..••.•••••.••.•.•.••.•••••••••.•.••.•••••.•. 26
7. Cirrus • ...... •...•...•••••.••.•....••... • • • • • • • • • 26 8. Cirro stratus .....•.•...•....•.....•.•.••...•.•...... ••.•.••. 27 v. DIURNAL VARIATION OF CLOUD .AMOUNTS, CLOUD TYPES, AND THUNDERSTOBMS .•• 27 A. J aI1uary • ••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 27 1. Cloud am.cunts •••.•..•••...••...••••••.•..•.••••••..•...••••••. 27 2. Cloud types .•...••.•.....•.....•••••••.•....•.....•...•.••... 28 B. February . ...••.•••.•...•••.....•••••..•.••.••••....•••••.•..•.•.. 28 1. Cloud amounts ...... ••....•..•.....•.•....•...... 28 2. Cloud types •••••••••••••••••• ...... 29 c. March • •.••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 29 1. Cloud amounts . ...••••.•.•...•....•.••.••••.....•...... •.... 29
2. Cloud types ...... •.•.....•••...... ••••...... • 30 TABIE OF CONTENTS (Cont'd).
?,age
D. April ••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 31
1. Cloud am.cunts ••••••••••••••••••••••••••••••••••••••••••••••• 31
2. Cloud types ••••••••••••••••••••••••••••••••••••••••••••••••• 32
E. May ••••••••••••••••••••••••••••••••••••••••••••••••••••• • ••••••• 32
1. Cloud am.o'Unt s ••••••••••••••••••••••••••••••••••••••••••••••• 32 2. Cloud types ••••••••••••••••••••••••••••••••••••••••••••••••• 33 F. June •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 33 1. Cloud am.ot2nt s ••••••••••••••••••••••••••••••••••••••••••••••• 33 2. Cloud types ••••••••••••••••••••••••••••••••••••••••••••••••• 34 G. July •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 35 1. Cloud am.cunts ••••••••••••••••••••••••••••••••••••••••••••••• 35 2. Cloud types ••••••••••••••••••••••••••••••••••••••••••••••••• 36
H. Augu.st •••••••••••••••••••••••••••• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 38 1. Cloud amount s • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 38 2. Cloud types ••••••••••••••••••••••••••••••••••••••••••••••••• 39 I. September ••••••••••••••••••••••••••••••••••••••••••••••••••••••• 40
1. Cloud am.cunt s • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 4o
2. Cloud types ••••••••••••••••••••••••••••••••••••••••••••••••• 41
J. October ••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 42 1. Cloud am.cunt s • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 42
2. Cloud t)1!> es ••••••••••••••••••••••••••••••••••••••••••••••••• 42
K. November •••••••••••••••••••••••••••••••••••••••••••••••••••••••• 43 1. Cloud am.cunts ••••••••••••••••••••••••••••••••••••••••••••••• 43 2. Cloud types ••••••••••••••••••••••••••••••••••••••••••••••••• 43 TABLE OF CONTENTS (Cont'd)
Page
L. December ••••••••••••••••••••••••••••••••••••••••••••••••••••••• 44
1. Cloud amoll?l.t s •••••••••••••••••••••••••••••••••••••••••••••• 44
2. Cloud types •••••••••••••••••••••••••••••••••••••••••••••••• 45
VI. S~Y •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 45 A. Introduction •••••••••••••••.••••••••••••••••••••••••••••••••••• 45
B. Cloud am.ollnt s • , •••••••••••••••••••••••••••••••••••••••••••••••• 46
c. Cloud types •••••••••••••••••••••••••••••••••••••••••••••••••••• 50 1. Stratocumulu s •••••••••••••••••••••••••••••••••••••••••••••• 52 2. Cumulus and cumulonimbus ••••••••••••••••••••••••••••••••••• 52 3. Altocllmulu s •••••••••••••••••••••••••••••••••••••••••••••••• 53 4. Cirrus and cirrostratus •••••••••••••••••••••••••••••••••••• 54 ACirn'OWI.zEDGEl-mNTS. • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 55
REFERENCES •••••••••••••••••••••••••••••••••••••••••••• , • • • • • • • • • • • • 56
.APPEND])( • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •, • 57 THE .ANNUAL AND DIURNAL VARIATIONS OF CLOUD AMOUNTS AND CLOUD TYPES AT SIX ARIZONA CITIES
ABSTRACT The annual and diurnal variations of cloud types and cloud amounts are des cribed for six first-order weather stations in .Arizona. These stations are Flagstaff, Winslow, Prescott, Phoenix, Tucson, and Yuma. Most of the data analysed were obtained on punched-cards from the National Weather Records Center at Asheville, North Carolina. The period of record was from January 1, 1948 through December 31, 1955. This information was supplemented by cloud amount data for the period from July 1, 1956 through March 31, 1958 taken from the supplement to the Local Climatological Data for the six stations. After a brief outline of the terrain features in the vicinity of the stations (section II), the type, quantity, and quality of the data are discussed. Emphasis is put on the homogeneity of the records, both temporally at a single station and spatially from one station to another. In section rv the annual variations of cloud amounts, cloud types, and thunder storms are described, without any reference to the more detailed diurnal variations, which are discussed later, month by month, in section V. Part of the summary (section VI) is devoted to an investigation of the re lation between the average cloud cover and the frequencies of occurrence of both clear and overcast skies at a "typical" .Arizona city.
I. INTRODUCTION. This is the first in a proposed series of papers dealing with the climate of six .Arizona cities, Flagstaff, Winslow, Prescott, Phoenix, Tucson, and Yuma. Be cause of the critical water situation in the arid southwest there has long been a need for such a study. However, until the Institute of Atmospheric Physics was established at the University of .Arizona in Tucson in 1954, no group existed within the state which had either the personnel or the equipment available to devote more than a fraction of its time to the problem. Even the operations of the Institute were quite limited in the first years of its existence because of a lack of proper facilities. But now that a Numerical Analysis Laboratory, with an IBM 650 Data Processing Machine, has been set up at the University there is no reason wny a complete investigation cannot be started. Most of the necessary data are already on punched-cards available from the National Weather Records Center at Asheville, North Carolina. -2-
In this first paper statistics on the average conditions of cloud cover, cloud types, and thunderstorms, based, in general, on data for the past five to ten years, will be presented for each of the six cities, Future papers of this series will cover the distribution and variation of wind speed and direction, temperature, dew point, and relative humidity. The selection of clouds and thunderstorms as the first subjects for study was dictated partly by the interest of astronomers in these elements and partly by a recent Institute report by Robert B. DesJardins on the distribution of clouds at
Tucson (1958). Astronomical interest in cloud cover was augmented in 1957 by an AURA (Asso ciated Universities forRes~archin Astronomy) project involving the selection of the best possible site in the Southwest for a new observatory, Two Institute re ports were prepared as a part of this investigation. The first, by Dr, James E. McDonald (1958), presented a large number of generalized cloud and precipitation statistics for the Southwest, The second, by the present writer (1958), contained a description of the average cloud cover for the six .Arizona cities mentioned above, based on eight years of data, It is the author's intent here to extend this latter paper. DesJardins' (1958) report, covering the distribution of clouds at Tucson, contains three basic parts. The first is an interesting discussion of the heterogeneities in the Tucson cloud data and the methods used to eliminate them. Further mention will be made of this subject later (section IIIC). The second and third parts discuss, respectively, the variation of cloudiness at Tucson and the relationship between cloud cover and precipitation amounts. The present study is essentially an extension of this work to all six of the first-order Weather Bureau stations in .Arizona. However, it would be hopeless, both timewise and effortwise, to try to cover the subject in as much detail as did DesJardins, Before discussing the nature of the data used (section III) and the annual and diurnal variations of cloud amounts and types and of thunderstorms (sections Dr and V), it seems advisable first to describe the topographical features of the six cities. -3-
This will be done in the following section, with the main emphasis placed on the presence or absence of high terrain in the vicinity. Some of the information given has been extracted from the "narrative climatological summary" included in the annual summary issue of the Local Climatological Data Weather Bureau publication for the various cities (1958).
II. TOPOGRAPHY OF THE SIX .ARIZONA CITIES.
A. Phoenix. Phoenix is located in south-central .Arizona in about the center of the Salt River Valley, a broad, oval-shaped, nearly flat plain. The Salt River runs from east to west through the valley but, owing to the Roosevelt and three other dams upstream, it is usually dry. Although bordered on all sides, except the southeast, by low-lying mountains, there are no sizeable peaks within about fifty
miles of Phoenix, which itself is at an elevation of only 1085 feet.* Beyond fifty miles the Mazatzal Mountains to the north-east rise to over 8000 feet. Although weather observations are still made in the city, the official Weather
Bureau station for Phoenix has been at the Sky Harbor Airport since October 1953. Also, most of the published records since July 1939 are based on airport data. The airport is located about four miles ESE of town and at an elevation of 1114 feet. B. Flagstaff. Flagstaff, at an elESvation of' 6907 feet, is the highest of the six cities. It is situated on a volcanic plateau in north-central .Arizona about 130 miles NNE of Phoenix and 10 to 15 miles south of the San Francisco Mountains. These mountains, with peaks rising to over 12,000 feet are the highest in the state. The city itself is located in the center of the largest standing forest of yellow pine in the United States. Since January 1950 weather observations for Flagstaff have been taken at the municipal airport, which is located about four miles south of town and at an elevation of almost 7000 feet.
* Unless otherwise indicated all heights in this section will refer to the elevation above mean sea level. -4-
C. Winslow. Winslow is located in north-central Arizona at the southern end of the Little Colorado River Valley and about 130 miles northeast of Phoenix. The Little Colorado River runs from south to north a couple of miles east of town. The Mogollon Rim, forty to sixty miles to the west and south of Winslow, contains several peaks rising close to 9000 feet. To the north and east lies the Colorado Plateau, which is desert-like and relatively flat, although punctured here and there by a large number of mesas and bluffs. Weather observations for Winslow have been taken at the municipal airport, about one mile west of town, since October 1931. The airport elevation of 4880 feet is only about thirty feet above that for the city.
D. Prescott. Prescott, at an elevation of 5380 feet, is situated in an inter mediate and comparatively mountainous and colorful region between the forested plateaus to the northeast and the arid desert region to the southwest. It is about eighty miles NNW of Phoenix in a small valley surrounded on all sides by mountains rising close to 2500 feet above the valley floor. However, there are no peaks higher than 8000 feet within fifty miles of Prescott. The immediate surrounding country is mostly rolling and covered with sparse desert-like-vegetation. Four small lakes, lying four to six miles to the south and southwest, are usually dry in years of below normal rainfall. Weather observations for Prescott have been taken at the municipal airport, about eight miles northeast of town, since August 1942. The airport elevation of 5014 feet is more than 300 feet below that of the city, so that weather records for the two locations should be only partially comparable. E. Tucson. Tucson, the southernmost of the six cities, is located on the fringe of the Salt River Valley at an elevation of 2376 feet. It is about 110 miles southeast of Phoenix. The normally dry Santa Cruz River runs from south to north just west of town. Within ten to fifteen miles of the city the terrain is flat or gently rolling, with many dry washes. The valley floor, which rises gradually in -
-5-
elevation from the north and northwest to the south and southeast is encircled by
the Santa Catalina, the Tucson, the Tanque Verde, and the Rincon Mountains. The
higher peaks, rising to 8000 to 9000 feet, lie to the north and east at a distance of between 20 and 35 miles from Tucson.
The airport, at which weather observations have been taken since October 1948,
lies about five miles SSE of town. Its higher elevation (about 2550 feet) and more open location give it a milder climate than that experienced within the city.
F. Yuma. Yuma, at an elevation of 142 feet, is the lowest of the six cities.
It is located in the extreme southwest corner of Arizona, on the east bank of the
Colorado River and 173 miles southwest of Phoenix. The Yuma Desert, with its im
pressive sand dunes, lies to the south. The only mountains in the vicinity with
peaks rising above 3000 feet are the Castle Dome Mountains, 45 miles to the north
east, and the Gila Mountains, 25 miles to the southeast. Fifty miles to the west lies the Imperial Valley of California.
The Weather Bureau office for Yuma has been located at the Yuma County Airport
since September 1948. It is about• five miles SSE of town and at an elevation of 200 feet.
III. NATURE OF THE DATA.
A. Elements •
1. Cloud amounts. Both the average cloud amount and the frequency of
occurrence of various tenths of cloud cover were determined for each city, for each
month, and for each of the eight observations times, 0020, 0320, 0620, 0920, 1220,
1520, 1820, and 2120 MST (Mountain Standard Time). These data are presented in the
48 tables in the Appendix. The upper right-hand column of each table lists the
average cloud cover in tenths for each month at the given hour and station. The
corresponding frequency distribution of amounts (in tenths) is given in part A of
each table, with the units used being the frequency of occurrence per thirty days
(or, roughly, per month). For each month (i.e., each row of the table) the sum of -6-
the individual frequency values should be thirty, plus or minus one-tenth (due to round-off errors). The number of observations upon which the statistics in the upper-half of the table are based is given in the next to last column.
The use of "per thirty days" rather than 11percent 11 in this paper deviates from the customary procedure and is based on the hope that the reader is more interested in the number of days per month that a certain event may occur than in the number of cases out of 100 that it occurs. A constant thirty day month was used in order to make the month-to-month frequency changes comparable. In the following "per month" will be used synonymously with "per thirty days".
Another deviation from the normal procedure in this paper is the use of the terms cloud cover or cloud amount rather than sky cover to signify that fraction of the sky obscured by precipitation or lithometers or covered by fog or the bases or sides of clouds (see McDonald (1958), page 37). Technically, the latter is probably more correct, but the former seems more meaningful, especially since obscuring phenomena rarely occur in the Southwest on more than one or two days in a month. Also, the most common type of obscuration at all stations, except possibly Yuma, is precipitation, in which case an overcase layer of clouds almost surely exists. The reader should therefore bear in mind that obscuring phenomena are treated as a cloud type in this paper and that their influence is included in the statistics of the upper half of the Appendix tables. 2. Cloud types. Part B of the tables in the Appendix contains data, for each hour and station, on·the monthly frequency of occurrence of the more common cloud types and fog, based on the number of observations given in the last column. Again the units used are "per thirty days". For the reader who is not too familiar with the various cloud types, a brief description of each is given below. These defini~ tions have been taken from the International Cloud Atlas, Volume I, (1956). (Volume II and the Abridged Atlas of this series contain many interesting pictures of the various cloud types. Ludlam and Scorer (1957) have also recently published a "pictorial guide" of clouds.) -7-
Fog (F) • A suspension of very small water droplets in the air, generally reducing the horizontal visibility at the earth's surface to less than 1 km (5/8 mile). When sufficiently · illuminated, individual fog droplets are frequently visible to the naked eye; they are then often seen to be moving in a somewhat turbulent manner. The air in fog usually feels raw, clammy, or wet.
This hydrometeor forms a whitish veil which covers the landscape; when mixed with dust or smoke, it may, however, take on a faint coloration, often yellowish. In the latter case, it is generally more persistent than when it consists of water droplets only.
stratus (st). Generally grey cloud layer with a fairly uniform base, which may give drizzle, ice prisms, or snow grains. When the sun is visible through the cloud, its outline is clearly discernible. Stratus does not produce halo phenomena except, possibly, at very low temperatures.
Sometimes Stratus appears in the form of ragged patches called fr.actostratus.
Stratocumulus {Sc). Grey or whitish, or both grey and whitish, patch, sheet, or layer of cloud which almost always has dark parts, composed of tessellations, rounded masses, rolls, etc., which are non-fibrous (except for virga) and which may or may not be merged; most of the regularly arranged small elements have an apparent width of more than five degrees.
Cumulus (Cu). Detached clouds, generally dense and with sharp outlines, developing vertically in the form of rising mounds, domes, or towers, of which the bulging upper part often resembles a cauliflower. The sunlit parts of these clouds are mostly brilliant white; their base is relatively dark and nearly horizontal.
Sometimes Cumulus is ragged. In this case it is called fractocumulus.
Cumulonimbus (Cb). Heavy and dense cloud, with a considerab11.e vertical extent, in the form of a mountain or huge towers. At least part of its upper portion is usually smooth, or fibrous or stra.i.ted, and nearly always flattened; this part often spreads out in the shape of an anvil or vast plume. -8-
Under the base of this cloud which is often very dark, there are frequently low ragged clouds either merged with it or not, and precipitation sometimes in the form of virga. Often hanging protuberances like udders are seen on the under surface of cumulonimbus clouds, in which case the name cumulonimbus mammatus applies.
Altostratus (As). Greyish or bluish cloud sheet or layer of striated, fibrous, or uniform appearance, totally or partly covering the sky, and having parts thin enough to reveal the sun at least vaguely, as through ground glass. Altostratus does not show halo phenomena.
Nimbostratus (Ns). Grey cloud layers, often dark, the appearance of which is rendered diffuse by more or less continuously falling rain or snow, which in most cases reaches the ground. It is thick enough throughout to blot out the sun. Low, ragged clouds frequently occur below the layer, with which they may or may not merge.
Altocumulus (Ac). White or grey, or both white and grey, patch, sheet, or layer of cloud, generally with shading, composed of laminae, rounded masses, rolls, etc., which are sometimes partly fibrous or diffuse and which may or may not be merged; most of the regularly arranged small elements usually have an apparent width of between one and five degrees.
Sometimes altocumulus clouds occur as altocumulus castellanus which presents, in at least some portion of its upper part, cumulus-like protuberances in the form of turrets which generally give the cloud a crenelated appearance. The turrets, some of which are taller than they are wide, are connected by a common base and seem to be arranged in lines. The castellanus character is especially evident when the clouds are seen from the side.
Cirrus (Ci). Detached clouds in the form of white, delicate filaments or white or mostly white patches or narrow bands. These clouds have a fibrous (hair like) appearance, or a silky sheen, or both. -9-
Cirrostratus (Cs). Transparent, whitish cloud veil of fibrous (hair-like) or smooth appearance, totally or partly covering the sky, and generally pro ducing halo phenomena. Cirrocumulus (Cc). Thin, white patch, sheet or layer of cloud without shading, composed of very small elements in the form of grains, ripples, etc., merged or separate, and more or less regularly arranged; most of the elements have an apparent width of less than one degree.
In this study nimbostratus has been combined with altostratus and cirrocumulus with cirrostratus. Al.so no effort has been made to differentiate between fracto stratus and stratus, between fractocumulus and cumulus, or between altocumulus castellanus and altocumulus. Considerable care must be taken in interpreting the cloud type statistics. As pointed out by DesJardins (1958), since July 1948 cloud types have been recorded by layers, i.e., the lowest layer, second layer, third layer, and fourth layer. With in each layer only the dominant cloud type or obscuring phenomenon is reported. This procedure was adopted by the Weather Bureau in order to make it possible to enter cloud type data on punched-cards, if only in an abbreviated form. As a result primarily of this change in observing techniques, DesJardins found a ficti tious decrease of about 15 to 20 percent from pre-1948 to post-1948 in the number of reported low and middle clouds (stratocumulus, cumulus, cumulonimbus, alto stratus, and altocumulus). No significant change was noted in the frequency of occurrence of high clouds (cirrus and cirrostratus). Before July 1948, all cloud types, irrespective of their density, were noted by the observers. The cloud type data available for this study did not begin until June 1951, so that no problem arises here due to this observing change. It should be noted however that the values given in the tables are to be interpreted technically as the average number of days (per thirty days) that a given cloud type is the dominant type in any of four possible cloud layers in the atmosphere. To get an I
-10-
estimate of the actual frequencies of occurrence, the values for low and middle clouds should be increased by roughly 15 to 20 percent while those for high clouds should be taken as they stand (assuming that DesJardins' results for Tucson can be extended to the other cities). Because from zero to four cloud types may be recorded at a single observation, the sum of the individual frequency values
for a given month will not be thirty (as for the cloud amount frequencies) but will vary over a rather wide range. For example, the sum of the frequencies for Phoenix at 0020 MST in June is 7.4, while that for Tucson at 1820 MST in July
is 79.9. Quite often the same cloud type was reported in more than one layer. How ever, when this occurred only the lowest layer of the cloud was counted. 3. Thunderstorms. The next to last lower-right-hand column of Part C of the Appendix tables, gives the frequency of occurrence (again, per thirty days) of thunderstorms for each month at a given hour and station. The number of observations upon which the averages are based is given in the last column. B. Period of Record. 1. Cloud amounts. Cloud amount data were extracted from two sources. The first was IBM punched-cards obtained from the National Weather Records Center at Asheville, North Carolina. These cards cover the eight year period from January l, 1948 through December 31, 1955. The second source was the Weather Bureau Local Climatological Data Supplement, from which cloud amount data were
available for July l, 1956 through March 31, 1958. Except for one or two missing observations, the complete nine or ten year record was available for Phoenix, Prescott, and Tucson. For the remaining stations the record is less complete, the periods of missing data being tabulated
below. -11-
Table 1
Periods of missing cloud amount and thunderstorm data.
Station Hour (MST) Periods of Missing Data
Flagstaff 0020 12/17/51 - 3/31/58 0320 12/17/51 - 3/31/58 0620 43 days from 12/20/51 - 3/16/52, 7/1/56 - 3/31/58 0920 5/1/49 - 5/3/49, 7/1/56 - 3/31/58 1220 7/1/56 - 3/31/58 1520 8/26/53 - 3/31/58 1820 8/26/53 - 3/31/58 2120 3/17/52 - 3/31/58 Winslow all hours 8/1/57 - 8/31/57 Yuma all hours 1/1/48 - 9/19/48 -12-
Obviously data are most scarce for Flagstaff, especially during the afternoon
and evening hours.
2. Cloud types. Cloud type data were available only from the IBM punched-cards and then only from June 1, 1951 to December 31, 1955, a period of
four and one-half years. The record is complete for Phoenix, Tucson, and Winslow.
For the remaining stations, the periods of missing data are listed on Table 2.
The data for Yum.a at all hours and for Flagstaff at 0020, 0320, and 2120 MST are
so scarce that they will not be considered further in this paper.
3. Thunderstorms. The period of record for thunderstorms coincides exactly with that for cloud amounts.
C. Homogeneity of the Data. There are several factors which might act in
such a manner as to preclude a direct comparison of the results for the various
stations. Among these are (1) changes in observing methods; (2) different periods of record for cloud amount and cloud type statistics; (3) the smaller number of observations available for Flagstaff than for the other stations; and (4) differences in observing procedures.
1. Changes in observing methods. As mentioned in Section III P2, a major change was made on July 1, 1948 in the method of reporting cloud types. In stead of reporting all visible cloud types, the observer was instructed at that time to report clouds by layers, selecting only the dominant type in each layer
(the number of layers not to exceed four). Since this change, which was initiated to facilitate the punching of the information on cards, occurred before June 1, 1951, when cloud type data became available for this study, it created no problem here.
Beginning in July 1957 observations were made en the hour rather than at twenty minutes after the hour. It is very doubt,ful that the effect of this change on the results could be significant.
2. Different periods of record for cloud amount and cloud type statistics.
The averages for cloud amounts are based on about twice as many observations as those for cloud types. Hence some care should be taken in comparing the two sets -13-
Table 2
Periods of missing cloud type data.
Station Hour (MST) Periods of Missing Data Flagstaff 0020 12/17/51 - 12/31/55 0320 12/17/51 - 12/31/55 0620 43 days from 12/20/51 - 3/16/52 1520 8/26/53 - 12/31/55 1820 8/26/53 - 12/31/55 2120 3/17/52 - 12/31/55 Prescott 0020 6/10/54 - 12/31/55 0320 6/10/54 - 12/31/55 0920 7/8/55 - 7/10/55 2120 6/9/54 - 12/31/55 Yuma 0020 all missing 0320 all missing 0620 6/1/51 - 8/1/55 0920 6/1/51 - 7/31/55 1220 6/1/51 - 7/31/55 1520 6/1/51 - 7/31/55 1820 6/1/51 - 7/31/55 2120 6/1/51 - 7/31/55 -14- of data. Fortunately, the relative distribution of cloud types appears to be almost the same from one year to the next. For example, if cirrus is reported twice as often as altocumulus at 0620 MST in January of one year, then that ratio tends to hold up in any other January at the same time. Similarly, if cumulus is reported twice as often at 1520 MST as at 1220 MST in July of one year, then that ratio also tends to hold up in any other July for the same hours. It is the actual frequencies of occurrence of the cloud types which are most variable from one period to the next and which would seem to require between three and five months of data to yield dependable means. A rough estimate of the reliability of the average frequencies of occurrence of the cloud types for Tucson, based on 4½ years of data, is given in Table 3. These results are compared with those of DesJardins (1958), who used data for the nine-year period from
July 1, 1945 to July 1, 1954. The values given in the table are the means of eight three-hourly average frequencies of occurrence. Unfortunately the same eight hours were not used in both studies (DesJardins analysed data for 0230, 0530, 0830 MST, etc.,), so that a small part of the discrepancies in the table should be due to this difference (which also precludes comparing the individual hourly averages). Only the five cloud types stratocumul~s, cumulus, altocumulus, cirrus, and cirrostratus are considered, with the latter two being combined. This was necessary because of the method of analysis and period of study used by DesJardins. The greatest difference between the two sets of averages is 2.5 days per month and occurs for altocumulus in March. In general, the agreement is poorer for altocumulus than for the other cloud types, with the shorter record giving higher frequencies of occurrence than the longer record in seven months. However the mean absolute difference between the two sets is only 0.9. This, together with the heterogeneity of the Tucson cloud data used by DesJardins, suggests that, even with respect to frequencies of occurrence, a period of record as short as 4½ years is sufficient to give a reasonably good picture of the long-term cloud distribution. -15-
Table 3
Comparison of daily average frequencies of occurrence (per 30 days) of certain cloud types for Tucson obtained (1) by this study using 4½ years of data and (2) by DesJardins using 9 years of data. Averages are based on 8 three-hourly means for each day and month.
CLOUD TYPES
Month Stratocumulus Cumulus Altocumulus Cirrus and Cirrostratus (1) (2) (1) (2) (1) (2) (1) (2)
Jan. 6.3 6.6 2.8 2.1 10.8 8.4 14.3 12.6 Feb. 3.6 3.9 2.5 3.3 7_.5 7.5 11.2 12.9 Mar. 5.2 3.9 4.8 4.5 11.2 8.7 14.1 14.1 Apr. 3.0 3.0 5.2 5.4 9.2 9.3 13.2 14.7 May 2.1 1.5 6.4 6.3 10.2 8.1 12.1 12.9 Jun. 1.6 1.8 5.8 7.2 8.6 10.5 9.3 9.0 Jul. 6.3 4.8 13.1 12.9 23.2 21.9 17.0 18.3 Aug. 5.7 4.5 13.4 11.4 19.3 21.0 18.2 17.4 Sept. 2.9 2.7 9.0 9.6 12.1 13.2 6.7 9.0 Oct. 2.9 3.6 5.4 6.3 9.2 8.7 10.5 10.2 Nov. 5.0 4.5 3.4 2.4 8.5 7.2 11.5 12.0 Dec. 4.6 5.7 1.7 2.1 10.4 9.0 15.4 13.8 Year 4.1 3.9 6.1 6.1 11.7 11.1 12.8 13.1 -16-
3. The smaller number of observations available for Flagstaff than for the other stations. The length of record for Flagstaff cloud amount data is roughly half of what it is for the other stations, being less than half for the nighttime observations and greater than half for the daytime observations. This brings up the question of whether it is justifiable to compare the means for Flagstaff with those of the other stations. Table 4 is presented in order to show that it is probably all right to do so. The values given for each month and station are the averages of the eight three-hourly means for the day. Except for Flagstaff, two means are given for each station. The first is the mean daily cloud cover based on a period of record concurrent with that for Flagstaff. The second is the mean based on the complete period of record. Because of missing data, it was not possible to get monthly means for Yuma concurrent with those for Flagstaff until September. The average difference between the two sets of means is 0.15 tenths and never exceeds 0.5 tenths (Tucson in September). In general, January, March, and August are slightly cloudier and September and December slightly clearer for the complete period than for the abbreviated period. However, the differences in all cases seem to be negligible so that there should be no hesitancy about comparing
Flagstaff's shorter record with the longer record of the other stations. Based on the average of the differences in the monthly means for the two periods for the five stations with a complete record, the mean monthly cloudiness for Flagstaff for the complete period may be estimated. These values are entered in parentheses in Table 4. 4. Differences in observing procedures. Although the Weather Bureau Manual of Surface Observations (1951) largely fulfills its purpose of securing uniformity in the taking and recording of weather observations, it still appears to be possible for two observers to report the same weather situation differently, This is especially true with regard to cloud types. Even the most experienced -17-
Table 4
Average total daily cloud cover (tenths) near six .Arizona cities for each month and for two periods of record: (1) four to eight year period for which data are available for Flagstaff and (2) complete 9½ year period. Averages are based on 8 three-hourly means for each day and month.
Station
Month Flagstaff Phoenix Prescott Tucson Winslow Yuma (1) * (1) (2) (1) (2) (I) (2) (1) (2) TIT (2) Jan. 4.9 (5.2) 3.5 3.9 4.2 4.4 3.9 4.2 4.4 4.7 3.5
Feb, 4.7 (4.8) 3.7 3,8 4.2 4.1 3.7 3.9 4.3 4.3 2.8 Mar. 4.5 (4.8) 3.8 4.1 4.1 4.3 4.1 4.4 4.1 4.3 3.2 Apr, 4.1 (4.o) 3.3 3,2 3.7 3.7 3.5 3.3 3.9 3.9 2.2 May 3.3 (3.4) 2.2 2.3 2.6 2.8 2.3 2.4 3.1 3.1 1.4 Jun. 2.1 (2.1) 1.5 1.6 1.8 1,8 1.8 1.8 2.1 2.1 o.8 Jul. 4.8 (4.9) 4.o 4.2 4.4 4.5 5.7 5.8 4.9 4.9 2.4 Aug. 4.1 (4.4) 2.9 3,3 3.6 3.8 4.3 4.6 3.8 3.9 2.0 Sept. 2.6 (2.3) 1.8 1.5 2.1 1.9 2.5 2.0 2.4 2.1 1.1 1.0 Oct, 2.6 (2.6) 2.3 2.3 2.4 2.4 2.4 2.4 2.4 2.6 1.6 1.5 Nov. 3.0 (3.0) 2.2 2.2 2.5 2.5 2.4 2.3 2.9 2.8 1.7 1.7 Dec. 4.4 (4.2) 3.7 3.5 4.o 3.6 4.o 3.7 4.1 3.9 3.0 2.9
* Estimated values for the complete period. -18- meteorologists will often disagree as to whether a certain cloud layer is, for example, stratocumulus or altocumulus. Or if they agree that it is altocumulus, they might not agree that it is present in only one layer instead of two separate layers. This, together with obvious differences in the inherent ability of observers, makes it rather difficult to judge whether an anomaly in cloud type conditions at a certain station is a reality or is due to a quirk of the observer. A possible example of the foregoing is differences in the number of reports of layers of altocumulus among the six stations. Table 5 gives for each month the ratio of the total number of reports of (1) altocumulus and (2) multiple-layered altocumulus for Tucson to that for Phoenix, Prescott, and Winslow. Any differences in the total number of observations have been taken into account. Lack of data prevents including ratios for Flagstaff and Yuma. What data are available do suggest, however, that reports of more than one layer of altocumulus at these stations are rare. The result that more than one layer of altocumulus is reported on the average, at least four times as often at Tucson as at any other station is a discrepancy that can only be partially explained by the greater frequency of occurrence of altocumulus at Tucson. There is a possibility that the mountains in the near vicinity might have some effect in stratifying the clouds but it is hard to believe that their influence could be very great.
The greatest number of reports of multi-layered altocumulus at Tucson occurs in July and August in the morning between 0600 and 0900 MST. At this time more than one layer is reported in one out of every three observations. Multiple decks of stratocumulus and cumulus are also reported occasionally at the six stations but not nearly as frequently as those of altocumulus.
Since the existence of more than one layer of a given cloud type was not considered in the body of this report, the above discrepancy can logically be over looked. However, the suspicion remains that if an excess of multiple layers of alto cumulus are reported at Tucson then an excess of layers of different cloud types -19-
Table 5
Ratio of the total number of monthly reports of (1) altocumulus and (2) more than one layer of altocumulus at Tucson to the same quantities at Phoenix, Prescott, and Winslow
Station
Month Phoenix Prescott Winslow (1) (2) (1) (2) (1) (2) Jan. 1.7 6.2 1.6 8.o 1.0 2.7 Feb. 1.7 4.o 1.6 1.8 1.0 2.4 Mar. 1.8 23.5 1.6 4.8 1.3 3.4 Apr. 2.0 7.7 1.6 15-3 1.2 5.1 May 2.0 16.5 1.4 7.3 1.1 7.3 Jun. 1.8 5.0 1.4 2.8 1.3 4.5 Jul, 1.6 7,7 1.7 5.1 1.4 3,9 Aug. 1.6 15.0 1.7 8.5 1.3 5.9 Sept, 1.6 11.2 1.6 2.7 1.6 6.2 Oct, 1.6 4.o 1.5 1,4 1.4 4.o Nov. 1.8 5.8 1.5 2.7 1.3 2.7 Dec. 1.6 7.1 1.7 5.0 1.1 2.7 Year 1.7 8.5 1.6 4.6 1.2 4.o -20- might also exist. In this case the frequencies of occurrence of all cloud types at Tucson would be too great, relative to the frequencies at the other stations. This point should be kept in mind in the discussion of Part rv and V of this report.
D. Nighttime Observations. Cloud observations taken at night are notoriously poor, both with respect to types and amounts. The latter, especially, are con sistently underestimated, usually resulting in a sharp, but often spurious, decrease of cloudiness at sunset and a similar increase of cloudiness at sunrise. Although a maximum of cloud cover is to be expected during the afternoon, when convective activity is best developed, it is very doubtful that it is as great (relative to the nighttime minimum) as indicated by the data. A nighttime decrease of cloudiness should be correlated with a similar decrease of precipitation. McDonald (1956), working with data for southeastern .Arizona, found that a minimum of rainfall occurs during the morning in summer (July and August), while in winter (January and February) a very weak minimum is noticed around midnight. These results are in general agreement with those obtained in this study, except that the winter nighttime cloudiness minimum is exaggerated. In Section V, dealing with the diurnal variation of cloud amounts and cloud types, the following procedure has been adopted. Whenever a sharr, increase of cloud cover is reported at a station at the first observation after sunrise followed by a decrease later in the day the station is considered to have a daytime minimum of cloudiness. The same holds for each of the cloud types. Double peaks of cloud amounts or types at sunrise and sunset are generally indicative of a broad maxi- mum during the night. Since all weather stations are affected equally by the hardships of nighttime cloud observations, a comparison among them would seem to yield more valid results than a study of each station individually, despite possible differences in inter pretation given by two observers to the same situation. Such a comparison will be one of the primary purposes of the following sections. -21-
IV. ANNUAL VARIATION OF CLOUD AMOUNTS, CLOUD TYPES, AND THUNDERSTORMS. A. Cloud .Amounts.
1. Average cloud cover. The upper part of Figure la gives curves of
the monthly variation of average cloud cover in tenths for each of the six stations.
Each point represents the average of eight hourly means, taken three hours apart through the day. The dashed horizontal lines indicate the annual means.
All stations display a double maximum of cloudiness during the year, one in winter and the other in summer. The winter maximum occurs in January at Flagstaff,
Winslow, Prescott, and Yuma and in March at Phoenix and Tucson. All stations have their summer maximum in July. The February dip in cloud cover, which is most noticeable at the more southern stations, and the October increase, present at all stations except Flagstaff, would probably disappear if a longer period were used to compute the means. This is especially true of the increase in October, which arose almost entirely from an extremely cloudy October 1957. Note, from Table 1, that data for Flagstaff, the only station not to show an October maximum, were not available for this month.
The winter and summer maxima are almost e4ual in magnitude at all stations except Tucson and Yuma. The summer maximum in July at Tucson represents the greatest mean cloudiness of any station for any month, while the July maximum at
Yuma is barely above the normal for the year and only slightly greater than the spring and fall minima at the other stations.
Major minima of cloud cover, of roughly e4ual magnitude, occur at all stations in June and September. The fall minimum generally lasts longer than the one in spring, especially at the more northern stations. As a result, except at Yuma,
May cloudiness is e4ual to or greater than that in September, October, and November.
For the year as a whole cloudiness is greatest at Flagstaff and least at
Yuma, in general decreasing southward. Tucson, with its large summer maximum, is the exception to the rule, having an annual mean only slightly less than that for Winslow and considerably higher than that for Phoenix. -22-
It is worthy of mention that all stations have their maximum rainfaill in the summer (July to September). In fact, at Prescott, Tucson, and Winslow there is about three times as much precipitation in the wettest summer month as in the wettest winter month. That this occurs in spite of the nearly equal average cloudiness in the two seasons (except at Tucson) probably reflects the greater liquid water content of the atmosphere over Arizona in summer than in winter, rather than any difference in the precipitation efficiency. 2. Frequency of occurrence. The lower part of Figure la gives the average frequencies of occurrence (per thirty days) of cloud cover of 0.l or less and 0.9 or more for each month and station. Again, the points represent the average of eight hourly means, taken three hours apart through the day. They can thus be interpreted, roughly, as the number of days during a given months that an average cloud cover of 0.1 or less, for example, may be expected at a particular station at an average hour. For simplicity, in the following cloud coverage of o.l or less will be referred to as clear, while coverage of 0.9 or more will be referred to as overcast. The dashed lines in the figures represent the respective annual means. These curves are very similar to those for average cloud amount, with, of course, those for the number of clear days being inverted. However, there are differences. For example, while for most stations winter and summer mean cloud cover are almost the same, only Tucson, with its high summer cloudiness, fails to have a winter maximum of overcast skies. This is due to the more stratified nature of clouds in winter than in summer. The greatest part of winter storminess occurs with large scale disturbances, whose associated cloud systems are spread rather uniformly over wide areas. This is not true in summer when convective activity is a maximum and cloudiness is about equally dependent on atmospheric and terrain conditions.
5 5
0 0
20 20
30 30
10 10
·25 ·25
15 15
NJ NJ
NJ NJ
JS JS
JS· JS·
Yuma Yuma
MM MM
MM MM
~ ~
JJ JJ
JJ JJ
N N
N N
JS JS
JS JS
Tucson Tucson
MM MM
MM MM
JJ JJ
JJ JJ
N N
N N
JS JS
Phoenix Phoenix
MM MM
MM.JS MM.JS
JJ JJ
JJ JJ
Month Month
Month Month
months. months.
N N
N N
by by
JS JS
JS JS
yea'r yea'r
Prescott Prescott
the the
MM MM
MM MM
for for
JJ JJ
JJ JJ
N N
N N
JS JS
JS JS
stia.tistics stia.tistics
e.r e.r
Winslow Winslow
✓
.. ..
MM MM
MM MM
.<:o
-,..---,,-..-,.-....,...--,,--"T"'""--,-
JJ JJ
JJ JJ
clo.uci clo.uci
\ \
N N
N N
of of
I I
JS JS
JS JS
Summary Summary
Flagstaff Flagstaff
MM MM
MM MM
la. la.
J J
J J
-
.. ..
Ui-il--r--r-,,....--r---:r--, Ui-il--r--r-,,....--r---:r--,
3 3
1 1
2 2
7 7
5 5
8. 8.
4-
0-'------
-
• •
-
-6 -6
~1>+-
°' °'
ol2 ol2
>~ >~
.. ..
a, a,
O O
301 301
::,o ::,o
'M 'M
-c, -c,
'M 'M
111 111
~ ~
~ ~
a, a,
C: C:
u u
a, a,
~ ~
u. u.
g g
11) 11)
:Fig. :Fig.
.µ .µ
~ ~
,-( ,-(
g g
f f
....., .....,
......
.. ..
u u
u u
>, >,
u u
o o o
a, a,
A. A.
C: C: ~ ~
a, a,
Ill Ill
11) 11)
.µ .µ
f: f:
-c, -c,
.c .c
......
>-a,~s >-a,~s
a, a,
>. >.
~ ~
~ ~
>, >,
0 0
a, a,
~....., ~....., tr..-! tr..-!
a, a,
6 6
!~~1 !~~1
< <
4-1 4-1
'M 'M
,-( ,-( .c .c -23-
Because of the periodic character of winter weather, i.e., short periods of
fine weather alternating with short periods of poor weather, and the spotty
nature of summer convective activity, it might also be expected that, with equal
amounts of cloud cover, a smaller number of clear days would occur in summer than
in winter. This is the case at all stations except Yuma, where the mean cloud cover in summer is relatively low.
The frequency distribution of cloud amounts is highly bimodal in all months, with double peaks at cloud coverages of 0.1 or less and 0.9 or more. For the year as a whole, roughly 75 per cent of the days fall into one or the other of these two categories. As might be expected, from what was said in the last paragraph, this percentage is higher in winter (about 80 per cent), and lower (about 70 per cent) in summer.
Most stations have an annual average of between two and three times as many clear days as overcast days. The ratio is greatest (6.6) at Yuma and smallest at Flagstaff (2.1). It is also smaller in winter and summer than in fall and spring, the seasons of least cloudiness. Only Tucson in July normally has more overcast days than clear days.
B. Cloud Types and Thunderstorms.
1. Introduction. Each row of curves of Figure lb gives the mean number of days per month that the indicated cloud type or thunderstorms can be expected at each station except Yuma during an average hour. The last qualifying phrase is necessary to emphasize that the plotted points are the average of eight hourly mean frequencies of occurrence taken three hours apart and not the average of the number of days per month that a particular cloud type is reported at any hour of the day. The latter quantity would always be equal to or greater than the former.
For example, if stratocumulus on the average occurred every day of a certain month at 0920 MST but never at any of the other seven hours, then its frequency of occurrence per month would be 30 days, while its frequency of occurrence per month -24- at an average hour would only be 30/8=3.75 days. As mentioned earlier, the cloud type and thunderstorm data for Yuma are too spotty to warrant presenting them here. Only the most common cloud types are considered in this and the succeeding sections. 2. Stratocumulus. stratocumulus is not a dominant cloud type in
Arizona, rarely occurring more than six days per month at a particular hour. It is most common at Winslow and Tucson and least common at Phoenix and Flagstaff, thus showing no definite regional pattern. In general, stratocumulus has summer and winter maxima, with the latter being dominant except at Tucson. At Flagstaff, Prescott, and Phoenix,the summer maximum is below the annual mean. The trend from one month to the next is very similar to that for the mean cloud cover (Figure la), even including the dip in February. 3. Cumulus. Cumulus is definitely most closely associated with the warmer half of the year, the peak activity coming in July and August at all stations except Flagstaff. The May maximum of cumulus at Flagstaff might be due to the scarcity of data for this station, but more likely it is the result of a combination of increasing insolation and the northward retreat of the upper level jet stream and its associated low level storminess. The southward decrease of the maximum would tend to support this hypothesis, however more data are needed. Cumulus is the dominant cloud type at Flagstaff and Winslow in late spring and summer. However, in July and August it is most common at Tucson, having an average hourly frequency of occurrence of thirteen days per month. 4. Cumulonimbus and Thunderstorms. a. Cumulonimbus. Cumulonimbus is typically a summer cloud type, all stations displaying a July or August maximum. However, its hourly average frequency of occurrence as the dominant type in a cloud layer is relatively low, never exceeding six days per month. The secondary maximum of cumulonimbus activity at Flagstaff in spring is associated with the maximum of cumulus at the same time. Fig. lb. Average hourly frequency of occurrence (per thirty days) of the prevailing cloud types for the year by months.
30 Flagstaff Winslow Prescott Phoenix Tucson 30 ., ::, ,....; § 20 20 ::, 0 0 +J., 10 10 +J.,"' ~ 0 ~ ;::?-s;;.;,r::y~ 0 J M "M J s N J J M M J s N J J M M. J s N J J M M J s N J J MM J s N J
., 20 20 ::, ,....; § ::, 10 10 0
0
30 3 "'::, I ~ rt .....e 20 2 i:: e:I 0 ~ ,....; (1) ~ 10 1 ~ ::, rt 0 0 I o-i-_,e:~;::,...+-, , ,A,., 1 ...... -r--.--,~ • , ,& . .--~,W.,r-,.... _,. 0 ~ J MM JS NJ.) MM JS NJ J MM JS NJ J MM JS NJ J MM JS N J G.I 0 i:: 30 G.I "' "'::, "'::, 0 +J 20 0 ., 0 .... +J"' C "'0 10 +J :>, ,-; 0 ., i:: (IJg. 0 (IJ Ji.."' 30 '30
.....::,"' 20 20 ::, ::,e 0 10 0 10 I\ ...... , 0 OJ MM J s N 1 .1 i1k5sN1.111 M} s ~ J 1 MM J s N 1 MM J N J 30 30
0 20 20 ~ -~"' 10 ...... -----+---=,..__ 10 0-1-~----- 0
30 30
r/l 3., 20 20 +J "'r/l e 10 10
·~ o~~~ ~ ;:-::;::;-~ ~ 0 .J M M J s N J J MM J s N J J M MT~7J J M M J s N J Month -25-
Two points worth noting are (1) the small frequency of occurrence of cumulo
nimbus relative to that of CU!D.ulus at Winslow and Tucson, and (2) the large
frequency of occurrence of cumulonimbus relative to that of cumulus at Phoenix.
Cumulonimbus is reported more often at Phoenix than at either Tucson or Winslow,
while the reverse is true of cumulus. A possible explanation for this, other than
differences in observing techniques, might be the following. Near Tucson and
Winslow the terrain is favorable for the development of a large number of cumulus
clouds almost directly overhead. These are easily spotted by the observers and
generally cover more of the sky than do any cumulonimbuses which might be present.
On the other hand, conditions are not conducive to very intensive cumulus build
ups near Phoenix, which lies at a rather low elevation in a broad valley. As a
result when an observer reports cumulus activity at Phoenix, it is normally
occurring at a considerable distance from the station and closer to the mountains.
He, therefore, is likely to see more of the sides of the clouds than their bases,
and, since the average cumulonimbus has a much greater vertical extent than the
average cumulus, it will appear to be the dominant cloud type.
b. Thunderstorms. As might be expected thunderstorms are most
common in summer and at the mountain stations of Flagstaff and Prescott, the same
stations that have the greatest cumulonimbus activity. The higher frequency of
thunderstorms at Tucson than at Winslow is probably a result of the fonner lying closer to mountains than the latter.
The smallest number of thunderstorms is reported at Phoenix, in spite of the
fact that it reports a relatively large amount of cumulonimbus activity. This
confirms the belief that most of this activity takes place at a considerable
distance from Phoenix and rarely penetrates into the city.
5. Altostratus. Altostratus is not a common cloud type in Arizona,
having an average hourly frequency of occurrence greater than two days per month
only at Prescott in winter. All stations have a mild peak of altostratus activity
during the colder half of the year, indicating that it is associated with the few well-developed frontal disturbances that pass through the area at that time. -26-
6. Altocumulus. Altocumulus is one of the two dominant cloud types in
Arizona, being the most common type at Phoenix and Prescott in summer (July through
September), at Tucson in every month, and at Winslow from December through April
and in July and August. At Tucson in July it has an average hourly frequency of
occurrence of more than 23 days. Although altocumulus is present at all stations
in sizeable amounts throughout the year, it is reported most frequently in
July and August.
The sharp decrease in the frequency of occurrence of altocumulus from Winslow to Flagstaff is difficult to explain without resorting to the small number of observations for the latter station. Since Flagstaff is normally cloudier than
Winslow it would seem that this scarcity of altocumulus at Flagstaff should be mad~ up by an excess of one of the other cloud types at the same station. This does not appear to occur.
Im alternate explanation, which is confirmed by the data, is that multiple
cloud layers are more often reported at Winslow than at Flagstaff. Thus, although
Flagstaff has a higher average cloudiness, Winslow reports more cloud layers and,
hence, more cloud types. This is more likely due to the inability of the Flagstaff
observers to see the higher cloud layers through a lower near-overcast than to any
actual difference in the number of layers at the two stations. 7. Cirrus. Cirrus is the dominant cloud type at Flagstaff, Phoenix,
and Prescott in all seasons except summer and at Winslow in October and November.
It is reported least often in September at all stations except Tucson, which has
a spring minimum. The time of the maximum varies from station to station, but in
general coincides with the time of maximum cumulus activity, especially at
Flagstaff, Phoenix, and Tucson. Most stations also display a secondary peak during
the colder half of the year. This is probably associated with the wintertime in
crease of frontal activity.
Cirrus is the most uniform cloud type, having an annual mean average hourly
frequency of occurrence of between 6.6 and 8.1 days per month at all stations. The minimum occurs at Tucson and the maximum at Phoenix. -27-
8. Cirrostratus. Compared with cirrus, the frequency of occurrence of cirrostratus varies greatly from one station to the next. It is rarely reported
at Flagstaff and Phoenix but is the third most common cloud type at Tucson. It is the only definitely winter-type cloud, having a maximum frequency of occurrence between January and April and a minimum in September at all stations.
The variance in the frequency of occurrence of cirrostratus, especially
between Phoenix and Tucson, may not be completely real but due partly to a difference in interpretation among the various observers. What one observer may see as cirrus, or predominantly cirrus, another may see as cirrostratus, or pre dominantly cirrostratus. The main difference between the two types, at least as far as occurrence is concerned, appears to be that cirrus is associated mainly with the convective activity of summer, and cirrostratus with the frontal activity of winter.
V. DIURNAL V.ARIATION OF CLOUD AMOUNTS, CLOUD TYPES, AND THUNDERSTORMS. A. January.
l. Cloud amounts. The average cloud cover in January (Figure 2, top row of curves) in Arizona is characterized by a general decrease from north to south (or, more properly, from northeast to southwest) and by a large diurnal
variation. As mentioned earlier, the diurnal variation is probably not as great as it appears due to the difficulties involved in observing clouds at night. The arrows beneath the upper curves of Figure 2, and the succeeding figures, indicate the time of the beginning of morning twilight ( t ) and the time of the ending of evening twilight ( 1). These times are approximately one and a half hours before and after sunrise and sunset, respectively.
The second row of curves shows the diurnal variation of the frequency of occurrence (per 30 days) of cloud cover of 0.1 or less (clear) and 0.9 or more (overcast) at each station. The three northern stations, Flagstaff, Winslow, and Prescott, are more likely to be overcast than clear from about 0900 until 1800 MST. -28-
This condition never exists at the southern stations, reflecting the southward decrease in intense frontal activity and cloudiness, 2. Cloud types, The diurnal variation of the five most common cloud types for January is presented, for each station, in the remaining sets of curves of Figure 2, Altocumulus and cirrus are the dominant types occurring about ten days per month during the daylight hours, Both have a nighttime decrease, which, especially for cirrus, is probably exaggerated. However, the daytime peaks are apparently real being derived from the same weak convective activity which pro duces moderate afternoon cumulus activity even at this time of year, Also., it should be noted that cirrostratus does not show any marked diurnal variation., while stratocumulus has an early afternoon minimum., flanked by morning and even ing maxima. The latter should probably be combined into one broad nighttime maximum., which may be at least partially related to an inversion which occasionally forms at night at an elevation of between 8,000 and 9,000 feet (i,e,, mountain top height), Note that the double peak is least well-developed at Phoenix., in the vicinity of which there is no high terrain, The mean diurnal trend in cloud cover can be explained largely by the corresponding trend in the occurrence of cirrus. The two sets of curves are re markably similar., considering that they are not based on the same amount of data, Of course., the other cloud types., especially altocumulus and cumulus., must exert some influence on the average cloud cover. But they are either not as widespread or not as common as cirrus, Thus the reality of the nighttime cloud cover minimum is largely dependent on the reality of the nighttime minimum of cirrus occurrence. B. February. 1, Cloud amounts. In February (Figure 3)., the average cloud cover reaches a maximum a few hours later than in January, occurring just before sunset at Yuma and no earlier than 1500 MST at most of the other stations. The result of this delayed maximum is to produce curves which are much less symmetric than Fig. 2. Summary of cloud statistics for January b? hO\lt"!'l.
I,. 7 Flagstaff Winslow Prescott Phoen:i.x '!'ucsnn Yuma 7
CJ9 .... 0 -1,J 0 cu 10 10 ,,...,, I,. Ill -1,J J'-.-._ >, Ill -...._ ~ ~ cu '0 0 0 0 l 0 ob I 1'2 • ob oo 12 00 o'O I 1'2 • do oo 12 (10 0 ("')
I,. 30 30 Q) Ill 20 20 CJ ;:l C .... Cl, I,. I,. 10 JO ;:l CJ~ CJ CJ _/' 0 ~ .... 0 ~ F-.~~ 0 C >, CJ 30 30 C Q) :, Ill C" ;:l Q; .... 20 20 I,. r,,.. CJ~ 0 10 1.0 ....-1,J ~ cu ,,-.r- ~ ~ 0 (l 00 12 00 00 12 00 00 12 00 00 12 00 00 12 00 30 30 Ill 20 20 ~ I,. •.-1 CJ 10 /' /\_ J\_ 10 0 ~ 0 30 30 Ul ;:l -1,J 21) cu 20 I,. -1,J Ill 0 10 10 I,. I,. ~ •.-1 ~ CJ 0 ~ 0 00 12 00 00 12 00 00 12 00 00------12 00 00 12 00 Hour (MST) Fi.g. 3. Surnmflry of cloud statistics for February by hours. 7 Flagstaff T;rinslow Prescott !'hoenix Tucson Yuma 7 6 6 5 4 3 2 1 t f t i 0 12 0000 12 00 0 12 0030 20 10 0 3() 30 ~,::l"' § 2() :W ::l 0 4-! 0 C .,_, 10 0 C/l 20 0 ::l I,. I,. •rl 10 0 0 ~ /\_ ~ ~ 0 0 30 .c"' in January. Cloudiness increases gradually from an apparent minimum near midnight to the late-afternoon peak and then decreases sharply after sundown. The jagged appearance of the curve for Flagstaff in the morning is probably not real, but a result of insufficient data. Cloudiness again shows a decrease from Flagstaff to Phoenix and from Tucson to Yuma. The mean number of overcast days exceeds that of clear days only at Flagstaff, and then only from about 1300 MST until the ending of evening twilight. All of the other stations, except Yuma, show a near equality of clear and overcast conditions in the afternoon, but the former are always dominant. 2. Cloud types. Other than a slight decrease in the frequency of occurrence of all cloud types, except cumulus, in February there is little change from January. Cirrus and altocumulus continue to be the most common types. The latter now shows a peak at all stations near sunrise and a secondary peak near sunset, thus having a diurnal variation similar to that for strato cumulus. The shift of the maximum cloud cover toward the late afternoon is reflected by a similar shift in the occurrence of cirrus, which now reaches its peak be tween 1500 and 1800 MST at all stations. It is likely that the increased cumulus activity in February, with a maximum between 1200 and 1500 MST, is sufficient to produce this late afternoon maximum of cirrus. In January any cirrus derived from the rather weak cumulus activity in that month is probably insignificant compared to that associated with frontal disturbances. One further difference between January and February is the development, in the latter month, of a nighttime minimum of cirrostratus at Winslow and Tucson, the only stations at which it is reported in any great amounts. C. March. 1. Cloud amounts. In March (Figure 4) there is a much smaller south ward decrease in cloudiness from Flagstaff to Tucson than in either January or -30- February. This is due mainly to a decrease of nighttime cloudiness at Flagstaff and an increase at Phoenix and Tucson. The result is a minimum cloud cover near midnight of between 3.1 and 3.6 tenths at all stations except Yuma. The maximum continues to occur in the late afternoon, so that the asymmetry of February is still present. However, the morning increase of cloudiness is less steady than in February, apparently occurring in two steps, one from midnight to near sun rise, and the other from about noon, or shortly before, until the late after noon. Between sunrise and midday the increase of cloudiness is slight at most stations, with there being some indication of a decrease around 0900 MST. Overcast skies are more common than clear skies in March in the mid afternoon at Flagstaff, Winslow, Prescott, and Tucson. In this respect March is more like January than February. At all stations the d.iurnal variation of the frequency of occurrence of clear skies is greater than that of overcast skies. The result is that skies are more likely to be reported as either overcast or clear at night than during the day. Whether they actually are or are not is difficult to say. It is probably that convective activity during the day could act in such a manner as to decrease the frequency of both clear and overcast skies. But, on the other hand, seeing conditions are such that at night most observers probably report an excess of both conditions. 2. Cloud types. Cirrus and altocumulus are the dominant cloud types in March at all stations except Flagstaff, where cumulus is most common. The correlation between the diurnal variation of cloud amounts and of cirrus is not as good as it was in February, probably because of the increasing importance of the other cloud types, especially cumulus. Why the peak cirrus activity at Winslow, and possibly also at Flagstaff, should occur so early in the day (near noon) is difficult to understand. It may be that the strong cumulus activity at these two stations during the afternoon tends to obscure higher clouds, and thus to reduce the number of reports of cirrus. Fig. 4. S=ary of cloud statistics for ~arch by hours. 1-1 7 Fla~staff Winslow Prescott Phoenix CJ Tucson 7 ~ 0 6 tJ 6 5 5 4 4 3 3 2 2 l t J ti i ! t t t i 1 0----~ono~•ti•~~·~'doon 0 Ill (!) 30()0 1?. .. oo do ' 1'2 ' do oo 12 u, 1-1 0030 Q) 0 ._.. .-< E 0 1-1 ~ ~ 2 Q) 20 6 j r11 10 C) I IJ-1--.-----,----,.-----, 0 30 VJ ::, '""::, E, 2 ::, 20 t) I+; 0 ,I.I C 10 ,.,. "'1-1 .j.l Ill Ill ~ ::,, '--- ~ ~ -0"' l?. 00 00 12 00 00 12 00 00 12 00 00 12 0rr, 1-1 30 (1, ,._,t', a, Ill 2 20 ~. ::, i:: 3 30 Ill 2 20 e1-1 •rl C) ...J",.. f'\ r\ J'\ 10 0 30 Ill ;:I .j.J 2 20 "'1-1 .j.J Ill 0 1-1 1-1 10 •rl ____...... __ C) 0 on ---12 00 00 12 00 00------12 0000----- 12 00 00 12 00 Hour (MST) -31- The minimum stratocumulus activity is reported at all stations in the early afternoon, just prior to the time of greatest buildup of cumulus (1500 MST), from which it is probably derived. Altocumulus shows somewhat the same behavior, although the midday minimum is not as well-developed as the one reported at night. The reality of the nighttime minimum of stratocumulus and altocumulus is questionable, especially that of the former. Also questionable is the large number of reports of altocumulus and cirro stratus at Tucson. The latter is "observed" more than twice as often in March at this station as at any other. It is possible that many of these observations should actually have been recorded as cirrus, whose frequency of occurrence at Tucson is suspiciously low. The decrease in the rate of cloudiness increase around 0900 MST can be ex plained by referring to the cloud type curves. It occurs at a time when strato cumulus and altocumulus are decreasing toward their midday minima and cumulus activity is just beginning to develop. D. April. 1. Cloud amounts. Nighttime cloudiness decreases sharply at all stations in April (Figure 5). On the other hand af'ternoon cloudiness is almost at the same level as in March at Winslow and Prescott and actually increases at Flagstaf'f, giving this station a very large diurnal range. At Phoenix, Tucson, and Yuma both days and nights are clearer than in March. As a result a strong north to south decrease of cloudiness is reestablished after a one month lapse. The decrease, or leveling off, of cloud cover noticed in March around 0900 MST is still present in April at most stations, being most marked at Winslow. Con sidering the questionable nature of the nighttime means, it may be that the actual minimum cloud cover occurs near sunrise or shortly thereaf'ter, rather than near midnight, as indicated by the data. This reasoning would be in line with McDonald's (1956) observation of a change from a weak midnight maximum of precipitation in winter to a stronger morning minimum of precipitation in summer in southern .Arizona. -32- There is still a short period in the afternoon at the three northern stations of Flagstaff, Winslow, and Prescott when overcast skies are more common than clear skies. The length of this period varies from about six hours at Flagstaff to 2½ hours at Winslow. Phoenix, Tucson, and Yuma are much more likely to be clear than overcast at all hours. 2. Cloud types. The frequencies of occurrence of all cloud types, except cumulus and, to a lesser extent, cirrus, decrease in April. As in the previous months cumulus activity is confined almost entirely to the daylight hours, particularly to the afternoon, reaching a peak near 1500 MST. The in crease in the number of reports of cirrus at all stations, except Prescott and Winslow, occurs in spite of the sharp drop in the number of frontal passages from March to April and may be attributed to the increased cumulus activity. Again the magnitude of the nighttime minimum of cirrus must be questioned. As in March stratocumulus is least well-developed near midday, although the minimum is not very pronounced at Prescott or Phoenix. The same is probably true of altocumulus, which continues to be one of the most frequently reported cloud types. However, its reported minimum for the day still occurs at night. E. May. 1. Cloud amounts. In May (Figure 6) the late afternoon maximum of cloud cover is maintained. The time of the minimum is still poorly defined, but ' appears to be near, or shortly after, the beginning of morning twilight at most stations. At Winslow and Yuma it almost definitely occurs several hours after sunrise (about 0900 MST). May is less cloudy than April at all stations, with the decrease being greater in the afternoon than at night, except at Winslow. Thus the diurnal variation of cloudiness is generally smaller than in April. For the first month no station reports an excess of overcast skies over clear skies at any hour. The three southern stations, Phoenix, Tucson, and Yuma, average more than twenty clear days and fewer than five overcast days during May. Condi tions are somewhat poorer further north, especially at Flagstaff, where near Fig. 5. Sunnnary of cloud statistics for April by hours. 7 Flagstaff Winslow Prescott Pho<'nix 'l'ucsnn Yuma 6 5 'v ,-...... g ,.c::"' 4 t; ~ QI QI I),'" ~ 2 cu'....,, ~ 1 t t t t j l ft j ! j ! 1 ~ ~ f:: 3~0... 0--12--00 o_o___,.._1~2--00 o-◊~-1~2~-'-,00 do ' 1'2 ' o'o do ' 1'2 ' rlo o_o.....;.,..-1~2--T--+00~0 "-' QI C 0 .... E2 1- ~ ~ 20 20 g .... C7' ti 10 1rr: _____ ~ () 30 30 20 10 o,-..-'v-.-=...... 00 12 00 00 12 00 00 12 30 20 10 0 30 10 20 10 000 12 00 do 12 do 00 12 00 00 12 00 Or-0-.--1.,..2--.--0-t-O O 30 10 "'e 20 :!O •rl"" _/ c, 10 .,,,..- 30 0 20 10 0.-...,.;;::;;,,,..-_~ -~---,---T--o 00 __.,12 00 00~ 12 00 00------12 00 00------12 00 00 12 00 Hour (MST) Fig. 6. Summary of cloud statistics for May bv hours. I- (l, 7 Flagstaff Winslow Prescott Phoenix Tucson Yuma 7 :> C t, 6 6 "1: ,-.. 5 5 :, I'll C ,.C ,..... ~ 4 4 (; t: (LI 3 3 (l, ~ OU'-' 2 2 ,.,ttt /\ /\ /'\ _/\ (LI 1 1 :> ttt t ! t i t i t i t i 0 I 0 I'll (Ll 00 12 0000 12 00 00 12 oo do ' 12 ' dooo 12 00 00 12 I'll,., 3 0~30 (LI 0 ""'0 ,... = 1-1 t t 2 w (LI :> .... a, ~ '--v 0 , • V V 10 ~ ~ ~ nr: -----'-- 0 30 30 I'll :, ,-; 2 20 iu ""' 0 0 ~ 10 10 ,-.. ,., 30 30 I'll 20 10 ~ ....I- CJ 10 ✓ _/\_ _;\ J\ :0 0 -0 30 . 30 I'll ;:I ~ 20 0 ,., 1500 MST the frequencies of occurrence of clear and overcast skies are practically the same ( about ten days per month) • 2. Cloud types. Cumulus, cirrus, and altocumulus are the most common cloud types in May as in each of the preceding months. They are all reported more frequently than in April at all stations, despite the decrease in mean cloud cover from the latter month. This discrepancy can only be partially explained by an observed decrease in the number of reports of the less common cloud types, stratocumulus and cirrostratus, from April to May. The only other possible ex planation is that, although the major cloud types are observed more often than in April, their sky coverage is less extensive. Cumulus activity apparently is very well-developed in northern Arizona in May, with some cumulus clouds reported at 1500 MST at Flagstaff and Winslow on more than twenty days of the month. Simultaneous with or slightly before the peak of cumulus activity most of the other cloud types show a decreased frequency of occurrence, with the minimum for the day of stratocumulus and altocumulus generally occurring at this time. Between 0900 and 1500 MST stratocumulus is hardly ever reported in Arizona. The maximum altocumulus activity is reported near sunrise at all stations. As mentioned earlier for stratocumulus, this maximum is often associated with a low level nighttime inversion which forms over Arizona at about 8,000 or 9,000 feet. Although the exact mechanism for the formation of the inversion is unknown, it is probable that radiative cooling from mountain tops and high plateaus at night helps to determine its intensity. The sunrise maximum of altocumulus prob ably marks the culmination of a gradual increase through the night rather than a sharp increase from the early morning as indicated by the data. Most of these clouds, as well as those left over from cumulus activity on the preceding day, tend to dissipate by midmorning under the influence of solar radiation. F. June. 1. Cloud amounts. The mean cloud cover decreases very sharply from May to June (Figure 7) at all stations. It never exceeds two tenths at Phoenix -34- or Yuma at any hour and just barely exceeds three tenths at Flagstaff and Winslow at about 1520 MST. The lack of clouds at Yuma in June may be emphasized by noting that its cloudiest hour (0620 MST) has a mean cloud cover of 1.1 tenths, which is the same as that at Flagstaff, Prescott, and Phoenix and less than that at Winslow and Tucson at their clearest hour. Although Flagstaff has the greatest average cloudiness of any station for the day as a whole, it is somewhat clearer than Winslow and Tucson at night. Most stations have a broad minimum of cloudiness in the early morning, followed by a maximum near 1500 MST. However, the diurnal range at Phoenix and Yuma is very small, amounting to less than one tenth, so that at these stations the mean cloud cover is essentially constant throughout the day. In line with the low mean cloud cover in June, all stations average fewer than five overcast days and more than twenty clear days during the month. At Yuma near midnight almost 28 out of every 30 June days are reported as clear. On the average, more than half of the days during the month will be clear at any given hour and station. 2. Cloud types. As might be expected from the decreased cloud cover, all cloud types are reported less frequently in June than in May. Cumulus during the day and altocumulus and cirrus at night are the dominant types. Stratocumulus is rarely reported except in the late evening and early morning at Winslow and Tucson, where it probably represents the remnants of daytime cumulus activity which is strongest in June at these two stations. Cirrostratus also has a low frequency of occurrence, as usual being reported most often at Tucson and even there on less than five days of the month. As in May, cirrus has a broad daytime maximum with a hint of a slight de crease infrequency just before and during the time that cumulus activity is strongest. It seems very likely that this minimum represents the actual minimum for the day, at least at Winslow and Tucson. Fig. 7. Summary of cloud statistics for June by hours. Winslow Prescott Phoenix Tucson Yuma 7 6 5 4 3 2 1 12 oo o'o 1'2 o'o oo 12 oo oo 12 oo oo 12 00~0 20 10 0 30 rt.I ::, ,-I § 20 u ""'0 0 ,I.I 10 - rt.I , u 30 C Gl rt.I g. .El 20 ~ ; u 0 ,I.I 1 10 ,-I 20 30 rt.I B G. July. 1. Cloud amounts. In July {Figure 8) for the first month there is a significant difference in the diurnal variation of cloudiness between the northern and southern stations, or, rather, between the mountain and valley stations. Except for the actual amounts, the diurnal variation at Flagstaff, Winslow, and Prescott is very similar to that in June, with an early morning minimum of cloud cover followed by an afternoon maximum. Note., however, that the extremes appear to occur about two or three hours later at Winslow than at the other two stations. At Phoenix and Tucson a very marked minimum of cloudiness is reported near midday., while the maximum appar~tly occurs in the early evening at Tucson and in the late evening at Phoenix. At Yum.a a broad afternoon minimum is followed by a maximum in the early morning., the exact opposite of the behavior at the northern stations. Apparently the diurnal variation of cloudiness in Arizona in July is very dependent on the station's elevation. The time of maximum., for example, is delayed progressively from midafternoon at the mountain stations {e.g • ., Flagstaff and Prescott), to the late afternoon at the high plateau stations {e.g • ., Winslow), to the early evening at the upper valley stations, (e.g., Tucson)., to the late evening at the middle valley stations {e.g., Phoenix), and finally to the early morning at the lower valley stations {e.g., Yum.a). There is a very plausible explanation for this behavior. Summer cloudiness is generally of convective origin., forming mainly over mountain regions, where the combination of forced ascent of moist air and intense daytime heating are most favorable for the development of strong cumulus activity in the midafternoon. As this activity increases toward evening it gradually spreads out aloft into the surrounding valleys. By sunset radiational cooling and do'WD.slope motion suppress cumulus development over the mountains and encourage it over the valley floors., mainly through convergence of the slope winds. Also at this time the down-valley winds -36- may actually import clouds from higher elevations, although this effect is probably small. The end result, then, is that the most favorable location for the development and maintenance of convective activity moves from mountain regions to gradually lower elevations from the midafternocn to the following morning. As it does so, its ability to support this activity decreases, so that at the lowest levels cumulus development is very poor and is mainly in the form of stratocumulus and altocumulus. The contrast between the average cloud cover at Yuma and that at the other stations is impressive, but it should be mentioned that, despite this, Yuma is, on the average, cloudier in July than the cloudiest station in June (Flagstaff). The greatest mean cloudiness for the year (about 0.7) occurs in the midafternoon in July at Flagstaff and Tucson. The nighttime cloud cover at the latter station is about as great as the maximum cloud cover at Prescott and greater than that at either Phoenix or Yuma. Only Phoenix and Yuma have a greater number of clear days than overcast days at all hours in July. The results for Tucson are quite interesting. It is more likely to be clear than overcast only between about 0900 MST and 1400 MST and even then the maximum number of reports of clear skies is less than the number of reports of overcast skies during most of the rest of the day. This is the only month of the year that this happens at any station. As might be inferred from the average cloud cover data, Flagstaff, Winslow, and Prescott report over cast skies more often than clear skies through most of the afternoon. 2. Cloud types. As might be expected, the frequency of occurrence of most cloud types is greater in July than in June. Only cirrostratus (not included in Figure 8) shows a slight decrease at several stations. The diurnal variation of all cloud types is more nearly the same from one station to the nex~ than is the diurnal variation of cloud amounts. One of the most interesting differences from the previous months is the shift of the maximum cumulus activity from the midafternoon to about noon at all stations. Fig. 8. Summary of cloud statistics for July by hours. 1-1 Flagstaff Winslow Prescott Phoenix Tucson Yuma (I) 7 7 ~ CJ 6 6 't:I ...... 5 5 :, I'll y 0 .i:: .... ,I.I 4 4 CJ i:: J\J'' (I) ~ 3 3 Q) .., J\ bO '-' rd 2 2 1-1 Q) 1 1 ~ 0 t ! t ! t ! t ! t ! t ! 0 I'll Q) ,<£0 12 00 00 12 00 00 12 00 00 12 00 00 12 0000 12 I'll 1-1 - 0030 ..... ,...Q) e0 0 1-1 ~ ~ 20 20 Q) 0> .... "' CJ 10 't:I :, :x:y. ....0 ~ ~ ~ CJ 11': 0 30 30 I'll ....:, :, 20 20 CJ5 ..... 0 C ,I.I 10 rd 10 '- ,.,, 1-1 I'll ,I.I >- I'll rd 0 't:I 00 12 oo oo~~~ 12 oo oo 12 oo do ' 12 ' dooo 12 0 C"l 30 1-1 30 Q) ,._,C, I'll 20 QI :, 20 CJ .... i:: Q) § 1-1 ::, 1-1 CJ 10 10 :, CJ CJ I 0 0 A 0 ..... 0 D > CJ I'll 30 i:: ;:I I rt 0'~ ! Q) s:: 20 1-1 C § rz. .... p.. rD 11 10 I'll ICJ rt I a0 00 12 00~000 12 00~000 I'll 30 30 I'll ;:I .... 20 20 ::,~ u 0 ....,I.I 10 ~ ~ 10 rd V Ar 0 0 30 30 20 "' 20 e1-1 .... 10 10 CJ V r\ j'v'\ /\ 0 0 00 12 00 00 12 oo do ' 12 ' do oo 12 0000 12 00 Hour (MST) -37- As a result the greatest frequency of cumulus is no longer reported at the time of maximum cloud cover, but, especially at Tucson, more nearly at the time of minimum cloud cover. The reason for this is that after about 1200 MST an in creasing proportion of the convective activity is reported as cumulonimbus. When the frequencies of occurrence of the two cloud types are combined the maximum activity is found to occur at about 1500 MST at all stations. Cumulonimbus, itself, is reported most often at about the time of maximum cloud cover, which, as mentioned earlier varies from about 1500 MST at Flagstaff to 2100 MST at Phoenix and Tucson. Other than to note the times of their respective peaks, there is probably not much justification for distinguishing between cumulus and cumulonimbus. Observing difficulties and differences are apparently such that stations re porting a maximum amount of cumulus activity usually report a minimum of cumulo nimbus activity (e.g., Winslow and Tucson) and vice versa. As mentioned in Section IV B4a this discrepancy is probably largely due to the location of the clouds with respect to the station. A mixed sky of cumulus and cumulonimbus directly overhead will usually be reported as predominantly cumulus, whereas the same cloud cover at a distance might appear to be mainly cumulonimbus. Before considering the other cloud types, it might be well to say a few words about the diurnal variation of thunderstorms (these data are superimposed on the cumulonimbus curves in Figure 8) • Thunderstorms are most numerous at Flagstaff and Prescott, with an average of more than six per .month being reported at these stations at about 1520 MST. The frequency of occurrence is about half as great at Tucson and Winslow, with only the former reporting more than three per month at any hour. Thunderstorm activity is at a near minimum at Phoenix and practically non-existent at Yuma (data not included in Figure 8). At all stations, except Phoenix, the greatest number of storms occurs at or just before the time of maximum cumulonimbus development. At Phoenix with its disproportionate number of reports of cumulonimbus, the peak occurs at about -38- 2100 MST, at least three hours after cumulonimbus reaches its peak. The delay in the time of maximum thunderstorm activity from about 1500 MST at the mountain stations to about 2100 MST at the valley stations is in very good agreement with the similar delay in the time of maximum cloud cover. For all stations, except Flagstaff, altocumulus is the most common cloud type in July. At Tucson it is normally reported on at least twenty days of the month at all hours, with the frequency of occurrence exceeding 27 days at 0620 MST. As in May and June, altocumulus activity reaches its maximum near sunrise and its minimum in the early afternoon. Only at Flagstaff and Prescott is this minimum lower than ten days per month. Cirrus is normally best developed in the late afternoon at about the same time that cumulonimbus activity is most pronounced. Its true low point probably lies near noon, rather than at night as indicated by the data. Tucson and Winslow, which have the greatest frequency of occurrence of cumulus (more than 25 days per month near noon), also report the most strato cumulus, suggesting that the latter is derived mainly from the former. This belief is strengthened by the observation that stratocumulus is reported in greatest numbers just after sunset, at a time when cumulus activity is rapidly diminishing. During the daylight hours stratocumulus rarely occurs. H. August. 1. Cloud amounts. August (Figure 9) is a somewhat clearer month than July, especially at Tucson and Winslow, but the mean cloudiness is still above the annual average at all stations except Yuma. The diurnal trend of cloud cover is very similar to that in July, with the biggest difference being that the maximum occurs a few hours earlier at Phoenix and Tucson. Although Flagstaff has the greatest average cloud cover at any particular hour (6.2 tenths at 1520 MST), Tucson still has the highest mean cloudiness for the day as a whole, mainly because of its relatively cloudy nights. Yuma continues to be by far the clearest of the six cities, its cloudiest hour being clearer than the clearest hour at any of the other stations. Fig. 9. Summary of cloud statistic~ for August by hours. 7 Flagstaff Winslow Prescott Phoenix Tucson Yuma 7 6 6 5 5 4 4 3 3 2 ?. t t t t ,.....;;.t __.,...;..+Q ! 1 12 00 00 12 00 00 12 00 00 12 00 00 12 0030 20 10 0 30 30 20 ?.() 10 to 000 "---12 oo oov 12 oo do~ ' :t'? ' do do~ ' h ' do 0-0--1.,:::2....,__o.... o 0 30 30 20 10 (\ 0-'------AA 30 6 I 20 4 1 :0-1-0-=--=::....,.12----...... ,00 o~o o,...o...,___,1,....2--r--.oo ~oo .... o~..,1;.,..2.....,..-0+0: 30 30 20 10 V rn 0+-'"T"""-,-"'T"'"".., --~.....-+o 30 30 20 10 ✓ /\ Jv\ n 000 12 oo oo 12 oo o'o t2 do oo 12 oo o....o---.--1T"2 ...... ---1-00° Hour (MST) -39- Clear skies are more common at all stations in August than in July. This is particularly true at Tucson, for which there is only a short period between about 1500 and 2100 MST when it is more likely to be overcast than clear. The afternoon excess of overcast over clear skies diminishes quite rapidly from Flagstaff to Winslow to Prescott and, except at Tucson, is non-existent further south. The diurnal range in the frequency of clear skies at Flagstaff, from more than 18 days per month at night to less than six days per month in the mid afternoon, is greater in August than in any other month or at any other station. In passing, it is interesting to note from the figures, that this range is greatest at the northern stations in summer and at the southern stations in winter. 2. Cloud types. Despite the reduction in average cloud amounts in August most cloud types are reported just about as frequently as in July, in dicating that they are probably not quite as well-developed. The diurnal variation of all cloud types is practically the same as in July, with the few differences that do occur being insignificant. Altocumulus at night and in the morning and cumulus in the afternoon continue to be the dominant cloud types, particularly at Tucson where they are reported on more than 25 days of the month during their respective peak hours. Thunderstorm activity is not quite as intense in August as in July. The greatest number of storms is still reported at Flagstaff and Prescott and the smallest number at Phoenix and Yuma. The diurnal variation is also about the same as in July, with the maximum activity preceding or coinciding with the peak of cumulonimbus development at all stations except Phoenix. The general sequence of cloudiness at a typical Arizona station during a midsummer day might be the following. The sun rises on a broken deck of alto cumulus, with a few scattered cirrus above and an occasional stratocumulus below. By midmorning cumulus activity has begun near the surrounding mountains, but altocumulus, although decreasing, continues to be the prevailing cloud type. -40- A few cirrus are still present at higher heights, however, the stratocumulus has dissipated. Cumulus dominates the sky by noon as the altocumulus activity gradu ally dies out. Cirrus begins to increase and the first cumulonimbus clouds start to build up over the mountains. The situation is nearly the same at midafternoon, except that the sky coverage has increased to the point where there are only a few patches of blue sky visible. Cumulonimbus activity is quite pronounced, with each thunderhead being surmounted by an expanding veil of cirrus. There are still enough cumulus clouds around, however, to make it the most prominent type. Altocum.ulus now reaches its low point for the day. As the sun slowly approaches the horizon in the late afternoon the cumulus clouds begin either to dissipate or to flatten out and become altocumuli, which, as a result, increase quite rapidly. Cirrus, derived from the still active cumulonimbus activity, now covers practically the entire sky. With the setting of the sun and the beginning of the night all convective activity ceases except possibly for a few widely scattered thunderstorms, detected only by an occasional flash of lightening in the distance. Most of the cloudiness is now in the form of altocumulus and cirrus, although a few stratocumulus clouds may also be present. This pattern is maintained through the rest of the night. I. September • 1. Cloud amounts. There is a very sharp drop in average cloud cover from August to September (Figure 10). This decrease is almost as great as the increase that occurs from June to July. As a result, September ranks alongside June as one of the two most cloud-free months of the year at all stations. As usual, Yuma is the clearest station, having an average cloudiness just barely exceeding one tenth at its most cloudy hour. Thanks to relatively strong activity in the midafternoon, Flagstaff replaces Tucson as the cloudiest station. Many of the characteristics of the diurnal variation of cloud cover present in August are maintained in September, despite the much clearer conditions. For example, cloudiness at the three northern stations, Flagstaff, Winslow, and Fig. 10. Summary of cloud statistics for September by hours. ,.. Flagstaff Winslow Prescott OJ 7 Phoenix Tucson Yuma 7 > 0 6 ti 6 'ti ,.., 5 5 ::I Ill .....0 .c:.... 4 4 ti C: OJ 3 3 OJ .... ,..CJ) ....., 2 ~ ~ 2 "'OJ 1 1 t ~ t i t t ~ 0 ~ ~ 0 Ill OJ 00 12 ooo 12 00 00 12 00 00 12 00 00 12 12 111 i..30 0030 .... OJ 0 0 ,.... e ,.. ~ OJ ~ ~20 20 > ~ 0 ..... ~ ti °' V 'ti 10 ....5 __...... _ ti ~ ~ ~ rr: 0 30 30 Ill ....::I i 20 20 ti .... 0 0 .... 10 10 ,.., ,.. Ill ,I.I"' >, Ill 0 ~ 0 'ti"' 00 12 00 00 12 oo 0A•21':'ao o~i'2 I (iQ 00 12 00 0 <"I ,.. 30 30 OJ ....,0. Ill 20 OJ 20 ti .e C: OJ ,.. i 10 10 ti (' ::I ti ti 0 .... 0 Jl ll h 0 0 >, ti Ill 30 6 C: ::I I M' C'~ •~ ::r' OJ C: 20 4 C: ,.. 0 ::I r,...... 0. (1) 11 10 2 Ill i M' ti 0 ~ I 0 .A~.,4-. Ill 00 12 00 00' 12 00 00 12 00 00 12 00 00 12 30 30 Ill .....::I § 20 20 ti ....0 10 10 ..... "-.r ~ f"J\ ~ "' 0 0 30 30 Ill 20 20 E,.. '"ti 10 10 0 ~~~ 0 00 ----12 00 00 12 00 00 12 00 00 12 0000 12 00 Hour (MST) -41- Prescott, still tends to reach its maximum and minimum somewhat earlier than at Phoenix and Tucson. There is no clearly defined diurnal variation of cloud cover at Yuma, but rather a broad maximum during the day and a somewhat questionable minimum at night. In contrast to the preceding two months, no station reports an excess of overcast skies over clear skies at any hour of the day in September. In fact, except at Flagstaff at 1520 MST, clear skies are at least three times as common as overcast skies. Although visually less spectacular than the onset of the summer rains in July, this rapid drying out is statistically just about as abrupt and noticeable. Almost overnight the warm, moderately humid, and cloudy days are replaced by bright, dry, and sunny days. Daytime temperatures remain at about the same level as in August, but the nights are considerably cooler, making the heat much more tolerable. 2. Cloud types. All cloud types are reported less frequently in September than in August. The decrease is strongest for altocumulus, cirrus, and cumulonimbus, with the frequency of occurrence of the latter dropping by more than seventy percent at most stations. Although much curtailed, there is, however, still quite a bit of cumulus development in September, at least during the first half of the month. The peak activity occurs in the early afternoon at all stations, that is, about three hours later than it occurred in August. This lag is obviously tied up with the failure of cumulus clouds, which would have developed into cumulonimbus clouds in July or August, to build up very much in September because of the much drier conditions. Thunderstorms are still most common at Flagstaff and Prescott, but even there they are observed on less than two days of the month at the hour of maximum development. As in July and August, the greatest activity is reported a few hours later at Phoenix and Tucson than at Flagstaff, Winslow, and Prescott. The diurnal variation of stratocumulus, altocumulus, and cirrus is very much the same as in the previous months. The former two cloud types continue to reach -42- an early afternoon minimum and morning and evening maxima, the morning maximum dominant for altocumulus and the evening maximum dominant for stratocumulus. September, representing a sort of lull period between the convective storms of summer and the frontal storms of winter, is the month of least cirrus and cirro stratus activity at all stations. Reflecting the dominance of moderate cumulus activity, cirrus is still reported most frequently near sunset. J. October. 1. Cloud amounts. October (Figure 11) is the second of three normally dry fall months. Although a bit less clear than September, especially during the night, the average cloudiness is still far below the annual average. The increase of cloud cover at night relative to that during the day results in a smaller diurnal variation than in previous months. The maximum quite clearly occurs in the midafternoon at all stations. The time of the minimum is harder to determine, but it most likely is near or slightly before sunrise, except at Winslow and Tucson, whose data suggest midmorning minima. The near coincidence of the time of maximum cloud cover at all stations indicates a return to the frontal-type disturbances of winter and a decrease of local convective activity. As in September overcast days are at a premium, rarely occurring more than five times per month at any hour or station. On the other hand, clear skies pre vail at least fifty percent of the time. At Yuma, as usual the clearest station, the cloudiest hour (1520 MST) is clear on about 20 days of the month and overcast on only three. 2. Cloud types. Cirrus is the dominant cloud type at most stations in October, undergoing about a forty percent increase in frequency of occurrence from September. It no longer shows a late afternoon peak, but rather a broad daytime maximum. This is consistent with the sharp fall off of cumulus activity (by about fifty percent) in October. Most of the c;:·irrus is now derived from southward advancing frontal disturbances. Fig. 11. Summary of cloud statistics for October bv hours. Winslow Prescott Phoenix Tucson Yuma (lJ 7 Flagstaff 7 "':> 0 6 6 u 'O ,-.. 5 5 ;:l 0 ..c:"' 4 4 ,-1 ,I.J u C (lJ 3 (lJ ,I.J oil .._, ~ /\_ ~ ~ 2 "' l ""(lJ 1 :> t t i t t t 0 i i t t 0 "' 00 00 12 00 00 12 00 00 12 00 00 12 "' ~ 3 00 12 00 00 12 0030 .... "'(lJ C 0 ,-1 E ?O (lJ 0 ~2 ~ "":> "" ~ 0 ,-1 a-, "--J ~ u 'O 10 ;:l 0 _..---..... ,-1 ~ u 11'0 ------0 3 30 "'::, ,-1 § 20 .... u 0 0 ,I.J 1 10 ,-.. "' "':>, ,I.J "" ~ 'O "' '-- ~ 0 "' 12 00 00 12 00 00 12 00 00 12 00 00 12 00 0 00 M --- 30 (lJ ""p. -..., (lJ 20 u "'::, C ,-1 (lJ 10 ::,"" ui u"" u 0 .... I' ~ r ~ 0 0 Jl :>, u 30 C (lJ ti'::, "'::, (lJ ,-1 20 "" ::, "" 5u C 10 ,I.J ,-1 Cl! --..r ~ ~ _r-\.._ 0 12 00 00 12 00 00 12 00 00 12 00 00 12 00 3 30 2 20 "'::, "" -~"' 1 10 ~ J\_ ---- ~ 0 30 "'::, ,I.I "' 20 ,I.I"' "'0 10 •.-1"' "'u ------r 0 00 12 0000 12 00 00 12 00 00 ~12 00 00 12 00 ----- Hour (MST) -43- Reports of stratocumulus and cirrostratus also increase at most stations in October. The latter shows no definite diurnal variation. However, strato cumulus still has the characteristic midday minimum and morning and evening maxima, the latter as usual being indicative of an actual nighttime maximum. Although altocumulus activity decreases slightly in October, it is still one of the major cloud types, even being reported more than cirrus at Tucson. The midday minimum, so evident in the summer months, is very poorly developed, except at Tucson, giving rise to a broad maximum during the day. This maximum is probably real, resulting from the convective activity, such as it is, which is present even at this time of the year, K. November. 1. Cloud amounts. The mean cloud cover is still below normal in November (Figure 12) and at Phoenix and Tucson it is even less than in October. Once more, a relatively strong decrease of cloudiness occurs from north to south. Next to Yuma, Tucson appears to have the clearest days and Phoenix the clearest nights. Flagstaff is generally the cloudiest station at all hours. All stations have a midafternoon maximum and a pre-dawn minimum of cloudiness, both characteristic of the colder half of the year. Although all stations report an increase in the frequency of occurrence of overcast skies in November, only Flagstaff, Winslow, and Yuma report a simultaneous decrease in the frequency of occurrence of clear skies. At Prescott, Phoenix, and Tucson,especiallyat the latter, clear days are actually more common than in October. 2. Cloud types. As in the preceding month, cirrus is the prevailing cloud type in November at all stations except Tucson. The characteristic winter time similarity between the diurnal variation of cirrus and the average cloudcover is present once more, indicating that the reality of the latter largely depends, again, on the reality of the former. Despite this predominance of cirrus in -44- November, its actual average frequency of occurrence is below the annual mean at all stations except Winslow and Prescott. Altocumulus is reported less frequently in November than in any other month of the year. Despite this it is still the dominant cloud type at Tucson, the only station at which it is observed on more than ten days of the month at any hour. The diurnal variation of altocumulus is poorly defined, although most stations have a weak morning maximum and a probable early evening minimum. Cumulus is not an important cloud type in November, generally being observed less frequently than either cirrus, altocumulus, or stratocumulus. It attains its maximum development in the early afternoon and is rarely observed during the night. Reports of stratocumulus increase by about forty percent from October to November. It is reported in about equal amounts at all stations and shows no diurnal variation, except for a weak minimum in the early afternoon. As in the other months of the winter half of the year, cirrus is reported least often and cirrostratus most often at Tucson. Unfortunately there is no way of checking this result unless data for some other southeastern Arizona station could be obtained. Lacking knowledge of any physical mechanism which could produce, for example, the reported strong increase of cirrostratus from Phoenix to Tucson, the only conclusion which can be reached at the present time is that the observed discrepancies are due primarily to differences in observing techniques. Admittedly this is the easy way to explain something which is not understood, but it certainly seems to be a better procedure to follow than to present some half-baked theory which many casual readers might misinterpret as a proven fact. It is unfortunate,that, in a study of this sort, so many of the more interesting results have to be questioned because of the nature of the basic data. L. ·December. 1. Cloud amounts. Cloudiness increases at all stations in December (Figure 13) as frontal activity becomes more intense. Yuma, the clearest station Fig. 12. Summary of cloud statistics for November by hours. 7 Flagstaff Winslow Prescott Phoenix Tucson Yuma 7 6 6 5 5 4 4 3 2 ~ _/\__ ~ ~ : ~---.;....t~t- t ! t l i l i l .--,--,--,'--+~ 12 00 ~ ~ 3,po 12 oo oo 12 oo oo 12 oo oo 12 oo do ' 12 do oo 30 Q.I 0 ~o ..... e ~ ~ ~ 20 20 g u 10 0 n1', i-+---...-...... --,....-- 30 "'::I 20 ~10 0 ,I.J 10 ,,..._ <11 Ill I-< >, ,I.J -V'- ~ - "cl<11 "' o,+.-..,...... ,...... ,...... ,--./ 00 12 0000 12 0000 12 0000 12 0000 12 ..,.,0 I,, 30 30 Q.I p.. '-' 20 0-+----.--...... -,.-,,-,.-, ,--.-,.-,.-, ,...... ,...... ,,...... ,,...... , .--,r-r--..-1"0 00 12 00 00 12 0000 12 00 00 12 00 00 12 00 30 30 20 10 0-...... 30 0 20 10 ~ 000 12 oo ob------' f2 ' ob oo~ 12 oo 00120'0 oo 2 Hour (MST) Fig. 13. Su=ary of cloud statistics for December by hours. 1-t a, Flagstaff Winslm•' Prescott Phoenix Tucson Yuma :, 7 7 0 () 6 6 "O ,-. 5 ::, ....0 ..c:...,"' 4 CJ C QJ ~ 3 QJ ..., ~ ~ bG '-"' J\ ('j 2 1-t QJ :, 1 ('j t t i t f t i i t ! 0 12 0000 12 00 00 12 00 00 12 0000 12 0030 .... 0 1-1 1-1 1-1 0 C 2 20 QJ:, 0 .... °' CJ 00 ~ ~ ~ 10 "Og ~ ~ ~ .... I l X () I 0 3 30 ...."'::, § 2 20 CJ .... 0 0 ..., ('j 1 10 ,-,"' ...,1-1 /'-...../ ~ -...../'-.....- :,, ~ ('j "' "O 00 12 oo do ' i'2 I o'O 00 12 0000 12 00 00 12 0 ("l 1-1 3 30 QJ p, '-' (!) "'::, 2 20 CJ .... C QJ 1-1 I 1 10 1-1 CJ ::, CJ CJ p 0 .... ~ ~ ~. 0 0 > CJ 3 30 C QJ ::, tr "'::, a, .... 2 20 1-t l;L, CJ~ 0 1 10 ...... , ~ /'-_ ('j ./"'- 0 12 0 00 12 oodo 12 do oo 12 0000 12 00 3 30 ., 2 20 ~ 1-1 .... 10 CJ 1 /" J\_ J\_ J\_ 0 30 ...,"'::, Ol 20 ...,1-1 "'0 1-1 10 ....1-1 CJ --..,. ~ ____...... 12 0000---- 12 00 00 12 00 00 12 00 00 12 00 O Hour (MST) -45- at all hours, has a mean daily cloud cover which is just barely below that of Flagstaff for November. The southward decrease in cloudiness is not as marked as in either January or November, with Prescott and Phoenix having about the same average cloud coverage. Both stations are clearer than Tucson. The early morning minimum and the midafternoon maximum of cloudiness are quite evident at all stations. Despite the increase of cloud cover in December, only Flagstaff at 1520 MST reports more overcast days than clear days. However, the excess of clear over overcast skies at this hour at all stations except Yuma is small. 2, Cloud types. Cirrus, at Flagstaff, Prescott, and Phoenix, and altocumulus, at Winslow and Tucson, are the dominant cloud types in December. Both are reported more frequently than in November. The same is generally true of cirrostratus and stratocumulus (the frequency of occurrence of the latter decreases slightly at Phoenix and Tucson). Both of these cloud types have very little diurnal variation, occurring just about as often at night as during the day. December is the month of minimum cumulus activity at all stations. It is especially deficient at Phoenix and Tucson, where cumulus clouds are reported on fewer than five days of the month at all hours. VI • SUMMARY • A. Introduction. One of the more striking results of this study is the similarity of the cloud cover and cloud type data among five of the six stations. Only Yuma must be set off from the rest because of its extreme dryness. It therefore seems worthwhile to consider what might be termed the mean annual and diurnal variations of cloud amounts and types at a hypothetical average Arizona station. To do this, data for four stations, Winslow and Prescott in the north and Phoenix and Tucson in the south, were combined. The results are presented in Figures 14 and 16, applying to cloud amounts and cloud types, respectively. -46- Data for Flagstaff were not included in-this averaging process for two reasons. First, the Flagstaff record is not as complete as that for the other four stations, and second, inclusion of Flagstaff would have biased the means toward the normally cloudier northern part of the state. In the following discussion an attempt will be made to mention those few points at which the individual stations deviate noticeably from the mean. In Figure 16 data for cumulus and cumulonimbus have been combined in order to avoid the discrepancy in the number of reports of the two types between Phoenix on the one hand and Winslow and Tucson on the other (see section VG2). Cirrus and cirrostratus (and cirrocumulus) have also been combined, this time because of the very suspicious-looking data for Tucson, which generally reported between two and three times more cirrostratus and about twenty percent less cirrus than any other station. B. Cloud amounts. Figure 14a shows the mean cloud cover in tenths for a typical Arizona station. It was obtained by averaging the values of mean cloudiness for Winslow, Prescott, Phoenix, and Tucson for each of the eight hours and each of the twelve months. In the figure the month of the year is given along the abscissa and the hour of the day along the ordinate. The lines are lines of constant average cloud cover (isonephs) drawn at intervals of one tenth. Those for values less than 0.5 are dashed in; all others are entered as continuous lines. In a similar manner, Figures 14b and 14c show the average frequency of occurrence (per 30 days) of cloud cover of 0.1 or less (clear) and 0.9 or more (overcast), respectively. Isolines for values of less than 15 days per month are dashed in. Typically, average cloud cover in Arizona rarely exceeds five tenths coverage during the year. It is a maximum (about o.6) during the mid and late afternoon of the last third of July, rapidly increasing and decreasing, respectively, before Fig. 14. (a.) Average cloud cover (tenths) for a typical Arizona city; average frequency of occurrence (per thirty days) of cloud cover of (b.) 0.1 or less and (c.) 0.9 or more for a typical Arizona city. Based on hourly surface data for Winslow, Prescott, Phoenix, and Tucson from January 1948 through March 1958. Hour (MST) (a.) 03 03 06 06 09 09 Hour 12 (b.) (MST) 12 15 15 18 18 15 211------21 00-----...... -----,.,,----..-...,.....,.....--,0"T"°..,.....,.,.....----.,...---,,,.,.----,- 00 03 ~; ')) ( (( ,-) .) 9 : \~ ~\) )) ( ," //,,,,, ... 09 { ,, // ) ,, I 09 Hour 12 \ ) \ r ' / LJJ 12 (c.) (MST) \ \ I \ \._,, ,I r,,, 15 , ) I /- ..., \ 15 ) I \ //?.\ \ \ \.__,/\ 18 18 -~,,. / /"\ I( I ' ) /J \ \\. 9 - /;1' I I \ \. / / \ .._9 21 ( \ - I /\ \ 21 _/ /3 3 6 \ 9 9 I I 3 \ -- 3 \ 6 001-'--r---.--.!::f:>ll,---r-~-r-.a<...l.~:L<.---r-i-'-...,..L.a<.-_~.-;::...-"'-",c--""-=-r--" 00 Jan Feb Mar Apr May .Jun Jul Aug Sep Oct Nov Dec Jan Month -47- and after this period. Simultaneously, the frequency of clear days reaches its annual low (about 7 days per month) and the frequency of overcast days reaches its annual high (about 14 days per month). The secondary maximum of cloudiness in the early morning hours of July should probably be more closely connected with the afternoon maximum of the preceding day. The only other period when the mean cloud cover exceeds five tenths and the number of clear days is less than 15 per month is during winter afternoons, from mid-December through March. Fall and late spring are remarkably clear, with upwards of twenty clear days and no more than five overcast days per month during these seasons. Differences among the individual stations are most marked in summer. At that time the maximum and minimum cloudiness tend to occur a few hours earlier at the higher elevations (mountains and plateaus) than at the lower elevations (valleys and plains). The reality,,of this result is confirmed by a similar behavior for thunderstorms, which are most active at the mountain stations in the midafternoon and at the valley stations near, or even after, sunset. It has been assumed that the effect of latitude is much less important than that of altitude in producing this lag. While the diurnal variation of cloudiness is generally in phase at the various stations during the rest of the year there are differences in the average cloud amounts. Primarily because of the north and northeastward intensification of frontal activity, most pronounced during winter and spring, mean cloudiness tends to decrease toward the southwestern part of the state. Here it is the latitude that is most important in determining a station's cloud cover regime, but its altitude must also be considered. For example, although Phoenix is further north than Tucson, it still has less cloudiness in winter because of its lower elevation. Before going on to the discussion of cloud types, a little time will first be spent considering the relationship between the average cloud cover and the -48- frequency of occurrence of clear and overcast days. That a relationship exists is quite obvious from Figure 14, but it is not obvious whether it is linear of curvilinear. Therefore, the average cloud cover data used to construct Figure 14 were first plotted as a function of the corresponding frequencies of occurrence of clear skies (Figure 15a) and then as a function of the corresponding frequencies of occurrence of over~ast skies (Figure 15b). The two relationships turn out to be very nearly linear, at least within the range of values plotted. The five circled points in each of the figures represent the values for 1220 and 1520 MST in July and August and for 1520 MST in September, i.e., the hours of maximum convective activity. They are set off from the rest of the points in such a manner as to indicate that during mid-summer afternoons, for a given average cloud cover, the expected number of both clear and overcast days is less than it is during the rest of the year. This result follows directly from the discussion of section IVA2.. That is, the predominantly cumuliform clouds of summer are associated with fewer completely clear or completely over cast days than are the mainly stratiform clouds of winter. The simple correlation coefficient between the average cloud cover and the frequency of occurrence of clear days is -0.986. That between the average cloud cover and the frequency of occurrence of overcast days is 0.973. These values were obtained using all 96 pairs of data (eight pairs for each of twelve months). Even better results are obtained when the summer afternoon data (circled points), are considered separately. Then for clear skies the correlation with average cloud cover is -0.999 on summer afternoons and -0.992 for the rest of the year. Similarly, the correlation coefficient of overcast skies with average cloud cover is 0.996 on summer afternoons and 0.986 during the rest of the year. lC less or Tucson 0.1 and (tenths) (a.) afternoon of cover Phoenix, (b.) cover cloud cloud Prescott, of average 2 3 4 5 6 7 8 9 of 1 rest Winslow, for occurrence 0 2 8 4 6 o~-...--....---....---,.----.__,-,.----.-----r-~---, 24 22 28 26 of 16 14· 12 10 > E! >.. 0 U Cl) 0 Cl) 1-1 0 1-1 0 C 1-1 :, u !II Ill -0 ,-; 0 -0 - Cl) u 0. :>-. Q) u S:::11-1 u 1-1 0 ::, 0 U s:: Q)ll-! Q) Cl) 1-1 :;I 1-1 1-1 g' r.r.. C"'I 11-1 00 '-"0'. averaged 10 frequency data 9 the and 8 surface 7 cover 1958. hourly (tenths) on year cloud of March 5 6 cover (a.) Based average 4 cloud through more. 3 1948 or between average 2 0.9 January 1 (b.) Correlation from and 0 2 8 4 0 6 28 26 24 22 20 18 10 16 14 12 15. . Cl) 0 Ill 0 Cl) 0 1-1 > u 0 Ill 1-1 u as >, Ill -0 ,-; 0 -0 ,-; 0. Cl) t:: >, 1-1 Cl) 0 t::11-1 Uli-l 0 0 U Cl) Cl) Cl) u :, 1-1 1-1 :, u go,-.. 1-1 r.r.. C"'I o,-; 11-1::, - Fig. -49- The lines drawn in the figures are based on the following least-squares regression equations. (a) For summer afternoons: number of clear days (.:s_ 0.1 cloud cover) per month at a given hour = 26.59 - 3.55 (average cloud cover for that hour in tenths) (_±0.12) (la) number of overcast days (? 0.9 cloud cover) days per month at a given hour = -2.33 + 2.17 (average cloud cover for that hour in tenths) (~0.16) (lb) (b) For the rest of the year: number of clear days(:=; 0.1 cloud cover) per month at a given hour = 28.05 - 3.38 (average cloud cover for that hour in tenths) (:0.34) (2a) number of overcast days(:::_- 0.9 cloud cover) per month at a given hour = -1.65 + 2.51 (average cloud cover for that hour in tenths) (±0.36) (2b) The numbers in parentheses are the respective standard errors of estimate in days. Note that the lines in the figure must eventually curve slightly in order to pass through the points 30 days and O tenths and O days and 10 tenths in Figure 15a and through O days and O tenths and 30 days and 10 tenths in Figure 15b. Thus they are not iinea.r over the complete range of cloudiness. The equations are based on mean values for four stations. To check their ability to estimate the frequency of clear and overcast days from the average cloud cover at particular hours at individual stations, they were applied to data for Flagstaff and Yuma. Since neither of these stations was used in developing Figure 14, and since one is the cloudiest and the other the clearest station in the state for which data are available, che test should be quite rigorous and conclusive. -~- The statisticolresults of applying equations (la), (lb), (2a), and (2b) to the mean monthly and hourly data for Yuma and Flagstaff are given in Table 6. Equations (la) and (lb) were used only for the five summer hours, 1220 MST and 1520 MST in July and August and 1520 MST in September. The results are very slightly better for Flagstaff than for Yuma and also better for clear days than for overcast days. The average absolute error is never greater than 0.76 day (or about 2.5 percent), which seems to indicate that the equations are quite applicable to individual stations. The biggest disc~pancy between observed and predicted values is that the latter are generally overestimated, especially during the afternoons of the warmer half of the year. This suggests that equations (la) and (lb) should be used more liberally, possibly for all daylight hours from June, or even May, through September. However, since the results are about as good as could be hoped for, it may be best not to try for any greater accuracy. The practical applications of equations (la), (lb), (2a), and _(2b) are quite limited, since, normally, any station for which average cloud cover data are .available is almost certain to have frequency distribution data also. Of course, use of the equations would eliminate the tedious process of computing these frequencies. It is also possible that the equations are applicalbe to most of the southwestern United States and not just to Arizona, but no test has been made of this. The principal purpose of these past few paragraphs, then, has been (1) to show the linear relationship between average cloud cover and the frequency of occurrence of clear and overcast skies, and (2) to emphasize the difference in cloud structure between summer and winter. c. Cloud types. The average hourly variation by months of the major cloud types is given in Figure 16. The lines are lines of equal frequency of occurrence per thirty days with those for values less than fifteen days being dashed in. Again, the month of the year is given along the abscissa and the hour of the day along the ordinate. Hour Hour (MST) (MST) 21 00 18 15 12 09 00 00 03 ~03 ,oo ,:V:I Jan Jan Dec Dec 1 6' P\, Nov Nov city. 1955. Oct Oct )a? J Sep (a2 \ 3 Arizona \ o• December Aug ~- ,,-..... cumulonimbus J typical Jul ,-\----~-,-,:;--:;, tJ\ 3 and Month through a at .Jun 1948 cumulus May types "-.. (b.) UI January Apr ,.,' cloud from Mar i 21 u\ Feb Feb I Tucson ui prevailing I Jan Jan 09 and 03~ 00 21 03 06 12 15 004::r;:;::-~::--r-:-::--r-::-~-..,.:.~-.--_J..4-_-.----.----.----J. 00121 18 06 09 00 the of 3 3 5 8 1 6 9 ~21 loo loo ~18 100 Phoenix, 6 " Jan • days) .Jan Z\ , \. \ 1 Z Dec Dec 0 • thirty 3 Prescott, Nov Nov \ \ / \ 3 ,\ \ \ ;· (per -✓ _.,,.,, Oct Oct ,...--....\ , --- // I Winslow, e i! Sep Sep '-~· 3 ) ii' JO 6, \ l I for ) occurrence , Pi4 __,------ Aug Aug , --"""- ..... it.lirS of ...... data 6 " ,,,.- ',,,../3(/ v Jul Jul l I /, ',,,~::::./--~ r, iirii / ¥3 J\ ( I altocumulus Month f("' l 01 stratocumulus Z: Jun .Jun surface \ • (c.) frequency ) " / (a.) May May ,/ . ...... J 3 hourly I hourly 1 I ...... Apr Apr )' on I r IZ:: ' ) \ ( 1 J Mar 0 Mar Z I /,6 3 Average Based \ \ ~-/ 3 / Feb Feb / \,_,/ ,.__,I , . . ' ( I I ' \ I 16. 6') 1 • • Jan Jan 01 ,cJ 1 1~6 oo Fig. Hour Hour (MST) (MST) I I I-' V1 (days) 3.6 7.6 15.2 20.8 hourly predicted mean and monthly (days) 3.1 7.3 15.0 20.4 mean observed mean the to (2b) absolute (days) 0.71 0.70 0.76 0.74 (2a), 6 average error Yuma (lb), Flagstaff Table (b) {a) (la), error 0.897 0.955 0.930 0.924 square root-mean- (b). equations Yuma applying and 0.980 0.964 0.980 0.951 of {a) correlation coefficient results Flagstaff days days days days for statistical. Predictand Clear data Overcast Clear Overcast -52- 1. Stratocumulus. Compared to the other major cloud types, strato cumulus (Figure 16a) occurs rather infrequently and ~hows very little variation, either diurnally or annually. It reaches a weak maximum for the year (about nine days per month) near sunset in the latter part of July, at which time it probably represents the remnants of dissipating cumulus. On the whole it is more common close to sunrise and sunset than at any other time of the day. From mid March to mid-November during the daylight hours reports of stratocumulus are rare, with the lowest point (0.2 days per month) being reached in the early afternoon in June. Among the individual stations Winslow and Tucson report the most strato cumulus and Phoenix the least (excluding Yuma), especially during the summer. The diurnal and annual variations are quite similar throughout the state. 2. Cumulus and Cumulonimbus. As might be expected cumulus and cumulo nimbus (Figure 16b) are much more common during the mid-to-late-afternoon from mid April to early October than at any other time of the day or year. The absolute maximum of slightly more th.an 29 days per month occurs near the first of August at about 1520 MST. The secondary maximum (about 20 days per month) in the first part of May reflects the considerable cumulus activity in the northern part of the state at this time. At Flagstaff this is the principal maximum for the year. Convective activity is at a minimum in early winter and during the night. In June, separating the May and July maxima, the air is too dry to allow much cumulus development, despite the increased insolation. The dryness must be associated with a shift of the upper level winds from a westerly direction in winter to a more southerly direction in mid-summer. Cumulonimbus is rarely reported except in July, August, and September. At this time it is most active in the middle to late afternoon, about three hours after the peak cumulus activity. The two types join together to produce the maximum near 1500 MST. -53- Cumulus and cumulonimbus are reported most often at Flagstaff and generally appear to be more common in the northern part of the state than in the south, especially during the colder half of the year. However, Tucson has the greatest hourly average frequency of occurrence for any given hour (about 29 days per month in July at noon). There is little difference in the reported diurnal variation of cumulus development among the five stations at any time of the year. On the other hand, summertime cumulonimbus and thunderstorm activity is at a maximum somewhat earlier in the day at the higher elevations than in the valleys and the plains. 3. Altocumulus. Altocumulus (Figurel6c) is the only cloud type that is reported more often in the morning than in the afternoon. Like cumulus, it reaches its annual maximum, about 25 days per month, near the first of August, but, unlike cumulus, the time of the maximum is in the morning between 0700 and 0800 MST. All things considered, it is very likely that most of the altocumulus is derived from cumulus and cumulonibmus clouds which have slowly dissipated during the night. It is also probable that the maximum extends farther back in the morning than indicated in the figure. At any rate, the period from about the first of July to the middle of September and from the late afternoon to near noon of the following day is the only time of the year when altocumulus is likely to be reported on more than fifteen days of the month. During the rest of the year, especially in winter, altocumulus is still quite common, never being reported on less than three days of the month. Typically, ~he maximum number of reports of altocumulus occurs near sunrise, particularly from about the first week in March until the end of October. At the same time there is a secondary maximum near sunset. In December and January, when frontal activity is the dominant cloud generating mechanism, the midday minimum in the frequency of occurrence of altocumulus disappears and is replaced by one broad maximum, extending throughout the daylight hours. The least alto cumulus activity apparently occurs at night in the spring and fall. -54- As in the case of stratocumulus, altocumulus is most frequently reported at Winslow and Tucson. At the latter station it is the prevailing cloud type in all months of the year, occurring on about 28 days of the month in July at 0620 MST. All stations display the summertime peak of altocumulus activity. Its diurnal variation is also nearly the same throughout the state in all months. 4. Cirrus and Cirrostratus. Cirrus and cirrostratus combined (Figure 16d) are generally more common in the colder than the warmer half of the year. In this respect they are similar to stratocumulus. But they occur much more often than stratocumulus (by a factor of about three) and together represent the most common cloud type at all stations. A broad daylight maximum of activity extends from about the first of November through May, with the greatest frequencies being reported in mid-December and late March (i.e., respectively, just before and after frontal activity reaches its peak.). Although cirrus and cirrostratus are not generally as common in summer as in winter, there is one exception. During the period of peak convective activity (mid-July to mid-August) large amounts of cirrus are cast off from the tops of cumulus and cumulonimbus clouds, producing a pronounced maximum extending from early afternoon probably to midmorning of the following day. The very sharp increase in the number of reports of cirrus and cirrostratus near sunrise and the corresponding decrease at sunset are quite suspicious. It is very likely that the nighttime minimum is much less pronounced than indicated, probably not existing at all in the summer. If this is the case, then it is also likely that the nighttime minimum of mean cloud cover is not as definite as shown in Figure 14a. Cirrus and cirrostratus do not have the same annual and diurnal variations. While the former is most often reported during the season of greatest convective activity, although by no means is it absent during the rest of the year, the latter is primarily a winter cloud type. Both types are generally reported least -55- frequently in September. Diurnally, cirrus shows a much greater variation than does cirrostratus, having a pronounced daytime maximum. The annual and diurnal variations of the two cloud types are nearly the same at all stations. The biggest difference is that the cirrus maximum is reported in May at Flagstaff and Prescott and in July or August at the other three stations. This reflects the similar trend in the annual variation of cumulus. As for all other cloud types except cumulus and cumulonimbus, cirrus and cirrostratus combined are most frequently reported at Tucson. Considering that Tucson is not the cloudiest station, its data must once more be questioned, although it is conceivably possible that its cloudiness is actually more fragmentary and multi-layered than that at any of the other stations. Only a more complete study than that conducted here could determine whether or not this is actually the case. ACKNOWLEDGEMENTS. This study would not have been possible without the cooperation of the National Weather Records Center at Asheville, North Carolina, which provided most of the data, and the University of .Arizona Numerical .Analysis Laboratory (particularly Mr. George Wright and Mr. Stanley Gale), which handled a large part of the analysis on its Type 082 IBM Card Sorter and its Type 402 Accounting Machine. Mrs. Ruby Christine Green, Mrs. Deanna Jo Thornburg, and Mr. David Graham provided valuable aid to the author in carrying out the large number of required hand computations and tabulations. The manuscript was ex.pertly typed by Mrs. Diane Lutz Davis and proofread. by Mrs. Thornburg. Parts of this report were sponsored by the Associated Universities for Research in Astronomy under the auspices of the National Science Foundation. -56- REFERENCES. DesJardins, Robert B., 1958: The distribution of clouds at Tucson, Arizona, with respect to type, amount, and time of observation. Sci. Rep, No • .§, Inst. of Atmos. Physics, Univ. of .Arizona, Tucson, 52 pp. McDonald, James E., 1956: Variability of precipitation in an arid region; a survey of characteristics for Arizona. Tech. Repts. on the Met. and Clim. of .Arid Regions, No. 1., Inst. of Atmos. Physics, Univ. of Arizona, Tucson, ee PP• -- , 1958: Cloudiness over the southwestern United States and its ------relation to Astronomical observing. Sci. Rep. No. I, Inst. of Atmos. Physics, Univ. of .Arizona, Tucson, 68 pp. Ludlam, Frank H., and Robert s. Scorer, 1957: Cloud Study~ Pictorial Guide. Royal Met. Soc., London, 80 pp. Sellers, William D., 1958: Eight-year average cloud cover for six .Arizona cities. Unpublished manuscript, Inst. of Atmos. Physics, University of Arizona, Tucson, 25 pp. United States Department of Commerce, Weather Bureau, 1956-58: Local Climatological Data. Natl. Wea. Rec. Center, Asheville. ---=-=--=--=---,---' 1951: Manual of Card Punching (WBAN), 5th ed. U.S. Wea. Bur., Washington, 78 pp. World Meteorological Organization, 1956: International Cloud Atlas, Volume 1. World Met. Org., Geneva, 155 PP• -57- APPENDlX. CLOUD STATISTICS FOR FLAGSTAFF, ARIZONA Nr 0020 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud Amounts (tenths) Month 0 1 2 3 4 5 p- 7 e 9 10 n tenths January 13.3 0.5 0.7 1.5 1.9 0.7 0.7 1.5 9.2 124 4.2 February 13.0 0.5 1.6 2.1 0.3 0.3 1.1 1.6 1.3 8.2 113 4.1 March 15.5 0.2 2.2 0.5 1.0 0.5 1.5 1.0 0.5 7.3 124 3.6 April 17 .5 1.2 1.2 o.8 0.5 o.8 o.8 1.5 1.0 0.2 4.5 120 2.7 May 20.1 1.2 1.7 0.7 0.2 0.2 1.2 0.7 0.5 3.4 124 2.1 June 24.o 0.2 o.8 o.8 1.0 0.2 0.2 1.5 0.2 1.0 120 1.2 July 12.8 1.2 1.7 1.2 1.2 0.2 1.5 1.5 1.5 7.3 124 3.9 August 17.7 0.5 1.2 1.2 0.7 0.5 1.0 1.2 0.2 5.8 124 3.0 September 21.5 o.8 1.2 o.8 o.8 0.2 0.2 0.5 o.8 3.2 120 1.8 October 22.3 0.2 0.5 1.0 0.2 1.0 1.7 0.2 0.2 2.7 124 1.8 November 20.5 0.5 0.2 1.0 o.8 o.8 0.5 1.0 0.2 0.2 4.2 120 2.2 December 15.1 o.6 1.1 1.1 o.8 o.8 o.8 1.6 0.3 o.6 7.2 109 3.6 c. Thunder- B. Cloud Types storms - Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 0 124 ' February 0 113 March 0 124 April 0 120 May 0 124 June 1.0 4.o 4.o 1.0 30 120 July 1.0 3.9 1.9 1.0 7.7 3.9 1.0 31 0.7 124 August 1.0 2.9 2.9 1.0 1.0 9.7 1.0 31 0.7 124 September 1.0 1.0 1.0 1.0 30 1.2 120 October 2.9 2.9 1.0 31 0.2 124 November 3.0 5.0 1.0 1.0 3.0 5.0 30 120 December 3.8 5.6 1.9 3.8 5.6 16 109 -58- CLOUD STATISTICS FOR FLAGSTAFF, ARIZONA AT 0320 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud .Amounts (tenths) Month 0 J. 2 3 4 5 6 7 -8 9 10 n tenths January 14.o 0.5 1.5 1.2 0.5 0.5 1.2 0.2 1.2 0.2 9.0 124 4.1 February 11.7 1.1 2 .• 1 1.6 o.8 0.5 0.5 1.1 0.5 10.1 113 4.5 March 15.2 0.7 1.0 1.0 0.2 0.5 1.7 1.7 1.2 6.8 124 3.6 April 18.0 1.0 1.0 0.2 0.2 0.5 o.8 1.0 0.2 7.0 120 3.0 May 20.2 0.7 1.0 1.2 0.5 0.2 0.5 0.2 1.2 4.1 123 2.2 June 24.2 0.2 0.5 o.8 o.8 0.5 1.0 1.0 0.5 0.5 120 1.1 July 14.5 0.5 1.5 1.5 0.7 0.7 0.7 1.0 2.4 1.0 5.6 124 3.6 August 17 .9 0.7 1.7 1.0 0.5 1.0 1.2 0.7 1.5 3,9 124 2.5 September 21.5 2.0 0.5 1.2 0.2 0.5 1.2 0.2 2.5 120 1.6 October 20.3 0.7 0.7 0.5 0.2 0.2 0.7 1.5 1.2 0.5 3.4 124 2.3 November 21.2 1.0 0.2 1.0 1.5 0.5 0.5 o.8 0.2 3.0 120 1.9 December 14.3 o.6 1.9 1.6 1.1 0.3 o.8 0.3 1.4 7.7 109 3.7 C, Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 0 124 February 0 113 March 0 124 April 0 120 . May 0 124 June 3.0 3.0 30 0.2 120 July 1.9 1.0 8.7 1.0 3,9 31 0.5 124 August 1.0 3,9 1.9 1.0 7.7 1.0 31 0.2 124 September 1.0 1.0 1.0 1.0 30 120 October 3,9 1.9 1.9 31 124 November 5.0 5.0 1.0 1.0 1.0 2.0 30 120 December 3.8 3.8 1.9 3.8 1.9 16 109 -59- CLOUD STATISTICS FOR FLAGSTAFF, ARIZONA AT 0620 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud .Amounts (tenths) Month 0 1 2 3 4 5 b 7 d 9 10 n tenths January 12.0 1.4 1.30 0.5 1.5 o.4 o.6 1.4 0.9 o.6 9.4 237 4.4 I' February 13.0 1.4 1.4 o.8 0.7 0.1 1.7 o.8 1.1 o.4 8.4 213 4.1 March 12.3 1.8 0.9 0.5 0.9 0.5 0.9 0.9 1.4 1.0 8.8 232 4.4 A:pril 14.2 1.2 1.2 1.2 o.8 0.5 1.2 0.5 1.2 1.2 6.5 240 3.7 May 16.1 1.2 1.7 1.6 0.7 0.5 o.8 0.7 1.2 o.6 4.8 248 2.9 June 20.l 1.4 1.0 0.9 0.5 o.4 o.8 o.8 1.8 0.5 2.0 240 1.9 July 11.6 1.9 1.3 1.0 1.3 0.5 1.6 1.6 1.5 1.2 6.5 248 4.1 August 14.2 1.8 1.0 0.5 1.6 o,8 1.2 1.8 1.8 0.7 4.6 248 3.4 September 18.9 2.0 1.5 1.0 1.1 o.4 1.0 o.8 o.6 0.2 2.5 240 1.9 October 18.5 1.9 1.6 0.2 o.8 0.2 o.8 1.3 0.2 o.8 3.4 248' 2.3 November 17 .2 1.0 1.5 1.4 0.9 1.0 o.4 o.6 o.8 0.9 4.4 240 2.7 December 13.7 1.2 1.2 1.2 o.6 o.4 1.1 0.9 1.1 0.2 8.3 245 4.o c. Thunder- B. Cloud Types storms Month F st Sc Cu Cb As Ac Ci Cs n freq. n January o.. 8 1.3 4.5 1.1 0.3 4.5 8.7 2.6 113 237 February Q.3 0.9 4.5 1.8 0.3 4.5 6.9 1.2 100 213 March -0.3 o.8 5.3 2.8 1.1 5,0 9.2 1.7 108 232 April 2.0 0.2 0.2 10.2 240 2.5 4.5 5.2 1.5 120 -· May 0~2 0.2 3.4 2.2 0.5 1.0 6.5 12.3 1.0 124 248 June 0.2 1.8 3.2 0.2 0.2 6.2 8.o o.4 150 240 July o.4 o.8 7.0 4.3 o.8 o.6 13.2 10.3 o.4 155 o.4 248 August 0.2 1.0 6.2 6.6 o.4 0.2 11.8 7.6 o.6 155 248 September _0.2 o.4 2.2 5.2 o.6 0.2 7.2 3.8 0.2 150 0.5 240 October o.6 4.1 1.7 o.4 3.7 8.o o.4 155 248 November 1.6 4.2 2.4 o.4 4.o 7,4 1.4 150 240 ' December . 0,2 1.2 5.7 o.6 o.4 4.7 7.9 2.2 152 245 -60- CLOUD STATISTICS FOR FLAGSTAFF, ARIZONA NJ:. 0920 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud Amounts (tenths) Month 0 ). ,, 2 3 4 5 6 7 e 9 10 n tenths January 8.3 1.8 0.7 0.7 o.8 0.2 1.3 o,8 2.1 o:r 12,3 248 5.7 February 11.7 1,9 0.7 0.7 o.4 0.5 1.3 0.3 1.2 0.9 10.5 226 4.7 March 11.5 1.7 o.8 1.2 0.7 0.7 0.7 0.5 2.1 1.0 9.1 248 4.6 April 11.9 2.5 1.5 1.5 0.5 0.5 1.6 1.4 1.4 0.9 6.4 240 3.9 May 13.8 2.8 1.7 0.5 1.0 o.6 1.1 1.2 1.6 1.0 4.7 245 3.3 June 19,2 1.6 1.5 0.5 1.0 1.0 0.9 o.4 1.1 o.8 2.0 240 2.0 July 11.4 2.1 1.1 1.6 1.0 1.1 1.8 1.9 1.7 0.7 5.7 248 4.o August 13.4 3.3 1.6 o.8 0.7 1.3 1.1 o.6 1.1 o.8 5.2 248 3.3 September 18.4 1.6 1.4 0,9 o.8 0.5 1.1 0.9 1.0 o.6 2.9 240 2.3 October 16.9 1.9 1.1 1.0 o.8 0.2 1.6 o.8 o.8 1.1 3.6 248 2.7 November 16.0 1.2 1.0 1,0 0.5 o.8 0.9 1.0 1.2 1.0 5.4 240 3.2 December 11.5 1.0 1.2 1.0 o.6 0.5 1.0 o.8 1.6 1.1 9.8 248 4.8 c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 4.1 4.8 1.5 1.5 5,8 10.9 2.4 124 248 February 1.1 3.7 4.5 0.3 3.7 8.5 o.8 113 226 March 1.2 3.9 6.5 0.7 3.9 8.o 2.4 124 248 April 1.5 4.2 8.0 0.5 0.5 3.8 12.0 o.8 120 240 May 0.2 14.5 0.7 0.5 4.4 11.1 1.7 124 245 June 0.2 8.8 0.2 0.2 5.8 8.o 0.2 150 240 July 1.0 3,9 17.7 o.8 10.7 8.3 o.8 155 248 August 1.0 2,5 17.7 1,2 0.2 10.1 6.4 o.4 155 o.4 248 September o.6 o.6 11.4 o.8 6.o 4.6 0.2 150 0.2 240 October o.4 2,3 6.o 0.2 4.7 8.9 o.8 155 0.1 248 November 1.2 4.6 2,4 1.2 3,2 10.0 1.0 150 240 December 0.2 2,1 7.6 o.8 o.8 4.8 10.5 2.5 155 248 -61- CLOUD STATISTICS FOR FLAGSTAFF, ARIZONA AT 1220 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud .Amounts (tenths) Month 0 1 2 3 4 5 b 7 t5 9 10 n tenths January 7.4 1.2 0.5 1.3 1.3 0.7 1.2 1.6 1.2 1.3 12.2 248 5.9 February 10.6 1.7 1.1 0.7 0.9 0.5 1.6 1.7 1.2 0.9 9.q 226 4.7 March 9.3 2.2 o.8 1.0 o.8 1.3 1.6 1.1 1.2 1.3 9.3 248 5.0 April 8.8 1.6 1.5 1.9 0.5 1.8 1.6 1.1 1.8 2.2 7.2 240 4.8 May 10.9 1.9 1.9 1.1 0.5 0.5 1.6 1.3 2.8 1.6 5.9 248 4.3 June 16.6 1.9 1.2 1.0 o.4 o.8 2.0 1.2 1.5 o.6 2.8 240 2.6 July 5.6 1.3 1.1 1.3 1.7 1.3 2,7 3.1 2.8 2,9 6.2 248 5.6 August 5.3 2.1 1.7 1.7 2.2 2.4 3.1 1.9 2.8 2.1 4.7 248 5.1 September 12.1 2.5 2.1 2.1 1.0 0.9 2.0 1.9 1.4 1.2 2.8 240 3.2 October 14.o 3.3 o.8 1.0 1.0 1.1 1.8 o.8 1.1 1.0 4.1 248 3.1 November 13.5 1.8 1.8 o.8 1.2 o.6 1.2 1.5 0.5 1.2 5.9 240 3.6 December 10.4 1.6 1.2 1.2 o.4 o.6 o.8 1.2 1.5 1.8 9.3 248 4.9 c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 2.4 4.4 3.9 1.2 5.1 12.1 3.4 124 248 February 0.5 2.9 8.2 0.5 2.6 8.2 1.3 113 226 March 0.7 2.4 12.3 0.5 1.0 3.6 10.9 1.5 124 248 April 1.0 1.5 13.5 1.5 0.5 3.2 11.8 2.0 120 o.4 240 May 18.4 2.7 2.4 9.9 2.2 124 0.2 248 June 13.2 1.4 0.2 4.8 7.8 0.2 150 o.8 240 July o.6 2.9 16.9 9.5 0.2 5.0 6.2 0.2 155 5.3 248 August o.4 1.2 17.7 9.7 o.4 6.2 5.0 o.4 155 5.6 248 September o.6 16.4 3.8 4.o 4.8 0.2 150 1.4 240 October 0.2 1.4 10.3 o.4 0.2 3.3 8.9 1.2 155 0.2 248 November 1.6 3.8 4.6 o.4 3.6 11.0 1.2 150 24o December 1.0 6.o 4.1 1.4 4.7 l2.0 2.7 155 248 -62- CLOUD STATISTICS FOR FLAGSTAFF, ARIZONA AT 1520 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud flmount s (tenths) Moni;n 0 l 2 3 Lj. J b '( 0 ':J lO n lteni;ns January 8.2 0.5 o.8 1.3 1.1 1.3 o.6 2.1 1.8 o.8 11.5 186 5.7 February 8.1 1.2 1.0 0.9 0.9 0.2 0.9 1.6 1.6 1.2 ·12.4 110 5.8 March 7.6 o.8 1.5 1.5 0.5 1.1 1.8 2.1 1.9 1.0 10.3 186 5.6 April 5.8 1.0 o.8 1.3 0.3 1.2 1.5 2.5 2.8 2.2 10.5 180 6.2 May 8.2 1.3 2.4 1.3 1.5 1.1 1.8 1.8 2.1 1.0 7.6 186 4.9 June 15.0 2.0 0.7 0.3 1.3 1.0 1.0 1.5 2.2 1.3 3.7 180 3.2 July 3.5 1.1 0.5 o.8 1.1 1.3 2.1 1.8 3.9 2.1 11.8 186 6.9 August 4.2 1.7 0.7 0.7 1.8 2.0 2.8 1.8 3.5 1.3 9.5 180 6.2 September 10.4 2.0 1.6 1.6 1.0 1.0 1.2 1.4 1.6 1.8 6.4 150 4.3 October 13.6 2.1 1.6 1.4 o.4 o.4 1.4 1.2 1.6 o.6 6.o 155 3.6 November 12.6 1.0 1.4 1.8 1.4 o.4 1.6 o.4 o.6 1.4 7.4 150 4.o December 8.3 1.0 1.9 1.6 o.6 o.6 o.8 1.4 1.2 1.2 11.6 155 5.5 c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 0.5 0.5 2.9 6.8 0.5 7.3 13.6 1.5 62 186 February 0.5 3.7 11.0 0.5 3.2 11.6 0.5 57 170 March 1.0 2.4 16.5 0.5 0.5 3.9 10.6 1.0 62 0.2 186 April 1.0 0.5 18.0 4.o 3.0 12.0 1.5 60 0.3 180 May 20.3 2.9 4.8 15.0 0.5 62 0.3 186 June 15.7 1.3 0.3 5.0 6.3 2.3 90 0.7 180 July 0.3 14.2 13.9 o.6 6.8 8.7 1.0 93 7.4 186 August 0.3 1.4 12.4 15.5 4.8 9.3 1.0 87 5.0 180 September 2.5 17 .5 3.5 6.o 6.o 60 1.8 150 October 1.9 8.2 1.0 5.8 10.2 1.5 62 1.0 155 November 1.5 3.5 8.5 1.5 4.5 10.5 1.5 60 150 December 1.5 4.8 8.2 1.0 4.8 12.6 2.4 62 155 -63- CLOUD STATISTICS FOR FLAGSTAFF, ARIZONA AT 1820 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud Amounts (tenths) Month 0 1 2 3 4 5 6 7 t) 9 10 n tenths January 9.0 1.6 1.1 2.1 1.5 0.3 1.6 1.6 1.0 o.8 9.4 186 4.9 February 8.3 1.1 1.2 1.9 0.7 0.9 1.9 1.8 0.7 o.4 11.1 170 5.4 March 7.9 1.3 1.3 1.3 2.3 0.5 0.5 1.6 2.6 0.5 10.3 186 5.4 April 7.0 o.8 2.3 1.5 o.8 0.5 o.8 2.3 3.0 2.3 8.5 180- 5.6 May 9.8 2.4 2.1 1.1 1.1 1.1 1.0 2.1 1.8 1.1 6.3 186 4.3 June 15.9 1.2 1.8 1.7 1.0 1.0 o.8 o.8 1.3 o.8 3.7 180 2.8 July 5.2 o.8 1.0 1.9 1.8 1.0 1.8 1.5 2.3 1.3 11.6 186 6.2 August 6.o 1.0 2.3 2.3 1.5 1.7 1.3 1.0 1.5 2.0 9.4 180 5.5 September 12.4 1.6 2.6 2.0 o.8 2.0 o.6 2.2 1.2 o.6 4.o 150 3.3 October 16.1 1.7 o.8 1.6 o.4 0.2 o.6 1.2 1.7 1.2 4.7 155 3.1 November 15.8 1.0 o.8 o.8 o.8 o.6 1.6 1.0 1.2 o.4 6.o 150 3.4 December 10.7 1.6 o.6 o.8 o.8 0.2 1.4 1.7 1.6 o.6 10.3 155 5.0 c. , Tb.under- B. Cloud Types storms Month F st Sc Cu Cb As Ac Ci Cs n freq. .n January 3.9 4.8 0.5 6.8 12.1 1.0 62 186 February 0.5 3.2 7.4 1.1 4.2 8.9 1.6 57 170 March 0.5 3.4 14.o 1.0 1.0 6.3 8.7 1.5 62 186 April 1.5 3.0 16.5 4.o 3.5 14.o 3.0 60 0.2 180 May 1.5 16.9 1.9 0.5 6.3 16.5 0.5 62 0.2 186 June 11.3 0.3 0.3 4.8 7.1 1.0 93 0.3 180 July 1.0 15.2 11.6 0.3 9.0 12.0 o.6 93 2.4 186 August 0.3 2.1 10.0 14.5 9.7 10.4 87 2.7 180 September 0.5 4.o 12.5 3.5 6.5 6.5 1.0 60 1.4 150 October 2.4 6.3 0.5 5.3 10.6 62 o.4 155 November 0.5 6.o 1.0 0.5 1!-.5 6.o 1.0 60 150 December 1.5 7.7 3.9 0.5 4.4 9.7 0.5 62 155 -64- CLOUD STATISTICS FOR FLAGSTAFF, .ARIZONA Nr 2120 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud Amounts (tenths) Month 0 1 2 3 4 5 6 7 -a 9 10 n tenths January 13.8 1.0 1.0 1.2 1.4 1.2 o.6 o.4 o.4 9.3 155 4.2 February 13.7 o.8 1.3 1.5 1.1 0.2 o.6 o.4 0.2 10.1 142 4.1 March 13.3 0.9 0.9 J.3 1.5 o.4 1.3 1.9 1.5 0.2 6.9 140 3.9 April 14.8 o.8 2.8 1.8 2.5 0.5 o.8 1.5 0.5 4.2 120 2.9 May 16.2 1.0 2.7 2.4 1.0 0.5 1.5 0.2 0.7 0.7 3.1 124 2.5 June 20.5 2.0 o.8 1.2 0.5 0.2 o.8 1.0 0.2 2.8 120 1.7 July 9.7 1.2 1.9 1.2 2.7 1.0 1.0 1.2 2.2 0.7 7.3 124 4.5 August 14.8 1.0 2.2 0.7 0.7 0.7 0.7 0.7 1.5 0.2 6.8 124 3.5 September 20.2 o.8 o.8 1.0 0.5 o.8 1.2 0.5 o.8 3.5 120 2.1 October 20.3 0.7 0.7 0.2 1.5 0.5 1.0 0.7 1.0 3.4 124 2.1 November 17.0 0.2 1.2 2.0 0.5 1.2 1.5 o.8 1.0 0.2 4.2 120 2.8 December 13.6 1.7 1.2 1.2 1.5 0.5 0.2 1.5 0.7 8.o 124 4.1 c. Thunder- B, Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 5.8 5.8 1.9 2.9 4.8 31 155 February 2.0 5.2 1.0 2.0 3.1 1.0 29 0.2 142 March 1.9 3.8 11.2 7.5 16 140 April 0 120 May 0 124 June 5.0 2.0 2.0 1.0 3.0 1.0 30 0.2 120 July 2.9 6.8 3.9 1.0 8.7 4.8 31 1.0 124 August 4,8 3.9 2.9 1.0 4.8 5,8 31 0.2 120 September 2.0 3,0 2.0 2.0 30 0,2 124 October 1.0 1.0 4.8 1.9 4.8 31 0.2 124 November 1.0 2,0 3.0 2.0 1.0 3.0 5,0 30 120 December 2.9 4.8 2.9 6.8 2.9 1.0 31 124 -65- CLOUD STATISTICS FOR WINSLOW, ARIZONA AT 0020 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud .Amounts (tenths) Month 0 l 2 3 4 5 6 7 e 9 10 n tenths January 12.4 1.2 1.7 1.7 1.3 0.3 o.8 0.7 1.4 1.0 7.6 310 4.1 February 16.2 0.9 1.0 o.6 o.4 0.5 0.5 1.3 o.6 o.6 7.3 282 3.5 March 14.5 1.9 o.8 1.8 0.5 0.7 o.4 o.6 1.1 1.0 6.8 309 3.5 April 16.9 0.7 1.4 1.3 o.8 0.2 o.8 0.7 1.1 1.2 4.9 270 3.0 May 18.9 1.7 1.1 o.8 o.6 0.9 o.4 0.5 o.8 1.0 3.3 279 2.2 June 21.9 1.3 1.3 0.3 0.3 0.3 0.7 0.2 0.7 1.0 1.9 270 1.6 July 10.6 1.5 1.2 1.4 o.8 a.a 1.0 1.9 1.4' 2.2 7.4 310 4.6 August 14.2 1.1 1.3 1.2 0.9 o.4 o.6 1.1 1.2 1.2 6.9 279 3.8 September 22.0 0.5 1.1 1.3 0.7 0.1 0.5 o.6 0.3 0.5 2.4 300 1.6 October 20.3 1.5 o.6 1.0 o.6 o.4 o.6 0.7 o.6 o.8 3.1 310 2.0 November 19.6 0.9 1.2 1.2 0.9 0.3 0.9 0.7 0.2 0.7 3.4 300 2.1 December 16.2 1.3 1.7 0.9 0.5 0.7 o.6 0.3 0.9 o.6 6.5 310 3.2 c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 0.2 0.7 3.6 0.2 0.7 7.3 3.6 6.5 124 310 February 0.3 4.2 0.3 5.0 2.4 2.4 113 282 March 0.5 4.6 1.2 1.5 6.3 2.9 4.1 123 309 April 4.2 0.2 0.5 6.8 1.8 3.2 120 270 May 3.1 0.2 0.2 1.2 6.8 3.1 2.7 124 0.1 279 June 0.2 1.4 o.4 0.2 4.8 3.8 1.4 150 0.1 270 July o.4 5.6 1.6 o.4 1.2 17 .3 3.5 1.2 155 0.3 310 August 7.8 o.4 1.0 o.6 14.7 3.7 1.7 155 0.2 279 September o.4 3.8 0.2 5.8 o.8 o.4 150 0.1 300 October 2.3 0.2 4.3 3.1 2.7 155 310 .. November o.6 1.4 3.8 o.6 o.6 4.4 4.o 2.2 150 300 December o.6 5.6 o.4 1.2 8.o 5.2 4.1 155 310 -66- CLOUD STATISTICS FOR WINSLOW, ARIZONA AT 0320 MST ave. Frequency of Occurrence (per thirty days} of cloud amount A. Cloud .Amounts (tenths) Month 0 1 2 3 4 5 b 7 ~ 9 10 n tenths January 12.5 1.0 1.6 1.4 0.9 o.6 o.6 1.4 1.4 1.1 7.7 310 4.2 February 15.7 0.5 0.9 1.0 0.5 o.6 0.9 0.7 1.3 o.6 7.2 282 3.6 March 15.2 0.9 o.6 0.9 1.1 0.3 1.2 0.9 1.5 0.9 6.8 310 3.7 April 17.1 o.8 1.1 1.1 1.2 o.6 0.3 0.9 1.0 o.8 5.1 270 2.9 May 20.0 1.5 1.5 1.2 o.4 0.1 0.5 0.5 o.B 0.5 2.9 279 1.9 June 22.8 0.7 1.1 0.9 o.4 0.2 0.2 0.2 o.8 o.6 2.1 270 1.5 July 10.8 1.7 1.6 1.2 1.0 1.2 1.4 1.6 1.5 1.3 6.7 310 4.3 August 16.0 1.7 1.2 o.8 o.6 o.4 0.5 1.0 1.0 1.1 5.7 279 3.2 September 22.2 o.8 0.9 o.6 0.2 0.3 o.8 1.2 0.7 o.4 1.9 300 1.6 October 19.6 1.1 0.5 1.2 1.0 0.2 1.0 0.7 0.5 1.0 3.5 310 2.3 November 19.8 o.~ 1.1 o.6 o.8 0.3 o.4 1.0 0.7 o.4 4.1 300 2.3 December 15.6 1.6 1.6 1.1 o.4 0.7 0.2 1.1 0.9 1.3 5.6 310 3.2 c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 0.5 1.7 5.6 0.5 1.2 6.8 2.2 6.5 124 310 February 0.3 2.9 0.3 o.8 5.6 2.1 2.6 113 282 March 0.2 4.8 0.7 1.9 5.8 2.7 4.4 124 310 April 4.o 0.5 1.5 4.o 3.2 3.2 120 270 May 1.2 0.2 0.5 8.7 3.1 2.7 124 279 June o.8 o.4 0.2 o.4 5.2 3.6 1.4 150 270 July o.4 4.5 1.0 1.4 17.7 2.7 1.9 155 310 August 0.2 6.o 1.2 o.4 14.4 3.5 1.0 155 0.1 279 September 0.2 2.0 o.4 6.4 o.6 o.8 150 300 October 1.6 0.2 4.7 1.7 2.9 155 310 November o.6 1.4 4.4 o.8 0.2 3.8 3.2 2.8 150 300 December o.4 o.6 5.2 0.2 1.4 7.2 4.8 3.7 , 155 310 -67- CLOUD STATISTICS FOR WINSLOW, ARIZONA AT 0620 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud .Amounts (tenths) Month 0 1 2 3 4 5 b 7 tj I 9 10 n tenths January 13.1 1.6 1.0 1.0 0.5 1.0 0.7 0.9 1.3 1.0 8.1 310 4.1 February 13.2 o.6 2.0 1.0 0.5 0.2 1.3 1.1 1.3 1.4 7.4 282 4.1 March 12.0 2.1 1.1 0.9 o.8 o.4 o.8 1.3 1.2 2.1 7.5 310 4.3 April 14.6 1.3 0.7 0.9 0.9 0.3 1.3 o.8 o.8 1.6 6.9 270 3.8 May 17 .5 1.7 1.1 0.9 o.6 o.4 o.4 0.9 0.9 1.1 4.5 279 2.7 June 21.3 0.9 1.4 1.1 0.2 o.8 o.6 0.2 0.2 1.4 1.8 270 1.6 July 10.3 2.0 1.5 0.9 1.3 0.9 1.2 1.2 2.2 2.3 6.4 310 4.5 August 14.6 1.6 1.7 1.1 0.5 o.4 1.1 1.3 1.0 1.6 5.1 279 3.4 September 20.2 1.3 1.0 1.3 0.3 0.3 o.8 0.5 1.2 o.8 2.3 300 1.9 October 17 .5 1.5 0.9 1.3 0.9 0.3 o.6 0.9 o.8 1.6 3.8 310 2.7 November 19.0 0.7 1.8 0.7 0.5 o.4 0.5 o.4 o.4 1.7 3.9 300 2.5 December 15.3 1.3 2.1 1.3 0.5 0.2 o.8 0.9 1.3 1.0 5.5 310 3.2 c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 0.5 0.7 5.8 0.2 2.2 8.2 2.9 7.0 124 310 February 0.5 4.8 o.8 o.8 8.5 4.8 2.1 113 282 March 0.5 6.8 3.1 1.9 11.9 6.5 4.4 124 310 April 0.5 5.0 2.0 o.8 11.0 9.8 3.5 120 270 May 0.2 3.4 1.9 0.5 0.5 14.3 13.1 2.9 124 279 June 0.2 1.8 3.2 o.6 o.6 10.2 10.6 1.6 150 270 July o.4 4.3 7.2 0.2 o.4 23.9 10.9 3.5 155 310 August 0.2 7.5 4.6 1.0 23.0 10.6 2.5 155 279 September o.8 4.o 3.6 o.6 0.2 13.2 4.8 o.8 150 300 October 3.9 2.1 0.2 9.9 8.o 2.3 155 310 November o.8 1.2 4.6 0.2 o.6 8.2 7.4 2.4 150 0.1 300 December o.4 1.7 5.8 0.2 1.0 11.4 5.6 3.7 155 310 -68- CLOUD srATISTICS FOR WINSLOW, ARIZONA AT 0920 MST ave. Frequency of Occurrence (per thirty deys) of cloud amount A. Cloud .Amounts (tenths) Month 0 l 2 3 4 5 b 7 lj 9 10 n tenths January 10.8 1.5 o.6 1.4 0.1 o.4 o.8 1.1 1.3 2.1 10.1 310 5.0 February 11.9 1.5 1.5 o.4 o.4 0.3 1.2 0.5 1.2 2.0 9.0 282 4.6 March 12.1 1.8 o.8 1.5 o.6 o.8 0.9 0.7 1.0 1.5 8.5 310 4.3 April 14.8 1.7 1.2 0.9 0.9 0.7 1.0 1.2 0.3 1.3 6.o 270 3.4 May 17 .5 0.9 1.0 1.4 1.3 o.4 1.0 1.2 o.8 1.3 3.3 279 2.6 June 21.9 0.7 1.3 o.a 0.3 0.2 o.8 0.7 0.7 0.9 1.8 270 1.6 July 13-.1 2.7 1,4 1.1 0.5 1.0 1.0 1.4 0.7 2.3 5.0 310 3.6 August 17.2 2.4 0.9 o.6 0.7 1.2 o.4 1.5 o.6 0.9 3.5 279 2.5 September 21.1 1.6 o.8 0.5 0.5 1.0 1.0 0.2 0.5 0.5 2.3 300 1.7 October 18.1 1.8 o.8 1.3 0.5 o.6 0.7 0.2 1.3 1.3 3.6 310 2.5 November 17 .9 0.9 1.0 o.6 0.7 o.4 o.6 1.0 0.7 1.3 4.9 300 2.9 December 13.6 1.5 0.5 1.2 o.6 0.7 o.8 1.0 1.1 1.2 8.1 310 4.1 c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 0.2 3.1 8.o 2.7 1.2 14.3 8.7 6.8 124 310 February 1.1 4.8 4.2 0.3 0.5 11.1 7.2 6.4 113 282 March 2.2 5.6 5.8 1.9 12.3 10.6 4.6 124 310 April 1.0 3.0 9.5 o.8 8.o 8,5 4.2 120 270 May 0.2 13.6 · 0.2 10.9 12,l 3.4 124 279 June 0.2 8.6 o.4 10.0 6.6 2.0 150 270 July 1.7 20.4 22.1 9.1 1.9 155 0.1 310 August o.8 2.5 20.0 0.2 ·o.4 21.3 9.5 .1.'4 155 279 September o.8 2.4 9.0 0.2 11.4 6.o o.6 150 0.1 300 October 0.2 1.4 7.2 8.5 10.3 1.6 155 0.1 310 November o.4 1.8 4.8 3.8 0.2 o.4 9.4 11.4 2.6 150 300 December o.4 1.7 8.1 1.9 1.6 12.2 14.o 3.9 155 310 CLOUD STATISTICS FOR WINSLOW, .ARIZONA AT 1220 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud fl.mounts (tenths) Month 0 1 2 3 4 5 b 7 8 9 10 n tenths January 8.4 1.6 o.8 1.5 o.6 0.7 1.4 o.8 1.6 1.9 10.6 310 5.5 February 10.2 1.4 1.5 0.9 0.7 0.9 1.3 1.1 1.1 1.4 9.7 282 4.9 March 11.1 1.8 0.5 0.7 0.7 1.2 1.0 1.3 1.2 2.0 8.6 310 4.7 April 11.4 1.6 1.0 1.2 o.6 1.1 1.3 1.4 1.7 2.2 6.4 270 4.4 May 12.7 2.5 1.2 o.8 0.9 o.8 1.1 1.0 1.5 1.8 5.9 279 3.8 June 17 .2 2.2 1.9 1.2 0.7 o.6 1.3 1.0 0.3 1.3 2.2 270 2.2 July 9.1 3.4 1.6 2.1 1.6 1.2 1.2 1.1 1.5 2.4 4.8 310 4.1 August 10.5 3.4 1.9 2.7 0.9 o.8 1.1 2.0 1.3 1.3 4.1 279 3.5 September 17 .6 1.9 1.8 0.7 o.8 0.9 1.1 0.9 1.2 0.9 2.2 300 2.3 October 15.8 1.6 1.4 1.0 0.9 o.6 1.1 0.9 o.8 1.7 4.4 310 3.1 November 15.7 1.6 o.6 0.9 . 0.5 o.6 0.7 1.3 1.3 1.4 5.4 300 3.4 December 11.8 1.5. o.8 o.8 0.7 1.1 1.0 0.7 1.3' 1.5 9.0 310 4.6 c. Thunder- B, Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq, n January 0.2 2.4 5.1 7.7 1.5 12.3 10.9 6.8 124 310 February 0.3 2.4 10.3 1.1 7,7 9.5 7.4 113 282 March 0.7 2.9 15.2 0.7 8.o 11.1 6.o 124 0.1 310 April 0.2 1.5 17 .5 o.8 6.2 11.0 6.2 120 0.1 270 May 0.7 22.5 0.5 0.2 6.8 10.9 4.4 124 0.2 279 June 0.2 18.2 o.4 0.2 6.2 8.4 1.6 150 o.6 270 July 0.2 o.6 26.4 2.7 0.2 12.6 10.3 1.0 155 1.3 310 August o.4 o.6 24.6 4.5 o.4 9.9 10.5 1.2 155 1.5 279 September o.8 21.6 o.8 0.2 5.8 6.o o.4 150 0.3 300 October 0.2 1.2 13.4 0.2 7.4 9.7 2,3 155 0.1 310 November 0.2 1.8 4.4 7.6 o.4 7.4 10.8 4.2 150 300 December 0.2 2.3 5.2 5.8 1.4 11.4 14.6 4.7 I 155 310 -70- CLOUD STATISTICS FOR WINSLOW, ARIZONA AT 1520 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud .Amounts (tenths) Month 0 1 2 3 4 5 6 7 8 9 10 n tenths January 9.4 1.3 0.9 o.4 1.5 o.6 1.1 0.7 0.7 3.1 10.5 310 5.4 February 10.2 1.5 1.2 o.4 0.5 0.9 1.4 1.5 1.3 1.9 9.2 282 5.0 March 8.4 1.9 0.7 1.0 1.4 1.4 1.6 1.1 1.2 1.8 9.6 310 5.2 April 9.3 1.2 1.6 o.8 o.6 1.0 1.7 0.7 2.0 2.9 8.3 270 5.1 May 10.0 2.3 1.7 1.2 1.2 1.3 1.0 1.0 1.1 2.2 7.2 279 4.4 June 14.1 1.8 1.4 1.1 1.4 o.8 1.6 1.8 1.2 1.6 3.2 270 3.2 July 4.2 1.6 1.8 1.7 o.8 1.5 1.6 2.0 2.3 4.1 8.3 310 6.1 August 5.5 2.7 1.5 1.5 1.8 1.9 2.3 1.5 1.7 3.7 5.9 279 5.2 September 14.2 2.2 1.6 1.1 0.7 1.2 1.2 1.6 0.9 2.2 3.1 300 3.1 October 15.2 2.0 1.4 0.5 1.0 1.0 1.2 o.6 1.4 1.5 4.4 310 3.1 November 15.5 1.2 o.6 1.0 o.8 0.7 o.6 1.3 1.1 1.5 5.7 300 3.5 December 11.9 1.3 1.1 0.3 0.7 0.5 1.2 1.1 1.4 1.9 8.8 310 4.7 c. Thunder- B. Cloud TY];les storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 1.5 3.6 11.4 1.2 13.3 9.9 8.2 124 310 February 0.5 2.6 12.5 0.3 6.9 9.3 7.4 113 282 March 0.5 2.4 18.4 0.7 0.5 9.4 10.4 5.8 124 310 April 2.2 19.8 1.0 8.5 9.2 7.0 120 o.6 270 May 0.7 24.4 0.5 0.2 6.o 9.0 5.3 124 o.4 279 June 0.2 21.8 o.8 0.2 4.6 8.o 2.6 150 0.3 270 July 0.2 24.4 6.o 11.6 12.0 o.4 155 2.3 310 August o.4 o.8 23.7 6.o o.4 9.7 14.4 o.4 155 2.2 279 September o.6 22.2 3.0 6.o 6.8 o.6 150 1.1 300 October 0.2 1.9 14.2 7.4 10.7 2.9 155 0.1 310 November 2.2 2.4 10.6 0.2 o.6 8.6 12.6 4.o 150 300 December 1.4 4.7 7.4 1.4 11.8 14.6 5.0 155 310 -71- CLOUD STATISTICS FOR WINSLOW, ARIZONA AT 1820 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud .Amounts (tenths) Month 0 1 2 3 4 5 6 7 8 9 10 n tenths January 10.4 2.0 0.7 1.1 o.8 0.9 1.2 1.6 1.5 9.9 310 5 .o February 11.2 1.5 0.9 o.6 0.9 0.5 0.9 1.3 2.1 1.7 8.5 282 4.8 March 9.1~ 1.7 1.1 1.3 1.0 0.7 1.0 1.1 1.4 1.6 9.9 310 5.1 April 8.8 1.3 1.0 1.8 1.1 o.8 1.1 1.4 2.0 1.9 8.8 270 5.2 May 10.1 2.0 1.5 1.3 1.2 1.6 1.1 1.6 1.2 1.6 6.8 279 4.4 June 15.8 1.5 1.8 1.l~ 0.9 o.8 o.4 0.9 1.1 1.5 3.8 270 2.9 July 4.7 1.4 0.7 1.5 1.3 1.0 1.0 1.3 2.0 3.5 11.8 310 6.6 August 8.1 1.7 1.4 1.1 1.0 1.1 0.9 1.3 1.3 2.7 9.6 279 5.4 September 14.9 2.8 1.3 1.6 1.2 0.7 0.5 1.2 1.6 1.4 2.8 300 2.8 October 17.2 1.7 1.5 1.1 0.9 o.4 0.3 1.0 1.0 1.1 4.o 310 2.6 November 16.9 o.8 0.7 1.0 0.7 0.3 o.8 1.2 1.4 1.3 4.9 300 3.2 December 12.8 1.5 1.0 o.8 0.5 0.7 0.9 o.8 o.8 1.5 8.9 310 4.3 c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 0.5 0.5 8.o 4.1 0.5 0.7 13.8 9.9 5.3 124 310 February 6.9 5.8 0.3 7.7 10.1 4.5 113 282 March 0.2 7.3 11.6 1.0 10.6 9.7 6.5 124 310 April 0.5 7.2 12.0 1.5 0.5 9.5 8.8 7.0 120 0.1 270 May 3,9 18.2 0.7 0.7 9,9 10.6 6.3 124 0.2 279 June 5.8 12.8 o.6 o.4 6.4 9.2 2.6 150 0.3 270 July 0.2 4.3 19.3 6.8 0.2 14.6 16.6 1.0 154 1.8 310 August o.6 4.8 17 .3 8.1 o.4 13,8 17 .1 1.2 155 2.2 279 September 10.4 9.8 2.6 0.2 8.6 9.4 o.8 150 o.4 300 October o.4 7.6 4.3 o.8 7.4 9.7 3.7 155 0.2 310 November 1.4 6.2 1.8 1.0 6.6 10.4 3.8 150 300 December 1.2 6.o 3 .. 1 1.6 9.7 10.3 4.8 155 310 -72- CLOUD STATISTICS FOR WINSLOW, ARIZONA AT 2120 MST ave. Frequency of Occurrence {per thirty days) of cloud amount A. Cloud J\mounts (tentns) Month 0 1 2 3 4 5 6 7 (j 9 10 n tenths January 12.4 1.2 1.5 o.4 o.8 0.5 1.1 l.2 1.5 1.5 8.1 310 4.4 · February 14.o 0.7 1.6 1.0 0.9 0.5 o.6 0.5 1.l 0.5 8.5 282 4.o March 13.3 1.2 1.3 1.7 0.7 0.3 1.0 1.1 1.5 1.0 7.i 310 3.9 April 15.4 Ll LO 1.3 o.6 1.0 0.7 LO L2 1.1 5.6 270 3.4 May 15.8 L4 Ll l.4 o.6 o.6 0.9 1.3 2.2 o.6 4.l 279 3.1 June 19.3 0.9 1.0 L3 0.3 LO 0.2 0.9 o.8 l.4 2.8 270 2.3 July 7.3 o.8 L5 L7 1.4 1.3 0.7 1.7 l.2 l.9 10.6 310 5.7 August 12.6 1.0 1.5 1.7 o.4 0.3 o.4 LO 1.5 1.3 8.3 279 4.3 September 20.3 1.3 o.4 0.9 o.6 0.5 0.7 o.6 1.0 1.3 2.4 300 2.1 October 19.7 L2 1.3 1.0 0.7 0.7 o.4 0.7 0.7 1.1 2.7 310 2.l November 18.9 0.7 0.9 1.2 o.8 o.4 0.7 o.a 1.1 0.9 3.6 300 2.5 December 14.3 1.4 1.0 1.4 0.9 0.5 0.7 0.5 0.7 1.1 7.7 310 3,8 c. Thunder- B. Cloud Types storms Month F st Sc Cu Cb As Ac Ci Cs n freq. n January 0.2 1.7 4.6 1.9 1.2 9.0 2.2 5.8 124 310 February 0.3 4.8 0.5 0.5 5.3 2.9 1.6 113 282 March 0.7 7.0 0.7 1.0 5.6 6.o 4.1 l24 310 April 5.0 1.0 LO 9.0 3.5 4.o 120 270 May 5.8 1.3 0.3 10.7 5.2 4.8 93 279 June 4.4 1.4 o.6 5.0 5.4 2.4 150 0.2 270 July 12.7 3.3 2.7 o.4 13.8 7.6 1.2 154 0.7 310 August l2.0 1.6 2.3 o.8 11.3 8.o o.8 155 o.6 279 September 6.o 0.2 o.4 0.2 5.6 1.8 o.8 150 0.2 300 October 0.2 J.l o.6 o.4 0.2 5.2 4.7 2.3 155 310 November 1.4 4.6 0.2 o.4 4.8 3,8 4.o 150 300 December o.4 o.4 4.3 o.6 1,9 7.8 5,6 3.9 155 310 -73- CLOUD STATISTICS FOR PRESCOTT, ARIZONA AT 0020 MST ave. ·- Frequency of Occurrence (per thirty days) of cloud amount A. Cloud .Amounts (tenths) Month 0 1 2 3 4 5 6 i 7 8 9 10 n tenths January 13.6 1.0 1.4 1.5 0.9 o.4 0.9 1.0 1.6 0.7 7.1 310 3.9 February 15.6 1.1 1.1 1.7 0.7 0.2 1.2 0.7 1.6 0.7 5.3 282 3.2 March 14.6 o.8 2.0 1.0 1.5 1.0 o.8 1.3 0.9 o.8 5.5 310 3.4 April 18.7 1.0 1.8 1.0 o.8 0.2 o.4 1.0 o.8 o.4 3.9 270 2.4 May 21.6 0.9 0.9 1.3 o.6 0.2 o.4 o.6 1.1 o.6 1.7 279 1.6 June 23.2 0.9 1.3 0.3 0.7 o.4 0.3 0.7 0.2 0.3 1.6 270 1.2 July 14.5 1.2 1.5 0.9 0.7 o.4 1.0 1.0 1.5 1.3 6.2 310 3.7 August 16.9 1.1 1.1 1.5 0.9 0.7 o.8 1.1 0.9 0.9 4.4 310 2.8 September 24.3 0.7 0.7 0.9 0.5 0.3 0.5 o.6 o.4 1.1 300 1.0 October 21.5 1.1 1.0 1.2 0.5 0.3 o.8 o.8 0.9 o.4 1.7 310 1.6 November 21.4 1.0 o.8 o.4 1.2 0.1 0.3 1.4 0.7 o.6 2.1 300 1.8 December 17.9 o.4 0.7 1.6 1.1 o.4 o.6 1.3 1.1 o.6 4.5 310 2.8 c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci· Cs n freq. n January 0.3 1.3 3.2 1.3 1.6 4.8 4.5 5.5 93 310 February o.4 2.1 0.7 0.1 3.5 4.6 1.8 85 282 March o.6 5.5 1.9 o.6 1.9 5.8 3.2 2.3 93 310 April 0.3 4.7 1.0 4.o 4.o 3.0 90 270 May 1.6 1.0 o.6 5.2 4.2 1.6 93 279 June o.6 2.7 4.2 1.8 100 270 July 2.6 1.3 2.3 4.2 11.0 2.9 93 0.3 310 . August 3.2 o.6 3.9 1.9 6.5 1.6 1.0 93 o.6 310 September 0.3 0.7 0.3 1.0 0.7 2.7 1.0 90 0.3 300 October 2.0 0.3 0.3 0.3 2.7 5.7 1.0 90 310 November 0.3 2.7 2.3 0.7 4.3 4.o 1.7 90 300 December 1.0 0.3 0.3 5.8 ' 1.9 3.9 4.5 2.3 93 310 -74- CLOUD STATISTICS FOR PRESCOTT, ARIZONA AT 0320 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud Pmounts (tenths) Month 0 1 2 3 4 5 b 7 tj 9 10 n tenths January 14.3 o.8 1.5 1.4 0.9 o.6 0.3 1.6 1.0 o.6 7.2 310 3.7 February 15.8 ot6 1.2 o.8 1.1 0.3 1.3 1.8 o.6 0.7 5.6 282 3.3 March 14.5 0.7 1.3 1.1 1.8 0.3 1.1 o.8 1.9 1.1 5.5 310 3.6 April 17 .4 1.3 1.1 1.0 1.0 0.2 1.0 1.0 1.4 o.6 3.9 270 2.7 May 21.3 o.8 1.5 1.2 0.2 0.2 o.6 1.2 0.5 0.5 1.9 279 1.7 June 24.o o.4 0.7 1.2 0.3 0.1 0.7 o.6 0.2 o.6 1.2 270 1.1 July 14.6 1.1 1.4 2.1 1.4 o,4 0.9 1.7 1.3 0.9 4.4 310 3.2 August 17 .1 1.3 1.3 1.5 0.7 o.6 1.0 1.4 1.1 0.7 3.6 310 2.6 September 23.8 0.9 1.3 0.7 o.4 0.1 0.5 0.7 o.4 0.1 1.1 300 1.0 October 21.5 o.6 0.9 o.8 1.2 0.1 0.9 1.0 o.8 o.4 2.0 310 1.7 November 21.7 0.7 o.6 o.8 o.8 0.2 0.5 o.6 0.9 o.4 2.8 300 1.8 December 18.8 o.4 1.4 1.3 o.6 0.3 0.5 0.9 0.9 0.7 4.5 310 2.6 c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January o.6 o.6 3.2 1.3 2.3 5.5 5.8 3.6 93 310 February 2.8 0.7 o.4 1.1 2.8 4.2 1.4 85 282 March o.6 5.2 1.6 0.3 2.6 5.8 4.8 3.2 93 310 April 0.3 3.3 2.0 1.0 4.o 3,7 2.3 90 270 May 0.3 o.6 o.6 0.3 0.3 6.5 4.2 1.0 93 0.1 279 June o.6 0.9 3.9 2.7 1.5 100 270 July 1.9 0.3 1.0 1.6 13.2 1.6 1.9 93 310 August 2.3 1.3 1.3 1.6 9.0 1.9 o.6 93 0.1 310 September 0.3 1.0 0.3 1.7 5,7 1.3 90 0.1 310 October 1.3 0.3 4.8 4.5 1.0 93 310 November 0.3 0.3 3.7 2,0 1.3 4.7 4.o 1.7 90 300 I I December 0.3 o.6 4.8 1.0 1.0 3.2 3.6 2.9 93 310 I -75- CLOUD STATISTICS FOR PRESCOTT, ARIZONA AT 0620 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud knounts (tenths) Month 0 1 2 3 4 5 6 7 8 9 10 n tenths January 13.5 2.0 1.0 1.3 0.7 0.9 l.2 o.8 1.2 0.9 6.8 310 . 3.7 February 14.2 l.4 1.2 o.8 1.3 0.3 o.8 1.4 1.4 1.1 6.o 282 3.6 March 11.7 1.5 1.6 1.1 1.4 o.4 1.0 1.9 1.4 1.5 6.6 310 4.2 April 15.1 L6 1.0 1.1 1.2 1.1 0.2 1.2 1.3 1.4 4.7 270 3.3 May 17 .2 1.3 1.5 0.9 1.1 0.3 1.5 1.2 1.7 1.1 2.3 279 2.5 June 21.2 0.9 1.2 1.1 1.1 o.6 o.4 o.6 o.8 0.9 1.2 270 1.6 July 9.9 2.8 1.6 1.0 1.5 0.9 1.5 1.9 1,8 2.2 4.9 310 4.2 August 12.2 1.8 1.9 1.5 1.1 1.6 1.1 1.8 1.5 1.7 3.7 310 3.5 '' September 18.5 2.1 1.6 1.3 1.4 0.9 o.6 o.6 0.9 o.4 1.7 300 1.8 October 16.2 1.7 1.5 1.4 1.3 1.2 1.6 0.7 0.9 1.1 2.5 310 2.5 November 18.2 2.2 1.2 1.2 1.0 o.6 o.6 0.9 0.2 0.7 3.2 300 2.2 December 16.1 1.0 :1.5 1.3 o.8 0.7 1.2 o.8 , o.8 o.6 5.5 310 3.1 c. ' Thunder- B. Cloud Types storms Month F St Sc Cu Cb As I Ac Ci Cs n freq. n I January 1.0 2.2 3.9 1.0 2.7 6.o 5.6 3.9 124 310 I I I February 3.7 1.6 1.1 5.8 6.9 1.3 113 282 March 0.2 6.5 1.9 0.2 1.2 8.7 11.4 3.1 124 310 April 0.5 3.8 2.8 0.:2 2.0 6.2 10.5 1.5 120 270 May 0.5 2.9 1.5 0.7 1.2 10.6 11.9 2.4 124 279 June o.6 1.8 o.6 o.a 9.2 10.0 1.4 150 0.1 270 July o.4 4.1 4.5 o.8 1.2 19.9 10.9 o.6 154 0.1 310 August 0.2 1.6 4.5 5.6 2.3 o.4 19.6 9.9 1.0 155 0,5 310 September 0.2 0.2 3.6 2.4 1.0 0.2 12.8 6.2 150 0.5 300 October 3.5 2.1 0.2 0.2 8.o 11.0 1.0 155 0.1 310 November o.8 4.o 1.0 0.2 1.4 6.2 8.8 1.0 150 300 i December o.4 5.8 1.4 I 1.9 6.o l 8.0 1.7 155 310 I -76- CLOUD STATISTICS FOR PRESCO'IT, ARIZONA AT 0920 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud Amounts (tenths) Month 0 1 2 3 4 5 6 7 8 9 10 n 1.1eiiths January 10.6 1.2 1.1 1.6 0.9 o.4 0.7 1.3 1.1 1.8 9.5 310 4.9 February 12.0 0.5 1.0 1.1 o.6 0.7 1.1 1.6 1.3 2.1 8.o 282 4.6 March 10.e 1.9 0.9 o.8 0.9 o.8 1.5 1.3 1.5 1.8 7.8 310 4.6 April 13.3 1.7 1.2 1.2 o.8 o.8 1.0 1.4 1.1 1.7 5.8 270 3.8 May 16.4 2.3 1.4 0.5 o.6 o.4 1.4 1.7 1.0 1.6 2.6 279 2.7 June 20.3 2.0 1.1 0.7 0.9 0.7 0.3 o.4 1.0 1.1 1.4 270 1.7 July 10.7 2.1 2.2 1.6 1.4 1.2 2.1 1.1 2.0 1.5 4.o 310 3.8 August 12,5 2.7 1.6 1.6 1.5 1.3 1.1 1.5 1.4 1.7 3.0 310 3.2 September 19.8 1.7 0.9 1.4 0.7 o.4 1.1 1.1 o,8 o.8 1.3 300 1.8 October 15,5 2.0 0.9 1.8 1.3 o.6 1.5 1.3 LO 1.5 2.6 310 2.8 November 16.0 2.1 1.0 1.5 1.0 o.6 1.1 1.0 1.4 0.5 3.8 300 2.8 December 12.8 1.4 o.6 1.3 1.8 0.2 o.8 1.2 1.1 2.0 7.0 310 4,1 I c. Thunder- B, Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 2.4 8.2 1.7 2.7 8.5 11.1 4.4 124 310 February 1.3 6.6 2.1 1.1 6.9 10.6 3.2 113 282 March 0.7 6.3 5.3 1.7 6.8 10.9 3,9 124 310 April 0.2 3.5 6.5 2.2 5.8 10.2 2.2 120 0.1 270 May 0.7 9,0 0,2 0.7 7.5 11.6 2.4 124 279 June o.4 5,8 0.2 o.6 8,o 7.8 2.0 149 0.3 270 July 0.2 1.2 15.1 o.6 o.8 19.1 9.3 1.2 152 0.5 310 August o.8 2.5 15.5 1.2 o.4 18.4 8.3 1.4 155 o.4 310 September o.6 1.0 5,6 1.4 0.2 10.6 4.4 0.2 150 o.6 300 October 0.2 2.5 4.1 0.2 o.6 8.5 10.1 1.4 155 0.1 310 November 0.2 1.0 5.2 2.4 0.2 1.8 6.8 13.2 1.0 150 300 December o.4 7.8 2,51 2.7 8.L 13.6 3.5 155 310 -77- CLOUD STATISTICS FOR PRESCOTT, .ARIZONA AT 1220 MST ave. Frequency of Occurrence {per thirty days) of cloud amount A. Cloud .Amounts (tenths) Month 0 1 2 3 4 5 6 7 t) 9 10 n tenths January 8.2 1.5 1.8 1.1 0.7 0.7 1.5 2.2 1.5 1.9 8.8 310 5.2 February 9.4 1.6 1.4 1.3 o.8 1.0 1.0 1.8 2.1 1.7 7.8 282 4.9 March 8.7 2.1 0.9 1.3 1.4 0.9 0.9 1.6 1.5 3.0 7.8 310 5.0 April 9.4 1.8 1.1 1.8 1.0 1.1 0.7 1.4 2.1 2.9 6.7 270 4.8 May 11.0 3.5 1.7 0.5 1.6 1.4 0.9 1.5 1.6 1.8 4.4 279 3.7 June 16.9 1.8 1.3 1.7 1.3 0.9 1.2 1.2 1.1 0.9 1.7 270 2.3 July 4.6 2.2 1.8 2.3 3.1 1.2 1.8 3.1 2.5 3.0 4.3 310 5.1 August 6.2 2.8 1.1 2.3 2.7 1.6 2.0 2.9 2.6 2.2 3.5 310 4.6 September 12.9 3.5 2.4 2.1 1.7 1.1 1.5 1.0 1.5 1.1 1.2 300 2.6 October 13.3 1.9 1.2 1.8 1.5 0.9 1.9 1.5 1.1 1.4 3.5 310 3.3 November 14.2 1.8 1.3 1.6 1.7 0.7 0.7 1.1 1.8 1.0 4.1 300 3.2 December 10.9 1.7 1.5 1.1 o.6 0.9 1.3 1.7 1.3 1.7 7.3 310 4.4 c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 0.2 1.5 5.1 6.3 2.9 8.2 11.6 4.6 124 310 February 0.5 4.o 7.4 1.3 5.8 9.5 4.o 113 282 March 0.5 3.9 11.l 0.5 1.2 6.3 11.9 3.9 124 310 April 0.5 3.8 13.8 1.0 o.8 5.2 12.2 3.2 120 0.2 270 May 1.5 18.4 0.7 0.7 6.o 11.9 2.7 124 279 June 0.2 0.2 13.4 0.2 o.6 7.2 8.4 1.8 150 o.4 270 July o.6 19.5 5.8 o.4 11.9 5.8 1.4 154 5.0 310 August o.6 1.2 18.6 8.3 1.2 9.5 5.6 o.6 155 5.1 310 September 0.2 18.2 2.4 0.2 7.8 4.o o.4 150 1.0 300 October o.8 10.7 o.6 0.2 7.0 9.9 2.3 155 0.7 310 November 3.6 6.8 1.4 6.6 13.4 2.8 150 300 December 5.8 5.6 0.2 1.9 8.9 13.4 3.5 155 310 -78- CLOUD STATISTICS FOR PRESCOTT, ARIZONA AT 1520 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud Amounts (tenths) Month 0 1 2 3 4 5 6 7 e 9 10 n tenths January 7.6 1.6 2,2 1.2 1.5 o.6 1.2 1.1 1.4 3.1 8.6 310 5,3 February 9.3 1.3 1.0 1.0 0.5 1.1 2.0 2.2 1.1 1.9 8.6 282 5.1 March 6.2 2.8 1.3 1.3 1.5 0.7 1.0 2.0 2.1 2.7 8,5 310 5,5 April 7.3 1.6 1.6 2.3 1.2 2.4 1.0 1.1 1.2 2.3 7.9 270 5.1 May 9.2 2.8 1.6 1.8 2.4 1.7 1.2 1.1 1.3 2.9 4.o 279 4.o June 15.4 2.2 1.6 1.4 1.1 0.9 o.8 1.6 1.6 1.3 2.1 270 2.7 July 3.7 1.5 1.6 1.5 2.3 1.4 2.1 3.2 3.0 3.5 6.3 310 6.o ' August 4.6 1.8 1.5 2.0 1.5 2.1 2.6 2.0 3.2 3.2 5.2 310 5.5 September 10.5 2,8 2,6 2,1 2.6 2.0 1.3 1.1 1.5 1.5 2.0 300 3.2 October 12.8 2.7 1.5 2.1 1.2 1.4 1.0 1.2 1.5 1.6 3.1 310 3.2 November 13.7 2.6 o.8 1.3 o.6 o.8 1.3 1.1 1.0 1.7 5.1 300 3.5 December 10.1 1.7 1.2 1.5 1.1 0.9 1.2 1.3 0.9 1.5 8.7, 310 4.7 c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb · As Ac Ci Cs n freq, n January 0.7 6.o 7.3 2.7 9.0 12.3 4.6 124 310 February 5,3 6,1 0.3 1.1 4.8 11.4 2.9 113 282 March 0.5 5.8 13.3 0.2 1.9 7.5 12.1 3.9 124 0.1 310 April o.8 3,8 16.0 1.0 2.2 7.0 10,8 3.8 120 0.2 270 May 1.2 19.1 1.2 0,5 7.5 14.5 2.7 124 0,3 279 June o.6 16.0 o.4 o.6 6.2 9,0 1.6 150 o.4 270 July 1.0 13.7 13,5 1.4 8.o 6.9 1.0 153 6,.5 310 August 0.2 1.2 15,3 13.6 1.7 8.1 6.2 1.4 155 5.5 310 September 0.2 18.0 3.6 5,8 4.8 o.4 150 1.8 300 October 1.6 10.9 o.4 o.4 7.2 10.5 2.7 155 0.3 310 November 4.8 6.6 0.2 1.0 6.8 14.o 3.2 150 300 i December : 6.2 4.5 I 3.5 ! 7.6. 13.6 3.7 . 155 310 -79- CLOUD STATISTICS FOR PRESCOTT, ARIZONA AT 1820 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud Amounts (tenths) Month 0 1 2 3 4 5 6 7 t) 9 10 n tenths January 9.6 1.1 1.8 1.5 1.2 0.9 o.8 1.7 1.0 1.9 8.6 310 4.9 February 9.5 2.0 1.5 1.6 o.8 1.1 1.4 2.0 1.7 1.9 6.4 282 4.5 March 8.1 2.3 1.4 2.0 1.4 1.1 1.0 1.9 1.9 1.3 7.6 310 4.8 April 9.1 2.3 1.8 1.2 1.4 1.4 1.6 1.4 1.3 2.7 5.7 570 4.4 May 10.5 3.5 2.0 1.4 1.5 1.0 1.8 1.6 1.5 1.5 3.5 279 3.5 June 16.3 2.3 1.6 1.6 0.9 1.1 o.8 1.2 o.8 1.1 2.3 270 2.4 July 5.5 2.6 1.7 1.8 0.7 1.3 1.8 1.8 2.3 3.0 7.4 310 5.5 August 7.3 3.9 1.5 1.5 1.6 1.0 1.4 2.3 1.8 2.1 5.6 310 4.6 September 15.1 3.2 1.7 1.8 1.0 1.4 1.0 1.3 o.8 1.0 1.7 300 2.4 Oct;ober 15.9 2.7 1.3 1.3 o.8 0.7 1.5 o.8 0.9 1.3 3.0 310 2.6 November 17.2 1.3 0.7 1.5 o.6 o.6 0.9 1.2 1.0 1.1 3.9 300 2.8 December 12.6 1.4 1.8 1.6 0.9 0.7 o.8 1.2 1.4 1.3 6.5 310 3.9 c. Thunder- B. Cloud Types storms Month F st Sc Cu Cb As Ac Ci Cs n freq. n January 0.2 0.5 6.5 2.7 0.2 2.2 7.0 9.7 4.1 124 310 " February 6.6 2.4 0.3 1.9 6.1 9.8 1.1 113 282 March 0.2 7.5 7.0 2.2 8.o 13.3 3.6 124 310 April 0.2 4.o 9.2 2.0 2.8 8.o 10.2 3.2 120 o.4 270 May 2.9 11.1 1.2 0.5 9.1 13.3 2.9 124 0.2 279 June 1.0 8.6 1.2 o.4 8.2 8.2 1.8 149 0.2 270 July 1.8 10.2 14.1 1.2 13.9 8.8 o.8 153 5.0 310 August o.4 2.9 12.8 12.2 1.7 10.7 10.1 o.8 155 3.7 310 September 4.8 7.2 4.2 10.6 7.4 o.4 150 0.5 300 October 0.2 4.1 3.3 1.0 o.4 6.8 8.9 1.9 155 0.5 310 November 6.4 1.2 o.4 1.2 5.4 10.8 2.6 150 300 December 4.8 1.6 3.1 6.6 12.2 2.5 155 310 -80- CLOUD STATISTICS FOR PRESCO'IT, ARIZONA AT 2120 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud .Amounts (tenths) Month 0 1 2 3 4 5 6 7 ts 9 10 n tenths January 12.4 1.7 1.3 1.5 1.7 o.6 o.8 1.3 1.1 o.8 7.0 310 3 .9 February 13.6 1.2 1.7 1.1 1.2 0.7 o.6 2.4 o.8 0.3 6.3 282 3.7 March 13.1 1.9 1.8 2.1 1.5 o.4 0.9 1.1 0.9 0.5 5.9 310 3.4 April 16.2 1.2 1.2 1.1 1.3 o.6 o.8 1.8 o.8 1.0 4.o 269 2.9 May 16.6 1.6 1.7 1.3 1.3 0.9 1.3 1.5 1.1 0.3 2.5 279 2.4 June 20.2 1.6 1.2 1.2 o.6 1.1 0.7 0.7 o.8 0.3 1.7 270 1.7 July 10.7 1.8 1.6 1.1 1.2 1.4 1.4 1.3 1.9 1.0 6.7 310 4.3 August 14.2 1.4 1.8 1.6 1.5 0.7 0.7 1.3 1.3 0.5 5.0 310 3.2 September 22.2 1.2 1.3 1.4 0.5 0.1 0.3 0.5 0.7 0.3 1.5 300 1.3 October 20.9 1.0 1.6 1.0 0.5 o.4 1.1 0.3 1.1 o.4 1.8 310 1.7 November 19.4 0.9 1.0 0.9 o.6 0.2 1.7 0.9 0.7 1.1 2.6 300 2.2 December 16.4 o.4 1.0 1.3 1.4 0.5 1.2 1.4 1.1 o.4 5.2 310 3.2 c. Thunder- B. Cloud Types storms Month F st Sc Cu Cb As Ac Ci Cs n freq. n January o.6 0.3 5.2 o.6 2.3 5.5 4.5 5.5 93 310 .. February 4.2 0.7 1.8 2.8 3.5 3.5 85 282 March 1.0 6.8 2.9 1.6 5.5 5.8 1.6 93 310 April 0.3 4.o 2.3 1.0 2.0 5.0 4.7 3.3 90 0.2 269 May 3.6 0,3 o.6 1.3 5.8 7.4 2.3 93 0.1 279 June 1.5 0.9 o.6 3.9 5.2 2.1 99 0.1 270 July 4.8 1.6 7.1 1.9 11.0 6.5 0.3 93 1.5 310 August 0.3 0.3 6.1 4.2 4.5 1.0 9.4 5.8 0.3 93 1.9 310 September 2.3 0.3 1.0 0.3 4.7 2.7 90 o.4 300 October 1.3 0.3 0.3 4.5 4.5 1.6 93 0.3 310 November 0.3 5.0 0.3 0.7 1.7 4.7 4.o 4.o 90 0.2 300 December 0.3 5.8 o.6 2.3 4.2 3.9 4.8 93 310 -81- CLOUD STATISTICS FOR PHOENIX, ARIZONA AT 0020 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud Amounts (tenths) Month 0 l 2 3 4 5 6 7 ~ 9 10 n tenths January 17 .1 0.7 0.9 0.9 o.6 0.5 0.7 o.6 1.3 o.8 6.1 310 3.2 February 18.4 0.7 o.6 o.6 0.3 0.2 0.9 o.6 0.7 0.5 6.3 282 3.0 March 15.6 1.5 1.5 o.8 o.6 0.2 o.6 0.9 1.5 1.0 6.o 310 3.3 April 20.0 o.4 1.4 o.a 0.7 0.1 1.1 0.7 0.7 0.7 3.4 270 2.2 May 22.0 1.2 0.9 0.5 0.8 0.9 1.0 0.3 0.3 2.2 279 1.5 June 24.3 0.1 o.4 0.7 0.3 0.2 o.4 1.0 o.6 0.2 1.7 270 1.3 July 13.0 1.2 0.7 1.3 1.1 0.7 0.9 0.7 1.1 1.4 8.2 310 4.2 August 16.3 1.5 1.3 1.0 0.5 o.6 o.4 1.2 o.8 0.9 5.8 310 3.1 September 23.4 0.7 o.6 o.4 0.5 0.3 0.7 o.4 0.7 o.6 1.7 300 L4 October 22.2 o.8 0.5 0.7 0.5 0.2 LO o.8 0.9 0.5 2.1 310 L7 November 23.2 o.6 o.a 0.5 o.6 0.3 o.4 1.0 o.4 o.4 1.8 300 L4 December 18.4 LO o.a 0.9 o.4 0.1 1.2 1.0 1.0 0.5 4.9 310 2.7 c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 0.2 4.8 0.5 LO 4.1 3.4 2.4 124 310 February 2.1 0.5 0.3 2.1 3.4 1.9 113 282 March 0.5 2.9 0.2 5.1 3.6 3.1 124 310 April 1.5 0.2 o.8 3.2 2.8 1.0 120 270 May 1.2 0.5 3.9 2.9 L7 124 279 June o.4 0.2 0.2 3.2 2.6 o.8 150 0.1 270 July 2.1 1.7 1.1 0.2 12.0 6.o L4 155 0.9 310 August 0.2 2.5 1.2 1.6 o.6 8.9 5.8 1.2 155 0.9 310 September 1.0 0.2 o.4 0.2 4.8 1.0 0.2 149 300 October 0.2 1.4 0.2 2.7 2.5 o.4 155 0.1 310 November 0.2 3.0 o.6 2.0 3.0 0.2 150 300 December 2.9 o.6 5.2 4.7 1.9 155 310, -82;. CLOUD STATISTICS FOR PHOENIX , .ARIZONA AT 0320 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud Amounts (tenths) Month 0 l 2 3 4 5 6 7 ts 9 10 n tenths January 17 .3 1.3 1.2 o.6 0.5 0.2 o.6 0.9 o.6 0.9 6.1 310 3.0 February 17.2 o.6 1.3 o.6 0.2 o.6 o.4 1.2 0.7 1.1 6.o 282 3.2 March 15.7 1.0 1.1 1.0 o.6 o.6 1.0 0.3 0.9 0.7 7.4 310 3.5 April 20.9 0.3 0.9 o.8 o.8 0.1 0.9 0.3 o.6 o.8 3.7 270 2.1 May 23.2 1.0 0.5 o.6 o.4 0.5 o.8 0.5 0.3 2.0 279 1.4 June 25.1 0.3 0.7 0.3 0.2 0.1 0.7 0.3 0.1 o.4 1.7 270 1.1 July 13.0 o.8 1.0 1.5 0.9 0.5 1.2 1.0 1.7 1.2 7.4 310 4.2 August 17.0 1.3 1.2 o.8 o.8 o.6 0.9 o.6 1.1 o.6 5.3 310 2.9 September 23.1 1.4 1.5 0.9 o.4 0.2 0.7 0.2 0.2 0.1 1.3 300 1.0 October 21.9 o.6 1.3 o.6 0.3 · 0.3 o.6 0.1 1.5 o.4 2.6 310 1.8 November 23.4 o.6 o.8 o.6 0.5 0.1 o.8 0.1 o.8 o.4 1.9 300 1.4 December 20.3 0.3 o.6 o.6 o.6 o.4 0.2 o.8 0.9 0.3 5.1 310 2.5 c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq, n January 0.2 4.8 0.7 0.7 4.4 4.4 2.2 124 310 February 1.6 0.5 1.1 2.4 2.6 2.1 113 282 March 3,9 4.8 3.4 3.4 124 310 April 1,5 0.2 0.2 2.8 4.2 1.2 120 270 May 0.7 0.2 3.9 3.4 1.2 124 279 June o.4 0.2 3.4 2.J:. 1.0 150 270 July 2.1 o.6 o.8 o.4 12.4 4.7 2.1 155 0.3 310 August o.6 2.5 1.4 o.4 1.2 9.9 4.8 o.6 155 o.4 310 September 0.2 0.2 o.4 o.4 5.2 1.2 150 300 October 1.4 2.3 3.5 0.2 155 0.1 310 November o.4 3.4 1.2 2.0 2.0 1.2 150 300 December 0.2 1.9 0.2 5.8 3.3 2.7 155 310 -83- CLOUD STATISTICS FOR PHOENIX, ARIZONA AT 0620 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud .Amounts (tenths) Month 0 1 2 3 4 5 0 7 ~ 9 10 n tenths January 15.9 1.3 1.3 o.8 0.2 0.3 0.9 1.1 0.7 o.6 7.2 310 3.4 February 16.5 0.9 1.7 o.6 1.2 1.0 1.1 o.6 0.7 0.5 5.2 282 3.0 March 13.8 1.2 1.1 0.7 0.1 o.8 0.7 0.7 1.2 1.3 8.o 310 4.1 April 15.8 0.9 1.2 0.7 0.9 0.7 1.1 o.8 1.3 1.9 4.8 270 3.3 May 19.1 o.8 0.5 1.1 1.1 0.3 0.9 o.8 1.4 1.1 3.0 279 2.4 June 21.9 1.8 o.6 o.8 o.4 0.3 o.6 0.7 0.9 0.9 1.2 270 1.5 July 8.o 2.0 1.3 1.0 1.3 1.2 1.3 2.9 2.0 3.0 6.1 310 5.0 August 12.8 1.6 1.6 o.6 o.6 o.8 1.4 0.9 1.5 2.5 5.7 310 4.o September 20.4 o.8 o.8 o.8 o.6 0.9 1.1 1.2 1.3 0.5 1.6 300 1.9 October 17.4 2.0 1.5 o.4 o.6 0.9 1.1 o.8 1.7 o.8 2.8 310 2.5 November 20.6 1.2 1.0 1.0 o.6 0.5 o.4 o.6 o.6 0.9 2.6 300 1.9 December 17.6 1.2 1.4 1.0 o.6 0.5 o.8 0.5 0.5 o.4 5.7 310 2.8 c. Thunder- B, Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 0.2 0.7 6.o 0.7 5.8 4.1 2.2 124 310 February 2.6 0.5 1.3 4.2 5.3 1.6 113 282 March 0.2 4.6 1.0 0.2 1.2 8.o 8.3 3.4 123 310 April 2.5 1.2 0.5 7.2 9.2 2.5 120 270 May 1.0 0.2 8.2 12.1 2.4 124 279 June o.6 o.6 7.6 9.6 o.4 150 0.1 270 July 0.2 2.3 3.3 1.0 0.2 22.l 11.3 2.9 155 0.1 310 August 1.0 2.3 3.5 1.6 o.4 20.0 12.4 1.9 155 o.4 310 September 1.2 2.2 o.4 o.4 13.4 4.2 150 0.1 300 October 0.2 0.2 2.7 0.2 0.2 7.8 8.3 1.0 155 310 November 0.2 0.2 3,4 o.4 o.8 5.4 7.6 1.0 150 300 December 0.2 0.2 2.3 0.2 0.2 7.4 5,4 2.1 155 310 -84- CLOUD STATISTICS FOR PHOENIX, ARIZONA AT 0920 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud Amounts (tenths) Month 0 l 2 3 4 5 6 7 tj 9 10 n tenths January ll.5 1.6 1.0 l.l 0.5 0.2 l.5 1.0 l.0 1.5 9.3 310 4.6 February 14.4 0.9 1.0 0.7 o.4 o.6 o.6 0.7 1.6 1.0 8.l 282 4.o March 14.o 1.0 o.6 o.8 0.7 o.6 0.7 0.7 1.0 1.8 8.2 310 4.2 April 15.2 1.8 l.O o.6 o.8 0.7 o.8 0.7 l.2 l.2 6.1 270 3.4 May 18.9 l.9 o.4 1.1 0.1 0.3 o.8 o.8 1.5 1.2 3.0 278 2.4 June 21.8 1.0 0.7 o.4 o.8 0.1 o.4 0.9 o.8 1.1 2.0 270 1.7 2.4 4.7 310 4.2 July 10.6 2.2 1.3 1.5 1.2 0.5 l.3 2.3 l.9 . August 14.7 l.4 1.9 0.7 0.9 0.7 1.2 l.2 l.2 2.3 4.o 310 3.3 September 21.2 l.2 0.7 o.6 1.3 o.6 0.5 o.4 1.4 0.7 1.4 300 1.7 October 17 .4 1.5 0.7 1.5 0.7 o.4 1.3 1.2 1.0 1.4 3.1 310 2.6 November 18.8 1.3 o.6 0.5 0.3 0.1 0.3 0.5 1.4 1.7 4.5 300 2.8 December 13.6 1.4 1.2 o.8 0.3 0.3 o.6 0.9 0.9 2.3 7.8 310 4.1 c. Thunder- B. Cloud TYJ es storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 0.5 0.5 5.1 1.5 1.2 9.8 10.0 3.4 123 310 February 0.3 2.9 2.1 0.5 6.9 9.5 1.9 113 282 March 0.2 3.6 3.6 1.2 6.8 9.4 4.1 124 310 April 1.2 4.o o.8 6.o 9.5 3.2 120 270 May 0.7 2.7 0.2 6.5 12.1 2.9 124 278 June o.6 1.6 0.2 6.6 9.0 o.6 150 270 July o.4 1.7 6.4 o.4 o.4 19.4 11.8 1.4 155 0.1 310 August o.6 1.9 1.6 o.4 1.0 16.9 11.4 1.9 155 0.1 310 September o.8 3.2 o.4 0.2 11.6 5.2 150 300 October 1.9 2.5 8.o 9.7 o.6 155 310 November o.4 4.o 1.6 o.6 7.0 9.6 2.0 150 300 December o.4 3.9 1.0 o.6 8.9 12.2 3.1 155 310 I -85- CLOUD STATISTICS FOR PHOENIX, ARIZONA AT 1220 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud .Amounts (tenths) Month 0 1 2 3 4 5 6 7 ts 9 10 n tenths January 11.5 1.0 o.8 0.7 0.7 o.8 0.5 1.4 1.4 2.4 9.0 310 4.9 February 12.4 1.2 1.1 1.0 0.1 o.6 1.3 1.4 1.5 2.0 6.8 282 4.3 March 12.2 1.6 1.4 0.9 0.5 o.4 1.2 1.3 1.4 o.8 8.5 310 4.3 April 13.7 1.4 1.2 o.8 o.8 o.8 0.9 o.8 1.8 1.4 6.4 270 3.9 May 18.0 1.4 1.0 0.5 o.6 o.8 o.8 0.5 1.2 1.4 3.9 279 2.7 June 21.3 1.0 o.6 1.1 o.4 o.6 0.1 0.7 o.8 1.3 1.6 270 1.7 July 12.6 2.5 2.0 1.0 1.1 o.6 1.3 1.8 1.5 1.8 3.9 310 3.5 August 15.5 1.7 1.9 1.5 0.7 0.7 1.1 1.1 1.6 1.7 2.4 310 2.8 September 22.0 1.0 1.3 0.9 0.5 0.1 0.7 o.6 0.9 0.5 1.5 300 l.5 October 18.1 1.3 1.1 1.2 0.7 0.3 o.8 1.1 1.5 1.5 2.6 310 2.5 November 18.0 1.5 0.7 o.8 o.4 0.7 0.5 0.7 0.5 1.1 5.1 300 2.8 December 13.6 1.3 1.2 1.2 1.2 0.3 o.6 o.6 o.8 1.3 8.2 310 4.o c. Thunder- B. Cloud Types storms Month F St Sc L,'U, Cb As Ac Ci Cs n freq. n January 1.2 4.4 5.8 1.9 8.2 11.9 2.4 124 310 February 1.6 5.6 0.3 4.5 8.7 4.o 113 282 March 0.2 3.4 8.7 0.2 0.7 7.0 9.9 4.6 124 310 April 1.2 9.5 0.5 5.0 11.2 2.5 120 270 May 10.6 0.7 4.6 12.8 2.2 124 279 June 0.2 4.4 0.2 o.4 4.4 9.8 1.0 150 0.1 270 July o.4 1.6 13.0 8.9 o.4 15.3 11.8 1.0 155 310 August 1.4 18.4 4.7 o.6 11.8 11.6 1.7 155 310 -September 0.2 12.6 1.0 0.2 8.6 5.0 0.2 150 0.1 300 October 1.2 6.2 o.4 7.0 9.7 o.8 155 310 November 0.2 3.6 5.8 0.2 7.0 10.8 2.6 150 300 December 3.1 4.3 . 1.4 7.8 14.4 2.7 155 310 -86- CLOUD STATISTICS FOR PHOENIX, ARIZONA AT 1520 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud Amounts (tenths) Month 0 1 2 3 4 5 6 7 8 9 10 n tenths January 11.1 1.5 1.2 1.0 1.2 0.3 o.8 o.6 1.1 1.7 9.7 310 4.7 February 10.7 2.1 0.5 0.9 0.3 0.3 1.8 0.7 1.9 2.2 8~4 282 4.8 March 9.5 1.5 1.5 1.8 o.8 0.9 1.2 1.1 1.5 1.7 8.6 310 4.2 April 13.3 1.6 1.2 1.4 0.9 0.9 1.1 0.7 1.1 2.1 5.7 270 3.7 May 16.4 1.0 o.8 1.3 1.1 0.5 1.0 1.0 1.9 1.4 3.7 278 3.0 June 20.7 1.2 1.0 0.9 0.2 0.3 1.0 o.8 0.9 0.9 2.1 270 l.9 July 12.1 2.6 2.3 1.6 0.7 1.2 1.5 2.4 1.1 1.2 3.4 310 3.3 August 14.5 3.4 1.8 1.5 1.2 0.9 0.9 0.9 1.4 1.5 2.l 310 2.6 September 21.0 1.3 1.6 1.1 0.7 0.7 0.7 0.7 1.0 1.2 300 1.4 October 16.5 2.1 1.5 0.7 o.6 o.8 0.9 0.5 1.2 1.7 3.7 310 2.8 November 16.4 1.0 1.5 1.2 o.6 0.2 0.7 o.8 1.6 0.9 5.1 300 3.1 December 10.9 1.5 1.0 1.1 0.7 o.6 1.2 0.9 1.1 2.2 8.9 310 4.8 c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 3.4 8.2 0.2 2.4 7.5 12.1 2.7 124 310 February 1.9 6.9 0.5 5.0 10.6 2.4 113 0.2 282 March 0.2 1.7 11.4 0.2 1.2 7.0 11.1 4.4 124 310 April 1.0 10.5 1.2 4.8 13.2 2.2 120 270 May 0.2 12.8 1.7 0.2 4.8 12.6 2.2 124 0.1 278 June 6.8 1.0 0.2 4.4 9.6 o.8 149 0.2 270 July 1.0 11.6 14.9 10.9 16.3 o.6 155 0.3 310 August o.6 13.2 14.4 o.4 8.o 15.9 o.6 155 0.1 310 September 0.2 14.2 4.8 0.2 6.o 5.8 0.2 150 0.1 300 October 1.0 8.1 1.0 7.2 9.5 1.6 155 310 November 0.2 3.0 5.2 o.8 5.6 11.4 2.8 150 300 December 0.2 4.3 2.7 1.4 6.o 13.6 3.9 155 310 -87- CLOUD STATISTICS FOR PHOENIX , .ARIZONA AT 1820 MST ave. F.requency of Occurrence (per thirty days) of cloud lam.cunt A. Cloud Amounts (tenths) Month 0 1 2 3 4 5 6 7 t) 9 10 I n uenths January 13.1 1.2 1.1 o.8 0.7 0.2 0.9 1.0 1.4 1.7 8.1 310 I 4.3 February 11.3 1.2 1.9 0.9 o.6 0.1 1.7 0.7 1.3 1.3 9.0 282 4.6 March 10.7 1.1 1.3 o.8 1.5 0.7 o.8 o.8 1.5 2.2 8.6 310 4.8 April 13.4 o.8 1.1 o.8 o.8 o.4 1.1 1.4 2.0 1.4 6.7 270 4.1 May 16.8 1.5 1.2 1.0 o.8 0.5 o.8 o.8 1.2 1.5 4.1 27g 2.g June 21.2 1.4 0.3 o.8 o.4 0.2 0.7 o.6 1.4 1.0 1.9 270 1.8 July 10.1 1.5 ·1.6 1.5 1.1 0.9 1.4 1.2 1.7 2.2 7.0 310 4.6 August 11.6 2.5 1.5 0.9 1.4 o.4 1.0 1.5 1.8 2.3 5.1 310 3.9 September 20.2 1.2 o.4 o.6 1.0 0.5 1.3 1.1 o.8 1.2 1.7 300 2.0 October 18.6 0.5 1.1 o.8 o.6 o.4 1.0 1.1 1.2 1.5 ~.4 310 2.7 November 18.8 o.6 0.7 1.2 0.5 o.6 1.0 1.2 1.3 1.1 3.0 ~00 2.5 December 14.2 1.5 1.0 0.7 I 0.5 : 0.7 0.7 o.6 1.2 0.9 8.2 310 ~.9 c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs ' n freq. n Januarv 0.2 4.4 2.2 0.2 1.7 6-8 11-4 2.4 12L. ~, () February 3.2 4.2 o.8 6.6 10.9 2.6 113 0.1 282 March 0.2 3.1 5.6 1.2 8.o 10.9 4.1 124 0.1 310 April 1.8 7.0 1.8 0.5 4.8 12.0 3.5 120 0.2 270 May 2.4 6.5 1.5 0.2 5.3 14.3 1.9 124 0.1 279 June 4.2 1.0 5.2 8.6 1.2 150 270 July 1.0 7.2 15.7 o.4 13.6 17 .5 o.6 155 o.4 310 August o.6 10.5 16.1 o.4 10.5 16.9 1.2 155 o.6 310 September 0.2 10.4 3.6 0.2 9.0 9.4 o.4 150 0.3 300 October 1.2 2.3 1.0 0.2 7.6 9.1 1.6 155 0.2 310 November 0.2 3.8 o.8 o.6 4.2 8.8 1.2 150 300 December 0.2 3.9 o.6 1.4 5.2 10.9 2.7 155 310 -88- CLOUD STATISTICS FOR PHOENIX, ARIZONA AT 2120 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud Amounts (tenths) Month 0 l 2 3 4 5 6 7 ts 9 10 n !tenths January 16.6 o.6 1.0 0.5 0.7 o.6 0.9 0.9 1.5 0.9 6.o 310 3.3 February 16.0 0.7 0.9 1.2 o.6 0.9 0.3 1.6 1.2 0.2 6.5 282 3.4 March 14.8 1.1 1.4 1.1 o.8 o.6 o.8 o.8 1.4 o.6 6.9 310 3' •. 6 April 18.8 0.7 0.7 1.8 0.7 o.6 0.2 0.7 1.3 0.9 3.8 270 2.5 May 20.0 o.6 1.0 o.8 o.6 0.9 o.8 1.1 0.9 1.1 2.4 279 2.1 June 23.2 o.4 o.6 0.9 o.6 0.2 o.8 o.4 0.7 o.4 1.8 270 1.4 July 10.4 o.8 1.4 1.1 1.0 0.9 1.5 1.5 2.1 1.5 7.9 310 4.8 August 13.7 1.4 1.6 1.1 0.7 0.3 1.4 1.8 1.5 1.5 4.9 310 3.6 September 22.5 1.1 o.8 o.6 0.7 o.4 0.5 o.4 o.6 0.5 1.9 300 1.4 October 20.9 0.7 0.7 0.9 1.3 0.2 0.2 1.1 1.3 0.9 2.0 310 1.9 November 21.1 o.8 0.7 1.0 0.7 0.3 o.6 0.5 1.0 0.3 3.0 300 1.9 December 17.2 1.0 1.0 0.7 1.3 0.5 0.7 0.5 1.1 o.8 5.4 310 3.0 l c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 0.5 2.9 1.0 4.8 3.9 1.7 124 310 February 1.9 0.3 0.3 o.8 4.2 2.6 2.1 113 282 March 0.2 2.9 1.0 0.2 0.5 4.4 5.1 4.4 124 310 Anril 1.0 0.5 0.2 o.8 3.2 4.8 2.0 120 270 May 0.1 0.7 0.2 0.2 4.4 6.5 1.9 124 279 June o.4 0.2 o.6 3.6 4.6 o.8 150 0.1 270 July 2.7 2.5 5.2 0.2 12.0 11.l 1.0 155 1.1 310 August 0.2 2.3 2.1 3.7 o.4 9.5 9.3 o.8 155 1.5 310 September o.6 o.6 0.2 5.6 3.2 0.2 150 0.2 300 October 1.0 o.6 o.6 3.5 3.9 1.0 155 0.1 310 November 0.2 3.4 o.4 3.2 5.4 1.4 150 300 December 2.7 0.2 1.2 4.5 4.8 3.3 155 310 -89- CLOUD STATISTICS FOR TUCSON, ARIZONA AT 0020 MST ave. Frequency of Occurrence {per thirty days) of cloud amount A. Cloud Amounts (tenths) Month 0 1 2 3 4 5 6 7 e 9 10 n tenths January 16.2 1.2 0.7 1.3 0.7 0.7 0.5 o.6 1.2 0.7 6.5 310 3.3 February 17.8 1.2 0.7 0.9 1.0 0.1 o.4 1.1 0.7 o.4 5.7 282 2.9 March 16.8 0.7 1.2 0.5 0.3 o.4 o.4 o.4 0.7 1.2 7.6 310 3.5 April 20.2 0.9 0.7 0.9 0.7 0.7 o.4 0.7 0.7 4.2 270 2.3 May 23.0 o.8 1.1 0.3 0.2 0.2 0.3 o.8 o.4 o.4 2.5 279 1.5 June 22.9 1.0 1.1 o.4 o.4 0.2 o.6 o.4. o.6 0.1 2.2 270 1.4 July 7.6 1.0 1.1 0.9 1.4 1.0 o.6 1.1 1.5 1.4 12.7 310 5.9 August 11.7 o.8 1.3 1.2 0.7 0.5 1.4 1.0 1.3 1.1 9.3 310 4.6 September 22.8 o.8 0.7 o.8 0.3 0.5 0.3 o.4 o.6 o.4 2.4 300 1.5 October 21.3 1.4 o.8 1.2 o.4 o.4 0.3 o.6 0.3 0.2 3.3 310 1.8 November 21.8 1.3 1.0 1.0 o.6 0.3 0.2 0.2 0.9 o.4 2.3 300 1.6 December 17.8 o.6 1.5 0.7 . o.6 0.2 0.7 o.8 o.4 , o.6 6.2 310 3.0 c. Thunder- B. Cloud Types I storms Month F St Sc Cu Cb As Ac Ci Cs n freq, n January 0.2 0.5 5.6 0.7 0.2 7.5 1.2 8.0 124 310 February 0.3 3.4 0.3 0.3 4.8 1.1 5.3 113 282 March 5.6 0.7 0.2 0.2 8.2 1.2 8.o 124 310 April 3.0 0.2 6.5 1.0 7.2 120 270 May 1.9 1.0 6.8 1.5 4.1 124 279 June 2.2 0.2 6.6 2.8 2.6 150 270 July 0.2 10.7 3.3 3.1 o.4 20.0 7.8 3.5 155 1.5 310 August 8.1 1.9 4.8 0.2 21.0 9.1 3.9 155 0.9 310 September 2.4 o.8 7.4 2,8 o.8 150 0.1 300 October 3.1 o.6 o.6 o.4 4.8 1.2 3.7 155 0.1 310 November 0.2 5.4 0.2 o.4 6,6 1.4 4.2 150 300 December 0.2 o.4 4.7 0.2 0.2 8.o 1.7. 7.2 155 310 -90- CLOUD STATISTICS FOR TUCSON, ARIZONA AT 0320 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud .Amounts (tenths) Month 0 1 2 3 4 5 6 7 tj 9 10 n tenths January 15.6 0.9 1.3 1.2 0.8 0.3 0.9 0.1 1.6 o.8 6.7 310 3.5 February 18.9 0.5 o.6 0.7 0.9 0.3 0.5 0.7 0.5 0.2 6.o 282 2.8 March 16.4 o.4 1.1 o.6 o.6 0.1 o.8 1.1 o.6 0.5 8.o 310 3.6 April 20.4 0.3 0.2 1.2 0.7 0.1 o.6 0.9 o.8 0.7 4.1 270 2.4 May 22.5 o.4 1.1 o.6 0.9 0.2 0.2 o.6 o.6 2.8 279 1.6 June 23.6 o.6 1.2 o.6 o.4 0.3 o.6 o.4 0.3 0.2 1.8 270 1.2 July 7.8 1.3 1.4 1.2 1.1 0.9 o.8 1.5 1.5 1.2 11.5 310 5.6 August 13.5 1.4 1.5 o.6 1.1 0.3 1.2 1.5 1.3 1.0 7.0 310 3.9 September 22.7 1.2 o.8 o.6 o.6 0.5 o.6 0.1 o.6 0.3 1.4 300 1.3 October 21.6 1.2 0.7 0.7 0.6 0.5 1.2 0.3 o.4 0.9 2 .. 1 310 1.7 November 22.6 o.8 0.7 0.5 0.7 0.3 o.4 o.6 0.5 0.5 2.4 300 1.6 December 18.6 o.6 0.9 1.0 0.5 0.1 0.7 0.5 o.8 o.8 5.7 310 2.8 c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 0.7 4.8 1.2 0.5 8.o 1.5 7.3 124 'UO FebruEirY 2.1 0.3 0.5 5.8 0.3 4.8 113 282 March 5.1 1.7 0.7 10.2 0.5 8.5 124 310 April 2.8 0.2 0.5 6.2 0.5 6.o 120 0.1 270 Mai 1.2 0.2 6.5 LO 4.6 124 279 June o.6 o.6 0.2 0.2 6.6 2.4 2.2 150 270 July 0.2 9.1 1.2 1.7 0.2 20.6 4.5 5.0 155 o.6 310 Au~st 8.1 1.9 1.9 J..6 ..7 6.6 5.4 155 o.4 310 September 2.0 0.2 o.8 6.4 1.6 o.8 150 0.2 300 October 2.3 0.2 o.4 4.7 1.0 2.9 155 0.1 310 November o.4 5.0 o.6 o.4 4.8 o.a 4.8 150 300 December 0.2 3.9 o.4 o.4 8.1 1.2 7.2 155 310 -91- CLOUD STATISTICS FOR TUCSON • ARIZONA AT 0620 MST ave. Frequency of Occurrence {per thirty days) of cloud amount A. Cloud .Amounts (tenths) Month 0 1 2 3 4 5 6 7 ts 9 10 n tentbs January 15.2 0.7 1.5 0.7 0.7 0.7 o.6 0.9 1.1 o.8 7.4 310 3.7 February 15.7 1.1 1.1 o.6 o.4 0.1 0.7 1.2 0.7 0.7 7.6 282 3.6 March 12.8 1.0 o.6 0.9 o.6 o.6 0.3 1.4 1.1 1.4 9.6 310 4.6 April 16.4 1.4 0.9 0.7 o.4 0.3 o.4 1.4 1.3 o.8 5.8 270 3.2 May 18.6 1.7 o.B 1.3 o.6 o.6 o.6 o.6 0.5 o.4 4.1 279 2.3 ~• June 2b.2 2.1, 1.3 1.1 o.4 o.4 1.0 1.0 0.3 0.7 1.3 270 1.6 July 5.6 1.2 1.5 1.4 1.1 0.7 1.5 1.8 1.7 1.5 12.2 310 6.2 August 9.7 1.6 1.7 1.8 0.9 1.1 o.6 1.0 1.5 1.2 8.9 310 4.7 September 19.6 1.5 o.8 o.8 0.5 o.6 1.1 1.2 0.7 o.6 2.6 300 2.1 October 17.7 1.7 1.6 0.5 o.6 0.9 1.3 1.4 0.7 o.4 3.3 310 2.4 November 20.2 0.9 0.5 o.6 o.4 0.2 1.0 1.0 0.9 1.0 3.3 300 2.3 December 16.9 1.3 o.8 1.0 o.8 0.3 0.5 0.5 0.9 o.6 6.6 310 3.2 c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq_. n January 0.5 6.o 1.2 11.6 3.9 9.2 124 310 February 0.3 3.2 0.3 0.3 8.o 2.9 6.4 113 282 March 0.1 8.o 1.7 0.2 14.8 3.4 10.6 124 316 April 4.o 1.8 0.2 13.8 5.2 10.5 120 270 May 2.4 1.2 0.2 15.2 7.0 7.5 124 279 June 3.0 1.2 0.2 12.8 8.o 3.8 150 O.l 270 July 0.2 9.3 5.8 o.8 o.4 27.5 13.2 8.3 155 0.3 310 August 0.2 0.2 10.1 7.4 o.6 o.4 25.4 12.2 7.6 155 0.2 310 September 4.o 2.8 0.2 16.4 3.8 2.0 150 0.2 300 October 4.7 1.6 0.2 0.2 12.2 6.8 3.9 155 310 November o.4 5.6 1.2 o.8 10.6 4.8 6.2 150 0.1 300 December 0.2 4.8 o.4 0.2 11.3 4.8 7.2 155 310 -92- CLOUD STATISTICS FOR TUCSON, ARIZONA AT 0920 MST a.ve. Frequency of Occurrence (per thirty d9¥s) of cloud amount A. Cloud .Amounts (tenths) MOnth 0 l 2 3 4 5 b 7 ~ 9 10 n tenths January 12.1 1.4 o.8 o.4 o.8 0.1 0.7 o.a 0.9 0.5 11.7 310 4.9 February 14.5 1.2 o.4 1.0 0.7 o.6 1.0 1.4 1.0 1.2 7.1 282 3.9 March 12.0 0.9 1.5 o.6 1.3 0.3 1.0 0.7 o.8 1.5 9.6 310 4.6 April 16.2 1.1 o.a 0.7 0.9 0.3 o.8 0.9 1.2 1.4 5.7 270 3.3 Mav 19.1 1.7 0.9 0.3 o.6 0.3 o.8 0.5 0.5 1.0 4.2 279 2.4 June 22.0 1.9 o.6 o.4 o.6 0.3 0.7 0.9 o.6 o.4 1.7 270 1.5 July 8.9 2.6 1.2 1.2 1.2 0.7 1.0 1.7 1.5 1.8 8.2 310 4.9 6.o 4.o Awrust. 11.8 1.7 1.5 1.3 1.1 0.7 l.5 1.2 1.5 l.7 310 September 22.4 o.8 o.8 0.5 o.4 0.3 0.7 0.7 0.7 0.7 2.0 300 l.6 October 18.7 1.5 1.0 1.3 o.6 0.2 1.2 o.8 0.5 o.8 3.7 310 2.3 November 18.2 2.2 0.7 o.6 0.5 0.2 0.3 0.9 1.0 0.9 4.5 300 2.6 December 15.0 0.9 0.3 o.6 0.2 0.3 0.7 o.6 1.2 l.2 9.2 310 4.2 c. Thunder- B. Cloud ~es storms Month .-.F,,,,. St Sc \Cu Cb As Ar! Ci Cs n freq_. n January 0.7 1.0 8.5 3 .4 0.2 14.3 6.8 9.4 124 310 February 0.3 4.8 2.9 0.5 11.1 5.8 8.7 113 282 March 0.2 5.8 5.3 0.7 13.6 3.9 11.1 124 310 A'Dril 1.5 6.8 11.0 6.5 8.8 120 270 May l.2 7.3 11.9 9.0 5.3 124 279 June o.8 4.2 0.2 10.4 6.8 3.6 150 270 July 2.7 17.3 26.2 12.4 6.o 155 0.1 310 August 0.2 4.5 20.4 24.2 12.0 5.6 155 310 · \ September 2.0 7.6 13.8 5.4 2.0 150 300 October 2.3 6.2 12.2 8.o 4.3 155 310 November 0.2 5.2 3.4 0.2 10.6 7.2 7.4 150 300 December 0.2 5.8 1.9 o.6 12.2 9.1 10.3 155 310 -93- CLOUD STATISTICS FOR TUCSON, ARIZONA AT 1220 MST ave. Frequencyi of Occurrence (per thirty days) of cloud amount A. Cloud Pmounts (tenths) Month 0 l 2 3 4 5 b 7 lj 9 10 n tenths January 12.0 0.9 1.1 0.9 0.7 o.4 o.6 o.8 1.4 1.1 10.4 310 4.8 February 12.2 1.9 1.0 0.9 0.2 o.4 1.0 1.0 1.4 1.6 8.5 282 4.4 March 11.2 1.5 o.4 0.7 o.8 o.8 1.1 1.1 1.7 1.0 9.8 310 4.9 April 13.8 1.4 1.6 1.2 0.9 o.6 0.9 o.8 0.9 1.4 6.6 270 3.7 May 17.7 1.4 o.8 0.9 0.2 0.5 o.6 0.9 1.4 1.0 4.6 279 2.8 June 19.8 1.9 1.1 0.7 0.3 0.7 o.8 1.2 o.a 0.9 1.9 270 1.9 July 8.6 3.0 1.7 1.6 1.4 1.0 1.5 1.8 1.7 2.2 5.3 310 4.4 August 10.6 3.1 1.3 1.9 1.3 1.2 1.5 1.5 1.5 2.0 4.2 310 3.8 September 20.1 2.2 0.7 1.0 0.7 o.6 o.4 1.1 o.8 0.5 1.9 300 1.7 October 16.6 2.3 1.5 1.2 o.4 o.8 1.0 1.2 o.6 1.4 3.2 310 2.6 November 18.2 1.5 o.4 0.9 0.2 0.5 0.9 1.3 0.5 1.1 4.5 300 2.7 December 13.2 1.5 o.8 o.4 0.5 o.6 o.4 1.2 0.9 1.7 9.0 310 4.4 c. Thunder- B. Cloud T:v,es storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 0.5 6.o 5.6 0.5 12.8 9.0 10.4 124 310 February 3.4 6.1 0.3 0.5 9.3 7.2 8.2 113 0.1 282 March 0.2 3.6 10.9 0.7 9.7 6.8 10.2 124 310 April 1.2 10.5 9.2 7.0 8.2 120 270 May 0.5 13.8 0.2 10.4 7.5 6.8 124 0.1 279 June o.4 12.2 0.2 9.4 6.2 4.2 150 0.1 270 July 1.0 27.9 1.2 23.5 10.7 4.8 155 0.5 310 August 1.2 27.5 1.6 0.2 21.0 12.0 4.3 155 o.4 310 September o.4 22.0 1g.8 5.0 1.4 150 0.1 300 October 1.0 14.o 0.2 10.3 8.9 4.8 155 0.1 310 November 0.2 4.2 7.4 9.0 7.2 9.4 150 0.1 300 December 0.2 4.1 4.3 0.2 12.8 10.7 11.6 155 310 -94- CLOUD STATISTICS FOR TUCSON, ARIZONA AT 1520 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud .Amounts (tenths) Month 0 1 2 3 4 5 6 7 lj 9 10 n tenths January 10.6 1.7 1.2 0~6 o.8 o.8 o.8 0.9 1.4 1.1 10.4 310 4.9 February 11.1 1.2 0.9 0.5 1.0 o.6 0.7 1.3 1.1 1.6 10.1 282 5.0 March 10.3 1.2 0.5 1.4 o.8 0.5 0.9 0.5 1.2 1.6 11.3 310 5.3 April 12.2 1.4 1.1 o.8 o.6 o.8 o.8 1.7 1.4 o.8 8.4 270 4.4 May 15.5 1.6 1.3 1.0 o.6 0.2 o.8 o.6 1.7 1.2 5.5 279 3.3 June 18.0 1.4 1.2 o.8 0.7 o.8 o.8 1.4 1.2 o.8 2.9 270 2.4 July 4.3 2.6 2.2 2.3 1.5 1.1 1.7 2.6 1.9 2.0 7.6 310 5.5 August 5.8 3.2 2.4 2.5 1.3 1.5 1.7 1.5 1.6 2.3 6.2 310 4.8 September 14.4 3.3 1.5 1.1 1.3 1.2 o.6 1.5 1.0 1.0 3.1 300 2.8 October 13.7 2.2 1.2 1.5 0.9 1.2 1.3 1.8 1.2 1.3 3.9 310 3.3 November 16.2 1.8 1.4 1.0 o.6 0.5 o.8 o.4 1.0 1.5 4.8 300 3.0 December 11.8 0.7 0.7 0.9 0.9 o.6 0.7 1.0 1.0 1.7 10.2 310 4.9 c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb A.s Ac Ci Cs n freq, n January 0.2 6.o 6.8 0.5 12.8 9.0 9.9 124 310 February 2.6 7.7 0.3 0.3 7.4 5.6 10.1 113 282 March 0.2 3.4 11.l 0.2 0.7 11.1 6.8 10.9 124 0.1 310 April 1.2 12.5 0.2 8.8 7.0 9.5 120 0.3 270 May 1.2 15.7 0.7 10.2 8.o 6.8 124 0.2 279 .June 16.2 o.4 7.6 7.2 4.o 150 o.6 270 July o.8 24.8 5.8 21.3 18.4 2.1 155 3.0 310 August o.6 24.l 5.8 0.2 22.l 20.6 2.3 155 2.6 310 September o.4 24.4 1.0 12.6 7.6 1.6 150 1.1 300 October o.6 13.8 0.2 9.5 9.1 5.6 155 0.1 310 November 0.2 3.4 9.6 10.4 7.8 8.6 150 300 December o.4 4.1 4.8 o.6 11.8 10.3 11.8 155 310 -95- CLOUD STATISTICS FOR TUCSON, ARIZONA AT 1820 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Clou:l. Amounts (tenths) Month 0 1 2 3 4 5 6 7 0 9 10 n 1-enths January 13.2 0.7 0.3 0.7 0.9 0.5 0.9 1.1 o.8 1.1 10.1 310 4.6 February 11.8 0.9 1.0 1.0 0.7 o.4 1.1 0.7 1.6 0.9 10.0 282 4.8 March 10.6 1.5 1.0 o.6 0.3 0.5 1.5 1.0 o.6 1.3 11.2 310 5.1 April 12.3 1.0 1.9 1.0 1.0 0.3 o.8 1.6 1.3 1.7 7.1 270 4.2 May 16.9 1.7 o.4 1.2 0.3 o.6 o.6 o.6 1.4 0.9 ~-3 279 3.0 June 18.7 1.6 1.3 1.0 0.7 o.8 0.2 o.8 0.7 o.4 3.9 270 2.3 July 4.o 1.6 1.1 o.8 1.2 o.8 1.3 o.8 1.6 2.3 14.6 310 6.9 August 6.4 2.1 0.9 1.0 1.1 1.0 1.4 1.4 1.5 2.3 11.0 310 5.9 September 15.9 1.9 1.4 1.0 1.1 0.3 o.6 0.7 1.1 1.3 4.7 300 3.0 October 16.2 1.6 o.8 1.2 1.2 1.2 0.5 1.3 o.6 0.9 4.7 310 3.0 November 18.3 0.9 1.0 o.4 1.3 0.2 0.7 0.9 1.6 1.5 3.2 300 2.6 December 13.6 0.5 o.8 1.0 1.1 0.5 0.5 1.4 1.5 1.1 8.1 310 4.2 c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 7.7 2.7 0.7 12.8 8.7 9.0 124 310 February 0.3 5.0 2.4 0.3 0.3 9.0 7.4 9.0 113 282 March 0.2 4.6 7.0 0.2 1.0 13.3 6.3 13.3 124 310 April 4.5 9.2 0.2 0.5 11.8 7.2 11.5 120 0.3 270 May 4.6 10.9 0.5 0.2 11.4 11.4 6.3 124 0.1 279 June 2.2 10.4 1.0 9.0 9.6 3.2 150 o.4 270 July 4.1 20.0 8.3 24.6 19.4 3.5 155 3.3 310 August 3.7 18.4 9.3 0.2 24.6 23.3 1.6 155 3.1 310 September 6.4 13.6 2.0 17 .6 11.6 1.6 150 1.5 300 October 5.4 5.6 o.4 0.2 13.2 10.7 5.0 155 0.3 310 November 6.o 4.2 o.6 10.0 7.2 7.8 150 300 I December o.8 5.0 1.4 1.0 11.8 8.1 11.3 155 310 -96- CLOUD STATISTICS FOR TUCSON, ARIZONA AT 2120 MST ave. Frequ..en~y of Occurrence (per thirty deys) of <:loud amount A. Cloud .Amounts (tenths) Month 0 1 2 3 4 5 6 7 ts 9 10 n tenths January 14.8 1.2 1.1 1.5 o.8 0.3 0.8 o.6 1.5 0.3 7.4 310 3.6 February 15.9 o.6 o.6 1.1 1.5 0.5 0.9 o.6 0.9 o.4 7.0 282 3.5 March 14.o 1.5 1.8 o.4 o.4 0.7 1.1 1.0 1.2 o.8 7.3 310 3.8 April 18.1 1.8 0.9 0.9 0.9 9.2 o.6 0.1 1.6 0.7 4.3 270 2.6 May 20.4 o.8 0.9 0.9 1.0 1.0 0.1 1.0 0.2 o.4 3.4 279 2.0 June 21.9 1.1 1.1 0.3 o.6 o.6 0.3 o.4 o.4 o.6 2.7 270 1.7 July 5.0 0.9 o.6 0.9 1.3 1.0 1.3 1.5 2.2 2.1 13.4 310 6.8 August 8.7 1.1 1.5 1.8 1.3 o.8 1.2 1.6 1.9 0.9 9.3 310 5.1 September 19.5 1.3 1.2 o.6 0.9 o.6 0.3 0.7 1.2 0.5 3.2 300 2.2 October 19.0 1.5 1.0 o.6 o.8 0.5 1.1 0.7 o.8 1.0 3.3 310 2.3 November 21.9 0.9 o.4 o.6 o.4 0.5 0.3 o.6 0.5 o.6 3.3 300 1.9 I ' i December 16.5 1.2 0.9 0.7 1.1 o.8 o.6 1.0 I 1.0 o.8 5.7 310 3.2 I c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 5.8 1.0 0.7 6.5 2.7 8.o 124 310 February 0.5 3.4 0.3 0.5 4.5 1.1 5.6 113 282 March 5.6 0.2 0.7 8.7 1.2 9.9 124 310 April 5.8 0.2 0.5 6.5 2.2 7.5 120 270 May 3.9 0.7 9.4 4.4 5.6 124 279 June 3.2 1.8 o.4 6.6 4.6 2.6 150 o.4 270 July 0.2 12.8 4.5 8.3 21.9 13.2 3.3 155 2.6 310 August 9.5 5.4 8.o 0.2 20.2 16.9 1.9 155 1.5 310 September 5.8 1.2 1.2 10.0 4.4 o.6 150 0.3 300 October 3.5 1.4 0.2 6.6 2.7 5.6 155 310 November 0.2 5.4 o.4 o.4 6.2 3.0 4.6 150 300 December o.8 4.1 o.4 o.4 7.2 2.1 8.7 155 310 -':Tf- CLOUD STATISTICS FOR YUMA, .ARIZONA AT 0020 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud Amounts (tenths) Month 0 1 2 3 4 5 b 7 tj 9 10 ·n tenths January 17.6 0.3 1.3 1.0 0.9 0.3 1.1 1.5 o.6 o.4 4.9 279 2.9 February 21.2 0.7 0.9 o.a o.6 o.4 o.8 0.2 o.8 o.4 3.1 253 1.9 March 20.2 o.4 1.0 o.4 0.3 0.5 o.8 1.3 0.9 0.5 3.7 279 2.3 April 22.1 o.6 o.8 1.1 o.8 0.2 o.4 0.5 1.2 0.1 2.1 240 1.6 May 24.8 0.7 o.6 1.3 o.6 o.4 o.8 0.1 o.6 248 o.8 June 26.6 o.8 o.8 0.2 0.2 o.4 o.4 0.1 0.5 240 0.5 July 19.5 1.1 1.0 1.5 1.0 o.4 1.1 o.8 o.6 o.8 2.4 279 2.0 August 21.7 o.4 1.1 1.3 1.0 0.2 0.5 1.4 o.6 1.7 279 1.6 September 25.8 o.4 0.7 o.4 o.6 0.1 0.2 0.3 o.8 0.1 o.4 290 0.7 October 24.6 1.2 o.6 o.6 0.5 o.8 0.3 0.3 0.2 0.2 0.9 310 0.9 November 24.o 1.0 o.6 0,3 0.2 0.1 0.5 0.9 o.8 0.2 1.4 300 1.2 December 19.7 1.6 1.0 1.3 o.6 0.2 0.9 o.8 o.8 0.1 3.2 310 2.0 c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq, n January 0 279 February 0 253 March 0 279 April 0 240 May 0 248 June 0 240 July 0 0.1 279 August 0 0.2 279 September 0 0.1 290 October 0 0.2 310 November 0 300 _ December 0 310 -98- CLOUD STATISTICS FOR YUMA, .ARIZONA AT 0320 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud Amounts (tenths) Month 0 1 2 ' 3 4 5 b 7 ~ 9 10 n tenths January 17.8 o.8 o.8 1.1 1.0 0.2 1.1 o.6 o.6 0.5 5.5 279 2.9 February 20.8 o.8 0.9 o.8 0.9 o.4 0.7 o.4 o.6 o.4 3.3 253 2,0 March 19.4 0.5 0.5 o.8 o.6 0.3 0.5 1.0 1.8 0.3 4.2 279 2.6 April 22.1 o.8 o.6 o.6 o.8 0.5 0.5 0.2 o.6 0.2 3.0 240 1.7 May 24.9 o.6 o.6 1.0 o.6 0.2 o.4 0.5 0.5 0.7 248 o.8 June 26.8 o.4 0.5 o.4 0.2 o.4 0.2 o.~ o.6 240 o.6 July 18 .9 o.6 0.9 1.1 1.2 o.4 1.3 1.2 0.9 0.5 3.0 279 2.3 August 22.0 o.6 0.9 o.6 o.6 o.4 0.5 o.6 0.9 0.3 2.4 279 1.7 September 26.3 o.6 o.6 0.3 0.5 0.2 0.3 0.5 0.1 0.5 290 o.6 October 23.9 1.4 0.2 0.9 o.6 0.3 o.4 0.2 0.5 0.2 1.5 310 1..1 November 23.3 o.6 o.6 1.1 0.7 0.5 o.4 0.2 o.8 0.1 1.6 300 1.3 December 21.2 0.7 1.0 0.7 0.2 0.3 o.4 o.8 o.6 0.5 3.8 310 2.0 ' c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 0 279 February 0 253 March 0 279 April 0 0.1 240 May 0 248 June 0 240 July 0 0.2 279 August 0 0.2 279 September 0 0.1 290 October 0 0.1 310 November 0 300 December 0 310 -99- CLOUD STATISTICS FOR YUMA, ARIZONA AT 0620 MST ave. Frequency of Occurrence {per thirty days) of cloud amount A. Cloud Amounts (tenths) Month 0 1 2 3 I 4 5 6 7 /j 9 10 n tenths January 16.7 1.5 1.9 1.3 0.5 0.5 0.5 1.2 1.0 0.5 4.2 278 2.7 February 19.9 1.5 o.8 0.5 0.7 o.4 0.2 1.4 0.7 o.6 3.2 253 2.1 March 14.3 1.8 2.3 1.4 o.8 o.6 1.2 o.6 1.2 0.9 4.9 279 3.2 April 17 .2 1.8 2.2 1.2 0.5 0.9 1.0 1.5 o.8 1.0 1.9 240 2.2 May 20.6 1.5 1.2 0.7 o.6 o.6 1.0 1.3 1.0 o.8 0.7 248 1.6 June 23.4 1.2 0.9 0.2 o.4 0.5 1.2 o.6 o.8 o.4 o.4 240 1.1 July 13.4 1.8 1.4 1.2 1.1 0.9 1.3 2.6 2.4 o.6 3.3 279 3.4 August 18.0 1.7 1.4 0.5 1.4 o.4 1.2 o.6 1.2 1.3 2.3 279 2.3 September 21.9 1.7 2.0 1.0 o.4 0.3 o.4 o.4 o.6 0.3 0.9 290 1.1 October 20.3 1.6 1.6 0.9 0.7 0.7 1.3 o.8 o.8 0.5 0.9 310 1.5 November 22.0 0.5 1.0 1.3 1.0 o.4 0.7 o.8 0.7 0.2 1.4 300 1.5 December 19.6 o.8 1.4 o.8 0.7 o.4 0.3 1.0 o.6 0.5 4.2 310 2.3 c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 0 278 February 0 253 March 0 279 April 0 240 May 0 248 June 0 240 July 0 0.1 279 August 1.0 5.0 3.0 18.0 12.0 4.o 30 279 September 1.0 9.0 30 0.2 290 October 1.0 3.9 8.7 31 0.1 310 November 1.0 6.o 8.o 30 0.1 300 December 1.0 1.0 1.9 5.8 1.9 4.8 31 310 -100- CLOUD STATISTICS FOR YUMA, .ARIZONA AT 0920 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud Amounts (tenths) Month 0 1 2 3 4 5 6 7 tj 9 10 n tenths January 13.3 2.2 0.9 1.2 0.5 0.5 1.2 0.9 1.3 1.3 6.8 279 3.8 February 15.8 1.3 1.5 1.4 0.5 0.5 1.3 0.5 1.7 1.5 253 3.0 March 14.7 0.5 1.1 1.0 0.9 1.1 1.0 1.3 2.0 1.3 5.2 279 3.6 April 17.2 2.1 1.8 o.8 1.0 0.1 o.8 o.6 1.4 o.6 3,6 240 2.5 May 22.3 1.1 1.2 0.7 o.6 0.7 1.1 1.3 0.1 o.8 248 1.3 June 23.8 1.0 o.8 0.9 o.4 0.5 0.5 o.8 o.8 o.6 0.1 240 1.0 July 15.7 1.9 1.3 1.9 1.0 1.0 1.0 1.5 1.3 1.0 2.5 279 2.6 August 17 .6 2.2 1.3 1.5 0.9 0.5 o.8 1.7 1.1 o.6 1.8 279 2.2 September 21.9 1.9 1.0 1.0 0.9 0.1 1.0 0.5 o.8 0.2 0.5 290 1.2 October 19.7 1.6 1.0 1.1 0.9 o.8 1.2 1.1 0.7 o.6 1.5 310 1.8 November 20.3 1.7 1.0 0.5 0.5 0.3 0.9 0.9 0.7 o.8 2.4 300 1.9 i Dec.ember 14.o 1.6 1.1 l o.8 0.9 i o.6 1.4 I 1.5 ! 1.4 1.8 4.9 310 3.6 ' c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 0 279 February 0 253 March 0 279 April 0 240 May 0 248 June 0 240 July 0 279 August 11.6 13.6 11.6 8.7 31 279 September 2.0 5.0 2.0 30 290 October 1.0 1.0 1.0 3.9 5.8 1.9 31 0.1 310 November 4.o 5.0 16.0 4.o 30 300 December 1.0 2.9 1.0 ,12.6 10.6 8.7 31 310 -101- CLOUD STATISTICS FOR YUMA, .ARIZONA AT 1220 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud Amounts (tenths) Month 0 1 2 3 4 5 6 7 ~ 9 I 10 n tenths January 12.9 1.4 1.5 o.8 1.3 o.4 1.3 1.5 1.5 1.7 5.7 279 3.9 February 14.1 1.1 1.3 1.4 0.9 0.9 2.0 1.9 1.4 1.4 3.4 253 3.3 March 13.8 2.2 o.6 1.2 o.6 1.4 1.2 2.2 1.3 1.0 4.6 279 3.5 April 16.5 1.5 1.9 2.1 o.6 o.8 0.9 1.1 o.8 1.5 2.4 240 2.5 May 20.4 2.1 1.1 1.7 o.6 o.6 o.6 1.1 0.5 0.2 1.1 248 1.4 June 24.1 0.9 1.1 o.8 o.4 0.2 o.8 o.4 o.6 0.2 0.5 240 0.9 July 15.9 2.5 2.3 1.4 1.2 o.B 1.2 1.6 1.4 o.4 1.4 279 2.2 August 17.8 2.2 1.5 1.0 1.5 1.0 1.0 o.4 1.2 1.3 1.2 279 2.0 September 23.1 1.3 o.4 1.2 o.6 0.7 0.7 0.5 0.5 o.4 o.4 290 1.1 October 19.1 1.1 1.6 1.6 0.5 0.7 1.1 1.3 1.2 0.5 1.5 310 1.9 November 18.7 1.7 1.2 1.0 1.1 o.6 1.1 1.4 1.0 o.4 1.8 300 2.0 December 12.9 i 1.7 1.5 1.2 0.9 0.9 1.5 1.7 1.4 1.3 5.1 310 3.7 c. Thunder- B. Cloud Types storms : Month F St Sc Cu Cb ' As Ac Ci Cs n freq. n January 0 279 February 0 253 March 0 279 April 0 240 May 0 248 June 0 240 July 0 0.1 279 August 19.4 1.9 8.7 9.7 5.8 31 279 September 5.0 3.0 30 290 October 1.9 3.9 6.8 3.9 31 310 November 2.0 2.0 1.0 4.o 11.0 4.o 30 300 December LO 2.9 1.9 4.8 I 11.6 9.7 31 310 -102- CLOUD STATISTICS FOR YUMA, ARIZONA AT 1520 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Clo~d Amounts (tenths) Month 0 1 2 '. 3 4 5 6 7 ~ 9 10 n tenths January 10.8 2.2 1.5 1.6 o.6 1.1 1.3 1.0 1.5 1.9 6.6 279 4.3 February 13.2 1.9 1.5 0.9 1.3 o.6 1.7 1.4 1.3 1.5 4.6 253 3.5 March 11.6 1.8 1.9 1.4 0.9 1.2 1.2 1.1 2.4 1.9 4.6 279 3.9 April 16.2 1.6 1.0 1.2 1.4 1.2 1.0 1.0 1.1 0.9 3.2 240 2.7 May 18.9 1.7 1.7 1.6 o.8 0.5 1.3 1.0 0.7 o.8 1.0 248 1.8 June 23.5 1.2 1.0 o.8 o.4 o.8 o.6 o.4 o.6 o.4 o.4 240 1.0 July 15.0 3.3 2.2 1.4 1.7 1.0 0.9 1.1 1.5 o.8 1.2 279 2.2 August 16.6 2.2 2.7 1.5 1.1 1.3 1.2 1.0 1.3 0.5 o.8 279 2.0 September 21.5 1.8 1.6 1.4 0.5 0.5 o.8 0.7 0.2 0.3 o.6 290 1.2 October 19.3 1.2 1.2 0.9 0.9 o.8 0.7 1.5 0.9 0.7 2.2 310 2.1 November 17 .o 1.9 1.9 1.2 0.9 o.8 0.5 1.5 0.9 1.3 2.1 300 2.3 i December 13.3 1.9 1.2 o.8 1.1 ! 0.5 , 1.0 1.2 2.1 1.5 5.5 310 3.8 c. Thunder- B. Cloud Types storms Month F 1 St Sc Cu Cb As Ac Ci Cs n freq. n January 0 279 February 0 253 March 0 279 April 0 0.1 240 May 0 248 June 0 240 July 0 0.2 279 August 22.3 4.8 7.7 9.7 4.8 31 279 September 9.0 1.0 30 290 .. October 5.8 1.0 4.8 3.9 31 0.2 310 November 2.0 2.0 5.0 13.0 6.o 30 300 I i I December 1.0 1.9 3.9 I 3.9 i 16.5 6.8 31 I 310 j -103- CLOUD STATISTICS FOR YUMA, ARIZONA AT 1820 MST ave. Frequency of Occurrence {per thirty days) of cloud amount A. Cloud /mounts (tenths) Month 0 1 2 3 4 5 6 7 e 9 10 n tenths January 11.1 1.5 1.9 i.4 1.0 1.0 1.1 1.3 1.5 1.4 6.9 279 4.2 February 12.8 1.7 1.8 0.5 1.1 o.8 0.9 1.8 1.5 2.• 4 4.7 253 3.8 March 11.7 1.6 1.6 1.2 1.0 1.2 1.3 1.5 1.9 1.0 6.o 279 4.o April 16.0 1.5 1.0 o.8 1.0 1.0 1.2 1.9 1.4 1.2 3.0 240 2.9 May 19.4 1.8 1.7 1.2 o.6 0.5 1.0 1.2 0.7 o.4 1.6 248 1.8 June 23.2 1.8 1.0 0.9 0.5 o.4 0.5 o.4 o.4 o.4 o.6 240 1.0 July 15.4 3.0 2.8 1.3 1.8 o.4 1.4 1.0 0.9 o.6 1.l~ 279 2.1 August 16.7 2.3 1.9 1.7 0.9 o.8 1.4 1.2 1.7 0.5 1.0 279 2.1 September 21.2 2.0 1.0 1.7 0.7 o.4 o.4 0.9 o.8 o.4 o.4 290 1.2 October 18.8 1.4 1.5 1.5 0.9 0.9 1.6 1.2 o.8 0.3 1.4 310 1.9 November 17 .6 1.2 2.1 1.4 o.8 0.9 1.1 0.9 1.0 0.7 2.3 300 2.2 December , 14.1 1.5 1.7 o.8 0.9 0.5 1.3 1.9 I 2.1 o.6 4.5 310 3.4 c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n January 0 279 February 0 0.1 253 March 0 279 April 0 240 ~ay 0 248 June 0 240 July 0 279 August 1.9 13.6 8.7 9.7 12.6 3.9 31 0.3 279 September 4.o 3.0 30 o.4 290 October 3.9 1.9 5.8 7.7 1.9 31 0.1 310 November 3.0 1.0 5.0 15.0 4.o 30 300 December 1.0 1.0 i 1.0 9.7 12.6 9.7 31 310 -104- CLOUD STATISTICS FOR YUMA, ARIZONA AT 2120 MST ave. Frequency of Occurrence (per thirty days) of cloud amount A. Cloud .Amounts (tenths) Month 0 1 2 3 4 5 6 7 13 9 10 n !tenths January 15.0 1.2 1.4 1.2 1.5 1.0 1.2 1.0 1.3 o.4 4.8 279 3.2 February 18.3 0.9 1.1 1.1 1.5 0.1 1.5 0.7 0.9 0.7 3.1 253 2.4 March 17.0 o.6 1.5 1.4 1.0 o.6 o.4 1.8 1.3 o.6 3.7 279 2.8 April 20.5 o.8 1.5 0.9 1.0 1.1 o.4 o.6 0.5 0.5 2.2 240 1.8 May 21.7 1.2 1.3 1.1 1.0 1.1 0.7 o.8 0.1 1.0 248 1.3 June 25.0 1.2 o.8 o.4 o.6 0.2 0.5 0.2 0.1 0.1 o.8 240 0.7 July 15.8 2.6 2.3 1.5 1.2 0.2 1.8 1.7 0.9 0.5 1.5 279 2.2 August 18.8 1.3 2.3 1.1 1.2 o.8 1.5 1.0 o.8 o.6 o.8 279 1.8 September 23.9 0.5 1.9 0.7 0.7 o.4 0.7 0.3 0.5 0.2 0.1 290 o.8 October 22.6 1.6 0.7 1.4 o.6 0.3 1.1 0.5 0.3 0.3 o.8 310 1.1 November 22.0 1.6 1.5 1.0 0.5 o.6 0.1 o.4 o.8 0.3 1.2 300 1.2 December 17.9 o.8 o.6 1.8 1.5 o.4 1.5 o.8 1.0 0.7 3.2 1 310 2.5 c. Thunder- B. Cloud Types storms Month F St Sc Cu Cb As Ac Ci Cs n freq. n I January 0 279 February 0 253 March 0 279 April 0 240 May 0 248 J1me 0 240 July 0 279 August 1.0 1.9 5.8 1.0 9.7 8.7 3.9 31 0.1 279 September 30 0.1 290 October 1.9 2.9 1.0 31 0.1 310 November 3.0 3.0 9.0 3.0 30 300 December ; 1.0 2.9 8.7 7.7 31 310: