240 BULLETIN AMERICAN METEOROLOGICAL SOCIETY

Some Dust Storm Conditions of the Southern High Plains

G. FREDERICK WARN

Texas Technological College, Lubbock, Texas

ABSTRACT

This paper summarizes some of the weather conditions which cause blowing dust in the Southern High Plains of Texas. These conditions include diurnal winds, whirlwinds and thunder- storms. A tentative classification of dust storms in this area is given in table form. The terms dust lift, dust swath, and dust ring are introduced.

INTRODUCTION anticyclonic winds. Only the first four of these are discussed in this paper. The following material is based on dust-storm observations and records made by the writer in The following classification (TABLE I) is pro- 1949 and 1950. The Southern High Plains in- posed tentatively for the various types of blowing clude part of the western two-thirds of the Texas dust which occur on the Southern High Plains, Panhandle and adjacent New Mexico, and the area based on the concept that they all fall in to a con- southward from the Panhandle almost to the tinuous series representing various degrees of in- Pecos River. This plains area is nearly flat, of- tensity. Such a scale might be useful for weather fering few topographic obstructions to the winds. observers in recording these phenomena. Much of the land is cultivated and planted to cot- ton and small grains, chiefly sorghums. Eleva- DIURNAL WINDS tion of the plains is approximately 3,200 feet above mean sea level. The climate is transitional Diurnal winds cause blowing dust of light in- between semiarid and humid, with an average an- tensity, especially during the dry months of late nual rainfall of about 19 inches during the past fall, winter and early spring. Dust and sand 39 years (Frost, 1950). The mean daily wind movement is usually in the form of traction (salta- travel at Lubbock is 149 or a speed of 6.2 mph.1 tion) and limited suspension. In traction, the par- Blowing dust is governed mainly by three factors: ticles bounce and skip along the surface in a form 1. Wind direction and speed. 2. Condition of the of mass movement. Traction movement is com- ground. 3. Precipitation. monly limited from two inches to three feet above The dust-blows of 1950 followed a year with the surface, with dust and sand particles traveling above normal precipitation of 29.36 inches. How- in isolated, wandering sheets and narrow streams. ever, much of the ground was dry and unprotected Local, sporadic dust movement, at heights of two by vegetation because of subnormal rainfall dur- inches to 15 feet above a dry surface, is caused by ing the period of October 1949 through April 1950. Total rainfall at Lubbock during these six months horizontal winds of 14 to 18 mph. Occasional ed- was 1.34 inches with only a trace recorded in dies and small whirls having 25 to 35 mph wind March. speed carry localized puffs and small clouds of Laboratory examination -of dust samples indi- dust to a maximum height of 75 feet. Gusts of 35 cated that the dust is composed of clay, silt and fine to 40 mph carry suspended dust to maximum sand. Most of the particles consisted of fine sand heights of 150 to 250 feet. Sustained winds of 25 1/16 to 1/8 mm in diameter. Clay particles (less to 30 mph are capable of holding dust in continu- than 1/256 mm diameter) made up from 25 to 40 ous suspension to a maximum height of 2,000 to percent of the individual samples (Warn and Cox, 5,000 feet.* Suspension tendencies are especially 1951). responsive to variations in stability and turbulence

DUST-STORM TYPES of the air. The amount of dust transported by diurnal winds is not appreciable unless considered Blowing dust is caused by diurnal winds, sur- over a long period of years. face eddies, whirlwinds (dust devils), thunder- storms, frontal winds, cyclonic winds, and some * Dust and cloud heights cited in this article are esti- mates, about one-third of which were corroborated by 1 Texas Agricultural Experiment Station Records, 1914— pilot reports; the heights are believed to be accurate 1950, Lubbock, Texas. within 500 feet up to 12,000 feet.

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TABLE I. CLASSIFICATION OF BLOWING DUST 2

Maximum Maximum Type Usual horizontal height of dust Horizontal No. Intensity Duration wind speed (ft. above visibility Effects of dust movement (mph) ground) (miles)

1 Light 5-30 min. 14-24 500 ft. 8-15 Sporadic blows of dust and sand at or near ground level. Occasional light yellowish haze of dust to 500 ft. Traction movement pre- dominant.

3 Mild §-3 hrs. 25-34 5000 ft. 2-7 Light, yellowish brown haze. Fine dust in con- tinuous suspension to 2,000-5,000 ft. Sunlight bluish gray. Blowing dust and sand to 75 ft.

5 Moderate 1-6 hrs. 35-44 12,000 ft. £-2 Dense, brownish haze. Occasional dust cloud effect. Dust in suspension to 8,000-12,000 ft. Lights and sun dimmed. Sky obscured.

7 Strong 3-12 hrs. 45-54 18,000 ft. i-i Dust usually approaches as a dark, churning wall. Sun barely visible as a weak blue light. Ground and air travel impeded.

9 Severe 6-24 hrs. 55-65 30,000 ft. o-} May approach as a dense, dark cloud on frontal winds or build up gradually to intense dust cover. Sun usually obscured. Infrequent.

2 Based on 110 observations of dust movement by wind during the period of study, 1949-50.

WHIRLWINDS The vortices of pre-thunderstorm clouds, in the cumulus congestus stage, create a pattern of con- Whirlwinds or "dust devils" are numerous on vergent winds with velocities of five to 20 mph. the Southern Plains during the late summer, fall As the storms approach maximum vertical de- and early spring. The observed whirlwinds va- velopment, updraft velocities of 30 to 45 mph are ried from one to 50 feet in basal diameter with possible.3 Surface eddies and small whirlwinds a few attaining a maximum width of 150 feet. loosen surface dust which is then caught by the The estimated range in height was from 25 to updrafts. The dust in the updrafts was observed 450 feet, with no consistent ratio between diameter and height. Velocities in the dust spirals were to rise to a maximum height of about 9,000 feet. estimated at 6 to 10 Beaufort. The rate of hori- These dust-laden updrafts, feeding into the vortices zontal movement was five to 30 mph. Very tur- of thunderstorms, are here termed dust lifts. Dust bulent weather in late winter and early spring lifts are frequent also in the turbulent updrafts of months of 1950 accounted for an unusual num- the shear zones in the front edges of thunder- ber of dust whirls. The maximum number ob- storms. Multiple updrafts, as many as six, were served on one occasion within a radius of 15 observed in isolated storms. miles was 16. Maximum duration of a single Strong downdrafts of chilled air develop as whirlwind was 20 minutes. Rows of whirlwinds internal precipitation begins in the thunderstorms. developed along the leading edges of several steep Downdrafts within thunderstorm clouds have been cold fronts. measured at 40-65 feet per second (Hilst and MacDowell, 1950; Byers and Braham, 1948). THUNDERSTORMS In the study area, downdrafts beneath the cloud Blowing dust is frequently caused by winds and mass attain sustained velocities of 25 to 55 mph. currents associated with air mass and frontal-type Upon striking dry, barren ground, the currents thunderstorms, many of which produce little or churn a swath of dust which rolls outward and no precipitation in the Plains area. Such "dusters" upward from either side of the line of impact. are of light to mild intensity, a few producing The dust swaths are initially 25 to 75 yards wide moderate dust storms. Nearly all of these storms and tend to lengthen in the direction of thunder- are of less than two hours duration. Dusters re- storm progression. sulting from large, isolated convective storms sel- 3 Large cumulonimbus clouds with vertical development dom last more than one-half hour. of 45,000 feet were numerous in the area during 1950.

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Downdrafts along the edges of the thunder- carried into vortices. The upper extremity of storm create cold, turbulent squall winds with each vortex, viewed from the side of the storm, velocities of similar range. These horizontal winds was clearly marked by the sudden appearance of under ride convectional (thermal) updrafts and a scud-like puff of cloud above the cirrus nothus cause numerous eddies and whirls. The dust layer. Dust rings developed along the storm stirred by these currents is carried in rising, arc- edges with the rapid lateral spreading of the like clouds, or rolls, away from the storm edges. storms. The complex result presented a picture The dust clouds were observed to reach maximum of whirlwinds, dust lifts, swaths and rings. The heights of 5,000 feet and to endure a maximum of outer edges of the upper parts of the storm cloud 30 minutes. These arc-like, rolling clouds of dust, produced a spreading, turbulent cover of cumulus spawned by thunderstorm' squall winds, are here mammatus, altocumulus and scud above the dust. termed dust rings. A dozen or more vortices were visible from the Large, isolated thunderstorm lines formed a underside. Several thunderstorm lines moved number of times in the area during the summer eastward as squall lines. Thunderstorm lines pro- months, usually along the western, eastern, or duced blowing dust of light to moderate intensity southeastern borders of the Southern Plains. Most with a duration of one-half to two and one-half of these lines contained from three to eight storm hours. centers and moved from southwest to northeast Frontal or squall-line thunderstorms are most with huge sheets of cumulus mammatus and cirrus frequent in the Southern Plains during late spring nothus (FIG. 1). When the storm lines reached and early summer, with sometimes a secondary maximum development, as many as 12 dust lifts recurrence in late summer or early fall. These were observed in simultaneous operation. Dust storms occur usually when a southeasterly circula- swaths developed suddenly beneath the southwest tion of mT air from the Gulf of Mexico enters a third of the storm lines and traveled eastward or trough between a waning cP anticyclone to the northeastward the entire length of the lines. Much east or east-southeast, and an mP or cP anti- of the dust was caught by adjacent updrafts and cyclone to the west. The importation of cold air

FIGURE 1. Blowing Dust Associated with a Thunderstorm Line. The individual storms begin on the left and move to the right with the upper winds in a broad, turbulent sheaf of cumulus mammatus and cirrus nothus. Numbers 1 through 7 indicate successive storm centers, with the current cumulus build-up feeding into No. 5 from the rear, or from an off-set position. In the final stages, the entire storm line spreads rapidly in a lateral and downwind direction with dust swaths and rings mingling in a single turbulent dust cloud beneath. The dust spreads laterally and progresses in a downwind direction as squall winds become dominant.

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from the west or northwest develops a rapidly light hail, occur during the maximum develop- moving cold front. The sudden lift of mT air ment of the dust cloud. Precipitation ranges from initiates a squall line 50 to 300 miles in advance light to excessive, with 0.01-0.02 inch usually of the front. Turbulence is especially noticeable sufficient to cause, first, a "muddy rain," and then when the cold fronts are steep or when they appear complete settling of the dust. aloft and trap mT air. A complex of dust whirls and dust lifts occurs along the pre-frontal squall REFERENCES line. Squall-line winds and eddies attain sustained velocities of 25 to 60 mph with gusts to 75 mph. Byers, Horace R., and Braham, Roscoe R., Thunder- storm structure and circulation. Jour. Meteor., vol. If no precipitation occurs in the early stages of 5, no. 3, pp. 71-86, 1948. squall development, the stronger fronts pass in the Frost, Harold L., Local Climatological Summary with form of a dark reddish brown, churning wall of Comparative Data, 1949, Lubbock, Texas. U. S. dust. This is one variety of the so-called "black Wea. Bur., Ft. Worth, Texas, 1950. Hilst, G. R., and MacDowell, G. P., Radar measurements duster" of the Southern Plains. Visibility fre- of the initial growth of thunderstorm precipitation quently drops to a minimum of two miles to zero cells. Bull. Amer. Meteor. Soc., vol. 31, no. 2, pp. within a period of 20 minutes. Height of the dust 95-99, 1950. cloud ranges from 4,000 to 8,000 feet, and quickly Warn, G. Frederick, and Cox, Wm. B., A sedimentary- merges with the overlying turbulent cloud cover. study of dust storms in the vicinity of Lubbock, Texas. Amer. Jour. Sci., vol. 249, pp. 553-568, Erratic sprinkles and rain squalls, with occasional 1951.

results seem to be an improvement over the original ones, CORRESPONDENCE since they are much more closely in agreement with ac- tual observed accelerations. The other results of the paper do not appear to be affected by the above discussion.— June 28, 1952. Northward Acceleration of Typhoons REFERENCES GEORGE P. CRESSMAN [1] Cressman, G. P., 1951: The Development and Motion Hq., Air Weather Service, Washington, D. C. of Typhoon "Doris," 1950. Bull. Amer. Meteo. Soc., 32 (9), 326-333. [2] Long, R. R., 1951: A Theoretical and Experimental In our study of Typhoon "Doris" [1] some work of Study of the Motion and Stability of Certain At- Rossby [3] was quoted concerning the northward force mospheric Vortices. J. Meteor8 (4), 207-221. on a vortex. In that article Rossby gave the total north- [3] Rossby, C. G., 1948: On Displacements and Intensity ward force on a vortex as the resultant of the Coriolis Changes of Atmospheric Vortices. J. Marine Res., VII: 175-187. forces acting on it. I have since received a letter from [4] Rossby, C. G., 1949: On a Mechanism for the Re- Dr. N. A. Phillips of the Institute of Advanced Study lease of Potential Energy in the Atmosphere. /. at Princeton pointing out that this result is not exactly Meteor., 6 (3), 163-180. correct, since there is a net pressure force acting on a [5] Philips, N. A., 1951: Letter to the editor. Tellus, 3 vortex which is very nearly equal to one-half the resultant (4), 301. of the Coriolis forces, and oppositely directed. Refer- ences to this may be found from Long [2], and indirectly The Radio Location of Thunderstorms from a paper by Rossby [4]. According to some unpub- E. T. PIERCE lished work by Phillips this is true even before northward motion of the vortex begins. This has since been pointed Cavendish Laboratory, University of Cambridge, by Phillips in [5]. Cambridge, England Since publication of the study of "Doris," Dr. H. Riehl The problem, discussed in a BULLETIN article [1], of gave a lecture at the Washington, D. C. branch of the locating thunderstorms by examining the waveforms of AMS, based on a study by Mr. L. Hughes of the Univer- the associated atmospherics, has been recently studied sity of Chicago. This work showed that the low level [2] at the University of Cambridge. The object has circulation of typhoons is much more symmetrical than been to derive the distance D by waveform analysis, and, had previously been supposed and seems to encourage with a single direction-finding station, establish the posi- the application of numerical computations. tion of the storm. The observations have covered day In view of these facts it seems reasonable to say that and night conditions and distances of up to 4000 km. as a first approximation the numerical results of the com- During the leader stages of lightning discharges the putations on p. 332 of [1] should be divided by two. The waveforms produced consist of irregular, high-frequency resulting accelerations would then be for examples 1, 2, impulses, and are unsuitable for analysis. Waveforms and 3, respectively, 2, and 8^ knots per day. These (Continued on page 246)

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