Peter C. Sinclair on the rotation Institute of Atmospheric Physics The University of Arizona ot dust devils' Tucson, Arizona
Abstract the dust devil from below. Thus, a dust devil rotating A number of authors have, in the past, been of the clockwise when viewed from above will appear to rotate opinion that dust devil direction of rotation is con- counterclockwise when viewed from below. His observa- trolled by the earth's rotation. While this contention tions of thirty small (not more than 2 ft in height) can be easily attacked through theoretical arguments, "disturbances" (rotation indicated by the motion of actual observations become the deciding factor. The leaves and feathers) showed that "in no case was the observations presented, believed to be the largest col- rotation other than anticlockwise2 (i.e., cyclonic)." lection on record, show quite conclusively that dust While he did not specifically state that he believed the devils in general have no preferred direction of rotation. earth's rotation controlled the dust devil sense of rota- tion, it appears this was the implication. Kukuski (1952), on the other hand, appeals directly During the past four years, considerable research on to the effect of the earth's rotation, for he states (p. 53): dust devils has been done in the desert near Tucson, Arizona. An important part of this research involved "the main element on which the direction of rota- a dust devil census in which environmental quantities tion will depend is the earth's rotation. The motion such as wind velocity, pressure, temperature, and dust of air in a thermal current" (I assume he includes devil quantities such as size, duration, direction of rota- dust devils) "is the same as that in a cyclone." tion, etc., were continuously observed and recorded. In addition, Brooks (1960) has perpetuated this concept One of the purposes of the census was to determine by finding from 100 dust devil observations that the from as large a sample as possible the percentage fre- sense of rotation was always cyclonic and thereby states quency of dust devils that have cyclonic or anticyclonic unequivocally: sense of rotation. The results of the census concerning dust devil rotation agree with some but are in conflict "Thus, it is concluded that earth rotation strongly with those from a number of other published sources. affects spin direction in full-size dust devils occurring Flower (1936), Humphreys (1940), Williams (1948), over level ground" (broken only by scattered trees Ludlam and Scorer (1953), Lacaze (1958) and Walling- and low buildings) "on calm days." ton (1961) all indicate that dust devils may rotate cy- The observations of Brooks are especially interesting clonically or anticyclonically. Flower and Wallington from several points of view: (1) of the 100 dust devils also indicate that large dust devils have a preference for that Brooks observed, not one exhibited anticyclonic cyclonic rotation. However, only Flower and Williams rotation, (2) Brooks' data were taken from the same present data, so it is not known whether the conclusions general region from which the author's data were of the other authors have been influenced by these ear- taken, and (3) his results represent the latest contribu- lier observations or whether they possess data yet un- tion to the meteorological literature on the subject of published or are appealing solely to theoretical argu- dust devil rotation. ments. 2 The reader should note that there is some advantage to On the other hand, Durward (1931), Kukuski (1952), the use of cyclonic and anticyclonic rather than counterclock- and Brooks (1960) appear to have the impression that wise (or anticlockwise) and clockwise for sense of rotation. dust devils have a strong preference for cyclonic rota- The former are accepted meteorological terms and are the tion. Observations from lower Egypt and Iraq during most meaningful if dust devil rotation data from both hemi- spheres are to be considered. Note also there is no ambiguity the late twenties indicated no preference for dust devil due to the low pressure center of the dust devil, since the sense of rotation. Durward (1931) thought this was a terms cyclonic and anticyclonic refer to sense of rotation "surprising result," which he attributed to the possi- rather than to a particular pressure distribution (Glossary of bility that some observers were using a point of ref- Meteorology, R. E. Huschke, ed., 1959). The existence of a erence above the dust devil, while others were viewing region of ambiguity along the equator is considered a very minor drawback in terms of the number of dust devils ob- i Work supported by the Office of Naval Research. served in this region. 388 Vol. 46, No. 7, July 1965
Unauthenticated | Downloaded 09/28/21 07:09 AM UTC Bulletin A?nerican Meteorological Society On purely theoretical grounds, one would expect that ticular size, especially in the case of very large dust the effect of the earth's rotation would be at least one devils. Since there is usually a direct relationship be- or two orders of magnitude less than that of the shear tween dust devil height and diameter, one might use or curvature terms in the generation of vorticity (Wil- either one or both to distinguish a large or small dust liams, 1948). There is no question as to the validity of devil. Table 2 presents these data as a composite of this analysis near and within the dust devil. Pressure Flower (1936), CDOP, Sinclair, and Williams (1948) measurements by Sinclair (1965) have shown that one data. In order to produce this composite tabulation, can expect a total pressure drop of 2 to 3 mb from the Flower's class intervals were employed. It is easily seen immediate environment (i.e., within a radial distance that, if diameter is used to distinguish a large or small of 10 to 100 m) to the center of the dust devil. Hence, dust devil, there appears to be a preference for cyclonic an order of magnitude analysis using the equation of rotation for the largest and smallest dust devils. For motion shows that near and within the dust devil the intermediate diameters, there appears to be no signifi- Coriolis acceleration may be 3 to 5 orders of magnitude cant preference for direction of rotation. The chi-square smaller than the accelerations produced by the pressure test indicates these features to be statistically significant gradient force. At large distances from the visible dust at the 5% level. On the other hand, if height is used as column these two analyses still show the effect of the a criterion for dust devil size, the data show that there earth's rotation to be insignificant by orders of magni- is a slight preference for anticyclonic rotation for tall tude. However, since no one has made measurements of dust devils and considerable preference for cyclonic ro- wind and pressure distributions over the entire region tation for short ones. For all heights between 20 ft and in which the circulation, that later leads to the visible 2000 ft, there is in general little preference for either dust devil, first develops, it would appear that the di- direction of rotation. While the chi-square test shows vergent opinion which has appeared in the literature these features to be statistically significant at the 5% concerning the importance of the earth's rotation on level, the significance of the test is not dependent on dust devil direction of rotation may be resolved by pre- the slight preference for anticyclonic rotation of tall or senting direct observational evidence as the ultimate large dust devils. Hence, from a statistical point of view, deciding factor. these data indicate that small (in terms of height or Table 1 contains data from five independent sources diameter) dust devils have a preference for cyclonic rota- of dust devil sense of rotation and constitutes the largest tion and large (in terms of diameter) have a preference collection of data known to the author (the questionable for cyclonic rotation. However, it seems reasonable to data of Brooks and Durward are clearly evident). The expect that, as more data are collected on large and data labelled CDOP (Cooperative Dust Devil Observa- small dust devils, the frequency distribution of either tion Program) were gathered by Institute of Atmospheric height or diameter would not be significantly depend- Physics personnel and other interested persons in the ent on sense of rotation and would show that dust devils Tucson area during 1960-63. The McDonald (1960) data of all sizes have, in general, no preferred direction of were collected by Professor James E. McDonald in the rotation. southwestern United States during 1960 and were ob- It is interesting to note also that other small-scale vor- tained by private communication. Flower (1936) made tices appear to have no preferred direction of rotation. his observations in Egypt and Iraq; Williams (1948) ex- Hissong (1926) cites some observations of very violent amined dust devils near Inyokern, California. Sinclair whirlwinds which formed over or at the edge of a large data were obtained by the author while making dust oil reservoir fire. Whirlwinds with cyclonic as well as devil measurements in the desert near Tucson during anticyclonic rotation were observed. In addition, Thor- 1960-62. Collectively, the data show rather conclusively arinsson and Vonnegut (1964) have observed numerous (even including the questionable data of Brooks and whirlwinds produced by the eruption of the Surtsey vol- Durward) that there appears to be no preferred direc- cano. They also found that the whirlwinds rotated cy- tion of rotation and hence factors other than the effect cyonically as well as anticyclonically. of the earth's rotation (such as terrain variations) are As pointed out previously, Brooks' data are especially responsible for observed dust devil spin directions. interesting in light of the above results, since his ob- The above conclusion is with reference to dust devils servations of 100 dust devils, all of which rotated cy- of all sizes. There exists, however, the possibility of a clonically, were taken in the same type of terrain and preferred direction of rotation for dust devils of a par- weather conditions as were the CDOP and Sinclair
TABLE 1. Observations of dust devil sense of rotation.
Direction of Flower McDonald Williams Brooks Durward Rotation (1936) CDOP (1960) Sinclair (1948) (1960) (1931) Total Cyclonic 199 53 9 60 9 100 30 460 Anticyclonic 175 35 29 84 12 0 0 365 Total 374 88 38 144 21 100 30 825
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Unauthenticated | Downloaded 09/28/21 07:09 AM UTC Vol. 46, No. 7, July 1965 data. Also, Brooks' arbitrarily chosen minimum dust- direction of rotation when the dust column is not dense devil size (i.e., diameter at least 5 ft near the ground enough to obscure the far side or when the observer is and a visible height of at least 100 ft) falls well within simply too far away for adequate observation. Although the range of values in Table 2 or the individual data Brooks' claim that "an observer's binocular depth- of CDOP or Sinclair. One must, therefore, conclude perception is unreliable beyond a few yards for detect- that Brooks' observations are either inaccurate or are ing the direction of spin in dust devils" seems overly so biased that nothing significant can be deduced from conservative, the possibility for error in determining his data. Even if one were to argue that there may be direction of rotation is real and must be considered in a peculiar wind field which always produces cyclonic all observations. Direct knowledge of this error in the dust devils in the region observed by Brooks, it is, of data presented here is unknown except in Sinclair's course, still not correct, in view of the large number data, where the observations were made by two or more of observations presented above, to conclude, as Brooks experienced dust-devil observers. Only those observa- has done, that "earth rotation strongly affects spin di- tions in which all of the observers agreed on the direc- rection in full-size dust devils occurring over level tion of rotation were recorded. In all cases, the dust ground on calm days." devil was pursued on foot or by vehicle for the closest One important fact brought out by Brooks concerns possible point of observation. This usually resulted in the ability of an observer to accurately distinguish the being able to observe the dust's motion on the near
TABLE 2. Dust devil frequency with respect to height, diameter and direction of rotation. Figures in parentheses are per cent of total dust devils in that interval. For example, there were a total of 62 dust devils with diameters smaller than 5 feet. Of this total, 37 or 59.7% had cyclonic rotation.
Height (ft) Diameter (ft) Height <5 5-10 11-20 21-40 41-80 81-150 151-300 301-500 501- 1001- 2001- Un- Total %in 1000 2000 4000 known Interval Dust devils with cyclonic rotation < 5 10 13 6 2 2 1 1 _ _ _ _ 2 37 (59.7) 5- 10 2 9 13 7 8 8 8 2 2 — — 9 68 (59.6) 11- 20 2 3 2 3 5 6 7 5 4 — — 3 40 (48.2) 21- 40 — — 6 5 5 8 3 3 3 3 2 3 41 (48.2) 41- 80 — — — 1 1 6 7 1 2 5 — 2 24 (42.1) 81-150 — — — — 2 2 4 5 7 2 3 2 28 (62.2) > 150 — — — — 1 1 2 1 1 — 2 8 (80.0) Total 14 25 27 18 23 32 31 18 19 11 5 23 246 % in interval (82.4) (73.5) (65.9) (48.6) (40.4) (49.2) (45.6) (51.4) (50.0) (52.4) (41.7) (74.2) (53.9) Dust devils with anticyclonic rotation < 5 2 7 6 4 1 2 1 2 25 (40.3) 5- 10 1 1 6 9 12 7 5 1 2 — — 2 46 (40.4) 11- 20 — 1 2 3 13 9 6 2 3 3 — 1 43 (51.8) 21- 40 — — — 2 6 12 8 3 8 1 2 2 44 (51.8) 41- 80 — — — 1 2 3 14 3 1 5 4 — 33 (57.9) 81-150 — — — — — — 3 7 5 1 1 — 17 (37.8) > 150 — — — — — — — 1 — — — 1 2 (20.0) Total 3 9 14 19 34 33 37 17 19 10 7 8 210 % in interval (17.6) (26.5) (34.1) (51.4) (59.6) (50.8) (54.4) (48.6) (50.0) (47.6) (58.3) (25.8) (46.1) All dust devils < 5 12 20 12 6 3 3 2 4 62 5- 10 3 10 19 16 20 15 13 3 4 — — 11 114 11- 20 2 4 4 6 18 15 13 7 7 3 — 4 83 21- 40 — — 6 7 11 20 11 6 11 4 4 5 85 41- 80 — — — 1 3 9 21 4 3 10 4 2 57 81-150 — — — 1 2 2 7 12 12 3 4 2 45 > 150 1 1 3 1 1 — 3 10 Total 17 34 41 37 57 65 68 35 38 21 12 31 456
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FIG. 2. Mobile instrumented tower with microbarophone for dust devil pressure measurements.
or height) will not be significantly dependent on sense of rotation.
References FIG. 1. Dust devil observed against terrain by Sinclair Brooks, H. B., 1960: Rotation of dust devils. J. Meteor., 17, in Arizona. 84-86. Durward, J., 1931: Rotation of 'dust devils.' Nature, 128, 412. Flower, W. D., 1936: Sand devils. London, Meteor. Office, side with the ground as a partial background (see Fig. Professional Notes, 5, No. 71, 1-16. 1) and/or to observe the motion of objects as large as Hissong, J. E., 1926: Whirlwinds at oil tank fire, San Luis cardboard boxes from a short distance. In addition, a Obispo, California. Mon. Wea. Rev., 54, 161-163. number of dust devil penetrations were made with an Humphreys, W. J., 1940: Physics of the Air. 3rd ed., New instrumented tower (see Fig. 2) from which the direc- York, McGraw-Hill, p. 154. tion of rotation was easily determined from the recorded Kukuski, J., 1952: Theory and Technique of Soaring. New wind data, which was also cross-checked with visual ob- York, Pitman and Sons, p. 53. servations from three different observers. In all cases, Lacaze, J., 1958: Tourbillons atmospheriques d'axes verti- these observations showed that for all dust devil sizes, caux. J. Scientifique de la Meteorologie, 40, 133-147. large or small, there appeared to be no preferred direc- Ludlam, F. H., and R. S. Scorer, 1953: Convection in the tion of rotation. atmosphere. Quart. J. R. Meteor. Soc., 79, 317-341. Sinclair, P. C., 1965: A microbarophone for dust devil pres- It is hoped that the results presented here will elim- sure measurements. J. Appl. Meteor., 4, 116-121. inate the confusion that exists in the literature concern- Thorarinsson, S., and R. Vonnegut, 1964: Whirlwinds pro- ing dust devil direction of rotation. Data from a num- duced by the eruption of Surtsey Volcano. Bull. Amer. ber of independent sources collectively show that dust Meteor. Soc., 45, 440-444. devils have no preferred direction of rotation. Only for Wallington, C. E., 1961: Meteorology for Glider Pilots. New the very largest or smallest dust devils (Table 2) might York, Pitman, p. 159. this conclusion be questioned. However, it is believed Williams, N. R., 1948: Development of dust whirls and simi- that, as more data are collected, the entire frequency lar small-scale vortices. Bull. Amer. Meteor. Soc., 29, 106- distribution of dust devil size (with respect to diameter 117.
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