42 BULLETIN AMERICAN METEOROLOGICAL SOCIETY

Some Examples of Chinooks East of the Mountains in

A. W. COOK AND A. G. TOPIL

U. S. Weather Bureau, , Colorado

ABSTRACT

The mechanics of chinooks as they occur in eastern Colorado were studied with the aid of upper-air data and cross sections of equivalent-potential across the Rocky Mountain region. The conventional model of the chinook involving precipitation on the windward side of the range adjacent to the chinook area, is discarded in favor of a model that does not require the precipitation. A number of cases are studied. One complete example with all important charts and diagrams is presented to show the mechanics of the chinook as it occurs in the Colo- rado area. Some data on other examples are also given.

INTRODUCTION 27, 1940 (FIG. 1) is an example of the extreme HE definition of chinook used in this paper rise that can occur during the night. In a short pe- is that applied by early settlers along the riod after midnight there was a rise of 25F°, the eastern base of the . temperature reaching 12° higher than the maxi- T mum of the previous afternoon. They used the term to apply to the warm, dry descending the eastern slopes of these mountains The maximum temperature in a chinook is pre- [17]. It is further restricted by usage of weather dicated by the potential temperature of the air de- forecasters in the Rocky Mountain area to refer scending the mountains; this may be different to cases where westerly are of sufficient from day to day according to advection of warmer strength to produce forced descent and adiabatic or cooler air at the level of the mountain crests and heating. passes, or by a greater velocity of upper winds The classical theory of the chinook is predicated stirring air down from an even higher level with upon the air rising on the windward slopes and greater potential temperature. There is a much cooling at the saturated adiabatic rate, then de- smaller diurnal range in temperature on a chinook scending on the leeside at the dry adiabatic rate. day (i.e., when the chinook blows all day) than In such a process the air arrives at the lee base of the mountains with an increase in temperature equal to the latent heat liberated during the con- densation while ascending the windward slopes. This theory is credited to Hann, who studied the in northern . That the foehn and chinook winds have much in common, there is no question. Several authors take more or less exception to portions of the original Hann theory [6] [7] [12] [13], principally in whether the ascending or precipitation phase is necessary, and what would cause warm air at high levels to descend and displace colder, more dense air at the surface. Hann later admitted that precipitation on the windward was not essential for a foehn [18].

SOME CHINOOK EFFECTS OBSERVED IN EASTERN COLORADO Chinooks in eastern Colorado can occur at any time, day or night. They are more pronounced at night because the normal temperature is lower FIG. 1. Thermograph trace showing a rapid rise in than in the daytime and the contrast between chi- temperature after midnight, produced by a . nook conditions and normal conditions is greater. The temperature rose far above the maximum of the The thermograph trace at Denver for January 26- previous afternoon.

Unauthenticated | Downloaded 10/05/21 10:16 AM UTC VOL. 33, No. 2, FEBRUARY, 1952 43

turbulence set up as the air spills over the mountain ranges. This is essentially a warm-frontal action but it will usually not show warm-front clouds and precipitation, because of subsidence in descending the mountain slope. Probably the simplest ex- planation of why the upper air descends the east slope is that the general pressure gradient in the lower cold air directs that air eastward, and it must be replaced by warmer air from the west. The second cause is subsidence. Air that de- scends along the mountain slope is warmed dynami- cally at the dry adiabatic rate. This should bring it to the base with the same potential temperature with which it started above the top of the range. Its previous history makes no difference, for the temperature above the mountain, as well as amount of compression heating, will be approximately the same whether the air came from a valley on the other side of the range or whether it has been at FIG. 2. Illustration of the extreme temperature drop the for several days and thousands of that can occur when a cold front from the north passes miles of travel. Observation shows the potential Denver at the same time that a chinook wind that has temperature at the bottom of the descent is not blown throughout the night ceases. quite as high as that at the level where the air started down; this discrepancy is probably due on an average day. There is often a rapid fall in principally to contact cooling and evaporation dur- temperature when the wind ceases, even for an ing descent, with mechanical mixing and radiation hour, at night, with an abrupt rise in temperature also contributing. The surface temperature dur- if the wind again reaches the surface. ing the chinook varies with the height from which The greatest temperature drop at the end of a the air is brought down. If upper winds are suffi- chinook occurs when it is coincident with the pas- ciently strong, they can bring down air from two sage of a cold front of polar air and the wind turns to five thousand feet above the main range of into the upslope direction. The magnitude of the mountains. closing temperature fall depends on the strength of the preceding chinook wind and the temperature contrast between the two air masses. The thermo- gram for Denver on January 17, 1940 (FIG. 2) shows an excellent example; in this case the tem- perature fell from 45°F to 17°F in the first hour and continued falling to 7°F below zero. The rise in temperature during a chinook may be attributed to three causes which are often con- current. The first, which is not essentially chinook effect but which is a component part of almost any type of warming, is replacement of a cold air mass by a warm air mass, i.e., by advection of warmer air. In many cases when a dense cold cP air mass moves out of Canada its western flank is pushed firmly against the wrestern mountain ranges. Weak disturbances often fail to dislodge this cold air mass and it remains until a strong pressure gradient de- velops with 40-60 mile per hour free-air westerly winds across southern and northern FIG. 3. Six consecutive raobs taken at Denver during Colorado. These winds remove the cold air mass a chinook wind situation. All show advection of warm partly through momentum and hydrostatic forces air up to the 400-mb level. The last two show removal of and partly by eddy excavation due to mechanical the surface inversion by the chinook wTind.

Unauthenticated | Downloaded 10/05/21 10:16 AM UTC 44 BULLETIN AMERICAN METEOROLOGICAL SOCIETY

The third cause of a large rise in temperature during chinook conditions is the preventing or destroying of the normal night-time ground in- version. The daytime maximum temperature in a chinook may be only a little higher than might ordinarily occur with regular daytime heating, whereas if the chinook wind continues strongly throughout the night, a ground inversion does not form and the usual nocturnal fall in temperature (average about 25°) is not realized. If an in- version has already formed during the early part of the night and the temperature has fallen to near a normal minimum value, the break through of a chinook wind to the ground causes a very striking rise in temperature. The temperature reached under such night-time conditions often approaches ordinary daytime maxima. After the chinook has FIG. 4. Thermograph trace showing day-by-day warm- once broken through to the ground during the ing during the chinook due to advection of warm air, and night and continues to blow the next day, the ad- the warm night of the 20th-21st during which the tempera- ditional rise in temperature due to insolational ture scarcely dropped below 50°. heating is usually small compared to the insola- tional effect on normal days. Where a chinook 2100 MST on January 18, the cold air mass, hav- wind at the surface lets up for a short period dur- ing just moved in, was at its lowest temperature at ing the night, even for less than an hour, the tem- all levels. In the four following soundings this air perature rapidly drops several degrees only to rise became steadily warmer at all levels—an excellent sharply as soon as the wind breaks through again. example of the advection of warm air aloft coinci- dent with a chinook. The last two raobs show the EXAMPLES OF COLORADO CHINOOK SITUATIONS characteristic lack of a ground inversion and the FIGURE 3 shows six consecutive raobs at Denver near-adiabatic below 12,000 feet MSL at 12-hour intervals during a chinook period. At once the chinook is well established. The thermo-

FIG. 5. Surface map for a chinook, showing the packing of isobars across southern Wyoming and western Colorado, January 20, 1943, P.M.

Unauthenticated | Downloaded 10/05/21 10:16 AM UTC VOL. 33, No. 2, FEBRUARY, 1952 45

FIG. 6. 10,000-foot map during a chinook, showing strong zonal flow (high index), Jan. 20, 1943. graph trace (FIG. 4) indicates a rising trend on It remains to prove that there was no release of the 19th, then on the 20th, continued and great latent heat for warming to the windward. The cli- rise due to a combination of advection and chinook matological data show that at 54 stations reporting (subsidence) with a relatively small drop in tem- over the upper Colorado River drainage basin, perature the night of the 20th-21st, when the wind which is to the windward of the Rocky Mountains continued to blow. continued high from Denver, there was only one measurable on the 21st, but rapid fluctuations of 10° to 15° shower on the 19th and three light showers on the occurred during the night of the 21st-22nd, when 20th. Thus the precipitation theory is discounted the wind decreased for short periods of time. in this case. Observations indicate that the ex- FIGURE 5 is the synoptic map on the evening of treme chinook days have only high thin cloudiness, the most pronounced chinook conditions, and incapable of producing substantial precipitation, FIGURE 6 the associated 10,000-foot chart. At even over the mountains. 10,000 feet the high zonal-index type of flow is To further discount the precipitation theory the present giving strong west winds with very weak case of February 8, 1945 (illustrations not in- ridge and trough developments. Air reaching cluded) may be cited. Comparing soundings it Denver comes from near or slightly north of was found that the air between 840 and 700 milli- Ely, Nev. Comparing the raobs at Denver on bars at Denver averaged 7C° warmer than at the evening of the 20th with those at Ely at the Grand Junction. Taking the specific heat of dry same hour and for 24 hours earlier (FIG. 7), shows air at constant pressure as .24 it would require 6, 0E, and w higher in every case, level for level, at precipitation of .4 cm or .16 inch to heat the air Denver than at Ely. This makes it very unlikely column by this amount. This would be required that much of the air came from low levels on the for comparable areas; that is if the northeastern west side of the mountains or was warmed by the one-fourth of Colorado were warmed 7C°, then latent heat of condensation. The only reasonable the northwest one-fourth should show an average possibility is that air from above 12,000 feet west of .16 inch of precipitation. Out of 52 stations in of the Front Range descended as a layer on the the Colorado River drainage basin of western east side of the mountains, retaining its equivalent- Colorado, only five reported showers on the 8th, potential temperature, but increasing its mixing none of which were heavy. Further, the sounding ratio and lowering its potential temperature at Grand Junction showed that neither the equiva- through evaporation of moisture into the air mass lent-potential temperature nor the moisture con- from the surface of the east slope. tent at lower levels was sufficient for such warming

Unauthenticated | Downloaded 10/05/21 10:16 AM UTC 46 BULLETIN AMERICAN METEOROLOGICAL SOCIETY

FIG. 7. Raobs at Denver, Ely, and Oakland. The potential temperature and equivalent-potential temperature at Denver are much higher than those at Ely, indicating that the air must have come from aloft and not from lower lev- els at Ely. w-mixing ratio; i?#-relative ; T°C-temperature centigrade; ^-potential temperature; ^-equiva- lent-potential temperature.

to be achieved by release of latent heat through CONCLUSIONS precipitation. Air in a chinook is warmed by compression as Pilots on flights over the San Luis Basin in it descends a mountain slope. This air will have southern Colorado report that warm air during at least the equivalent-potential temperature that chinook conditions does not enter even the larger it had at the crest of the mountain range from intermountain valleys. This Basin is bounded by which it was brought down, and at times it is the Continental Divide on the west and the Sangre brought from several thousand feet farther up and de Cristo Range on the east, the crests of the so has a higher equivalent-potential temperature ranges being about 70 miles apart across the valley. than air on the other side at the level of the crest The pilots state that when strong westerly winds of the range. Horizontal advection of warm air of chinook type blow across the valley above the is often just as important as vertical movements level of the tops of the ranges, very little wind or during cases of chinook warming. Chinook warm- turbulence is found below the level of the tops. ing is more pronounced at night, because the wind However, a downdraft is encountered on the east destroys or prevents the formation of a nocturnal ground inversion. The lapse rate is nearly dry side of the Sangre de Cristo Range where the wind adiabatic during chinook conditions from the descends to the . There is a possi- ground up to 10,000-16,000 feet MSL. bility of approximating the Hann model by refer- A precipitation shield on the windward side of ence to the sounding at Oakland on the 19th the Colorado mountains (heating of the air due (FIG. 7) which showed a moist layer above 750 to release of latent heat of condensation) is not mbs. If some of the lower air at Oakland was lifted necessary for a chinook and is in fact a rare ac- up the Sierras, then remained at levels above companiment in the Colorado area. Moisture is 12,000 feet over the Great Basin, to descend the gained rather than lost by the air in its transit east side of the front range of the Rocky Moun- across the mountains; the gain is due to evapora- tains, the old theory may in a sense apply. Even tion while descending the eastern slope. so, all chinook air would have had to come from above 8,000 feet over Oakland, judging by the REFERENCES equivalent-potential temperatures. No attempt [1] Kendrew, W. G.: The Climates of the Continents. Oxford Press, 1937. was made to ascertain whether rain fell on the west [2] Landsberg, H.: Physical Climatology, 1941. side of the Sierras as data were not readily [3] Milham, W. I.: . MacMillan Co., 1913. available. [4] Talman, C. F.: A Book About the Weather. Corn- wall Press, 1931.

Unauthenticated | Downloaded 10/05/21 10:16 AM UTC VOL. 33, No. 2, FEBRUARY, 1952 47

[5] Climate and Man. Yearbook United States De- [13] Willett, H. C.: Descriptive Meteorology. Academic partment of Agriculture, 1941. Press, 1944. [6] Humphreys, W. J.: Physics of the Air. McGraw- [14] Byers, H. R.: General Meteorology. McGraw-Hill, Hill, 1929. 1944. [7] Shaw, Napier.: Manual of Meteorology, Vol. IV. [15] Ward, R. DeC.: The Climates of the United States. Cambridge Press, 1942. Ginn & Co., 1925. [8] Meteorological Glossary. Air Ministry, Meteoro- logical Office, 1930. [16] Ives, R. L.: "Frequency and Physical Effects of [9] Admiralty Weather Manual. 1938. Chinook Winds in the Colorado High Plains Re- [10] Gregg, W. R.: Aeronautical Meteorology. Ronald gion," Annals of the Association of American Press 1930. Geographers, Vol. XL, Dec. 1950, pp. 293-327. [11] Berry, F. A.,' Bollay, E., Beers, N. R.: Handbook of [17] Thiessen, A. H.: Weather Glossary. U. S. Weather Meteorology. McGraw-Hill, 1945. Bureau, 1946. [12] Blair, T. A.: Weather Elements. Prentice-Hall, [18] Hann, J.: Handbook of Climatology. MacMillan, 1942. 1903.

Michigan Department of Conservation, Ann Arbor, Mich., NEWS AND NOTES writes: "From records obtained on more than four thousand fishing trips scattered over every day of Michigan's 135- day trout season for two years, little evidence can be Fishing Weather? found in support of the predictions of various fishing Dr. George W. Bennett, Aquatic Biologist, State calendars or of predictions of various fishing calendars or Natural History Survey Division, Urbana, 111., writes: of predictions based on changes in barometric pressure. "A number of years ago Dr. David H. Thompson, . . . Fishing was about as good when the barometer was formerly with the Natural History Survey, obtained a falling as when the barometer was rising."—Michigan twelve-year creel census from a private fishing club Conservation, May-June 1951, p. 4. located near St. Louis. As the record was a complete James G. Sieh and John Parsons calculated the average tabulation of all the fish caught from the club lake, catches per hour on a rising, falling and steady barometer. Thompson believed it would be worth while to spend the "There is a suggestion of increased activity of the yellow bass during periods of a rising barometer and decreased necessary time to check the catch records against changes activity during periods of a falling barometer [and] the in barometric pressure as recorded at the St. Louis opposite . . . with the yellow perch . . . [but] the dif- Weather Station. After about six months of work, ferences were not statistically significant. Walleye and Thompson came to the conclusion that there was no bullhead activity appeared almost constant during the correlation between fish catches and highs, lows, or three periods of barometric tendency. changes in barometric pressures. Because he could show "An attempt was made to correlate wind direction, wind no correlation whatever, he never published a formal re- velocity, sky cover or cloudiness, and thundershowers or port of this work. During the twelve years in which rain with fish activity. The date indicated that none of the catch record was made, fishermen were not aware of these variables could be correlated with movement the barometric theory so were not influenced in their phenomena."—Excerpts from "Activity Patterns of some selection of fishing time by changes in the barometer." Clear Lake, Iowa, Fishes, "Iowa Acad, of Sci., v. 57, Dr. Edwin L. Cooper, Institute for Fisheries Research, 1950, pp. 515-516.—C. F. B.

Unauthenticated | Downloaded 10/05/21 10:16 AM UTC