S. B. Idso, R. S. Ingram, and J. M. Pritchard an american haboob U.S. Water Conservation Laboratory and National Weather Service, Phoenix, Ariz. 85040 1. Introduction by thunder and/or rain after a time interval varying up One of the world's most awesome displays of blowing to 2 hr. dust and sand is the legendary "haboob" of Khartoum in Satellite photographs have shown that the squall line the Sudan. Raised by strong winds often generated storms that develop south and east of Tucson appear by the outflow of rain-cooled air from a cumulonimbus to originate from large dense masses of clouds over the cloud, the leading edges of these storms take on the Sierra Madre Occidental of northern Sonora, Mexico. appearance of "solid walls" of dust that conform to These cloud masses over Sonora seem to form some the shape of a density current head and rise on the days rather explosively in the deep semi-tropical air average to between 1000 and 2000 meters (Sutton, 1925; mass. During this season of the year, the Bermuda High Lawson, 1971). The average speed of advance of the often extends westward into eastern Arizona, and during Khartoum haboobs is 32 mph, with the greatest speed the afternoon some of the Mexican activity has been seen being about 45 mph (Sutton, 1925). Maximum duration to move northwestward, steered by variations in the approaches 6-1/2 hr with a peak between 30 min and easterly flow (possibly easterly waves) on the bottom side one hour, the average duration being about 3 hr (Free- of the lobe of the Bermuda High. man, 1952). During the passage of an haboob the air On radar, squall lines of the Sonora type begin to pressure may fluctuate considerably, or sometimes only appear generally just after noon as isolated cells over little. With its arrival, however, there is invariably a Santa Cruz and southern Cochise counties, and move fall in temperature and rise in relative humidity. In severe cases, Lawson reports that the temperature drop may be as great as 15C. In the arid southwestern United States, dust storms of a very similar nature often occur. Although much less frequent than the Sudanese haboobs (2 or 3 per yr at Phoenix as compared to about 24 at Khartoum), they are equally as dramatic. Most of them are caused by downdrafts from thunderstorms which develop over the mountain complex straddling the Cochise and Pima County boundary and the Mogollon Rim. (see Fig. 1). The air mass thunderstorm activity seems to be gener- ated in the convergence of moist, semi-tropical air raised by up-canyon winds on east-facing slopes in the late morning and the up-canyon winds on highly heated west-facing slopes during the afternoon. Those that develop southeast of Tucson usually move unimpeded up the Santa Cruz Valley as an organized squall line. This type generally arrives at Phoenix between 1700 and 2100, while the rest arrive mostly between 1500 and 1700. Two out of three of the dust storms are followed i Joint contribution from the Soil and Water Conservation Research Division, Agricultural Research Service, U.S. De- FIG. 1. Map of southern Arizona showing the primary partment of Agriculture, and the National Weather Service, geographical features discussed in the general characteriza- National Oceanic and Atmospheric Administration, U.S. De- tion of Arizona dust storms. Mountainous areas are desig- partment of Commerce. nated by xxxx. 930 Vol. 53, No. 10, October 1972 Unauthenticated | Downloaded 09/26/21 02:11 PM UTC Bulletin American Meteorological Society slowly northwestward. During the afternoon new cells continuing on towards Phoenix. From reports of its develop and generally form an arching squall line near arrival time at several sites along its path, its mean speed Tucson. This line, now composed of multiple cells, some- of advance was calculated to be 29 mph. times continues to intensify as it moves up the Santa General characteristics of the storm are reported in Cruz Valley towards Phoenix. The line itself with cloud Table 1 for the several sites where data were available. tops in excess of 40,000 ft often extends 100 mi or more The mean windspeed more than quadrupled upon the as it advances with surface dust clouds spreading out storm's arrival from 7 to 33 mph and matched well with ahead of it. Each moving cell probably contributes its its computed speed of advance. In addition, maximum own downdraft pattern, but the compactness of the cells gusts observed averaged 95% greater than the frontal in the line often causes these macro-lobes to merge in advance speed of 29 mph, in excellent agreement with what appears to be a solid wall of dust, reported by air- the general observation of Lawson (1971) that an craft to extend upward to 8000 ft. haboob's speed of advance is approximately half the The average time that these squall lines have reached maximum windspeed behind its front. Tucson is about 1600, and they are attended by strong Relative humidity jumped from 33 to 74% with the gusty south to southeast winds and pressure jumps. It passage of the front, and the temperature dropped over takes them about 3-1/2 hr to then travel up the Santa 13C, indicative of a quite severe haboob. Indeed, the Cruz Valley to Phoenix at an average speed of 30 mph. average time required for these humidity and tempera- When the dust storm arrives, the surface air tem- ture changes as derived from liygrothermograph records perature drops about 7C, on the average, although was only 7 min. To give some idea of how these changes some drops have been as small as 3C and others have compare with those of a normal day, Fig. 2 contains been as great as 13C. Average maximum wind velocity plots of humidity and temperature for the north within the dust storm is 48 mph, but has been recorded Phoenix station for the day of the storm plus the as high as 72 mpli. Minimum visibility averages about preceding day. Also included is a barograph trace from 1/4 mi and usually takes about 1 hr to return to 6 mi. Luke Air Force Base, about 12 mi further on in the However, this return has been as short at 12 min and as direction of the haboob's advance. long as 3 hr. Due to the large number of observations available in the Phoenix area, a rather detailed picture of the storm's 2. The Arizona haboob of 16 July 1971 leading edge was able to be constructed, as shown in On 16 July 1971, a sand and dust storm displaying Fig. 3. These frontal outlines were developed primarily classic haboob characteristics formed southeast of Tucson from four sets of photographs taken from sites 8, 16, 17, and began moving northwestward. It passed through and 18 in the directions of the arrows emanating from Tucson at 1530 and moved into the Santa Cruz Valley, those points. The first, from site No. 16, is a single TABLE 1. General characteristics of the Arizona haboob of 16 July 1971. Wind (mph) Temperature (°C) Rel. Humidity (%) Time of Station No. Arrival Before After Max. Before After Before After Tucson Int. Airport 1 6 55 81 34.4 20.6 33 81 1530 Williams A. F. Base 2 10 38 46 38.5 28.5 22 56 1810 Sky Harbor Int. Apt. 3 11 34 52 40.5 29.4 19 53 1840 Luke Air Force Base 4 16 32 46 39.0 26.7 22 70 1925 Gen. Motors Prvng. Grnd. 5 37.8 27.8 46 74 U.S. Water Cons. Lab. 6 35.0 23.5 34 63 U of A Citrus Exp. Sta. 7 37.8 25.6 27 78 Mummy Mtn. Observ. 8 40.0 26.0 23 48 1900 White Tank Mountains 9 32.2 17.2 2000 Flushing Meadows Sta. 10 39.0 23.5 28 60 1900 Maricopa Co. Health D 11 6 32 41.0 26.7 58 100 Scottsdale 12 2 22 39.0 24.4 40 93 N. Phoenix 13 2 33 41.0 25.6 30 78 1900 S. Phoenix 14 4 28 41.0 26.7 34 80 W. Phoenix 15 8 23 39.0 24.4 47 100 1900 Pritchard Photo 16 Ten Harkel Photo 17 Gilbert Photo 18 Average 7 33 56 38.3 25.1 33 74 931 Unauthenticated | Downloaded 09/26/21 02:11 PM UTC Vol. 53, No. 10,, October 1972 FIG. Air temperature and relative humidity for 15 and 16 July 1971 at the north Phoenix station plus the similar record of air pressure at Luke Air Force Base. photograph, reproduced in Fig. 4,* showing the leading undercutting a less dense fluid. In part C, where the edges of two distinct macro-lobes of the cold air out- two parts A and B are superimposed, it is observed that flow churning up the surface soil and dust. The second the surface dust cloud definitely conforms to the shape picture, shown in Fig. 5,# is a panorama of the central of the density currents so characteristic of the Sudan macro-lobe in the foreground of Fig. 4„ composed of five haboobs. separate photographs taken at site No. 17. From this unique combination of photographs, it was The third picture, shown in Fig. 6,* is a side view of a possible to determine that the leading edge of the macro-lobe (in this case the northeastern one), being surface-generated dust cloud was not really shaped as taken almost perpendicular to the local path of advance one large arc, but rather that it consisted of an arc of of that specific lobe against a partially clear sky from several associated macro-lobes of which at least three site No.