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Oil Haboobs in the Egyptian Sudan W. H. SCHEMPFf HE PRESSURE distribution over especially during the transitional the Sudan undergoes marked period that Khartoum experiences the T seasonal changes. The position most severe "haboob". of the thermal equator as it moves Dust and sand storms are very com- Northward during the Northern Hemi- mon in many parts of the world and sphere summer greatly influences the their occurrence is determined first pressure distribution, and the com- of all by the availability of suitable bined effect produces distinct changes loose material on the ground. Bagnold2 in the upper air currents. This an- makes a definite distinction between nual variation is best described by sand and dust storms in which he L. J. Sutton:1 says "a sand storm proper is a low "In the winter the pressure over cloud of flying sand rarely higher middle Egypt is high, while the Equa- above the ground than two meters, torial Belt is a region of low pressure, whereas a dust storm is the result of and a current of air from between North and Northwest blows from mid- carrying up of particles of much dle Egypt almost continuously to the smaller size than sand grains". A Southern Sudan. The center of the sample of dust in the "haboob" 50 feet Sudan low-pressure system begins to move Northward and by April has above the ground at Khartoum con- reached almost to Khartoum, where tained mainly quartz crystals plus the winds are still North to Northeast. humus, vegetable matter, clay, and The depression then moves toward other crystals of from 0.01 to 0.07 Arabia and from June to September, while it is East of the Red Sea, the millimeters in diameter. "Haboobs" Southwesterly wind following in its are most common at Khartoum, Kas- rear sweeps over the Sudan beyond sala, and Tokar which are all centers Khartoum as far North as Latitude of population and soil cultivation. 20 degrees." Kassala and Tokar border a seasonal It is then that conditions are dis- river while at Khartoum two great turbed, thunderstorms and "haboobs" rivers, Bahr el Arraq and Bahr el occur, and Khartoum experiences its Abyad, join to form the Nile. These short rainy season. Toward the end factors plus their close proximity to< of September the depression begins desert regions form the locale for to return and is approximately in the "haboobs". same position in October as it was in Aside from dust content, the prin- May. October and May are transi- cipal characteristic of the "haboob" tional months at Khartoum and dur- is its association with thundery con- ing them the wind may blow from ditions and the consequent large in- either Southwest or North and there creases of gustiness. There are un- is sometimes rain. As the depression doubtedly many factors which con- retraces its path toward the Equator tribute to the formation of "haboobs". the rain-bearing Southwesterly winds First they occur mainly during the are gradually replaced by the dry rainy season and during days present- Northerlies, and from November to ing rainy conditions. Sixty pilot-bal- April the wind at Khartoum blows loon ascents made at Bahrdar Georges with scarcely any variation from be- near Lake Tana from April, 1923, to tween North and Northeast. It is March 1924 indicated that on rainy fWritten when the author was an aviation days the winds at 3,800 meters above cadet in the meteorology course at New York University, as a theme in an essay course. sea level were easterly with a south- Unauthenticated | Downloaded 09/26/21 03:05 PM UTC FIGURE 1. Sounding at Khartoum, April 20, 1936, on pseudo-adiabatic chart. The heights are in decameters, relative and specific humidities are entered at right of each of the points of the dry-bulb curve (solid) and the wet-bulb curve is to the left (dashed). Temperatures in °C, pressures in mb. erly component, while on dry days toum and Bahrdar Georges, on dry they were easterly with a northerly days the surface has a smaller slope component. At Khartoum during the and is associated with subsidence and rainy seasons of 1926 to 1928 rainy some mixing at the boundary of the days showed a sharp discontinuity air mass, while there is a greater in wind direction from SW to NE slope on wet days. This deduction is between 2,000 to 3,000 meters and on purely hypothetical and is apparently dry days a gradual veer through of no forecast value. that region. J. S. Farquharson3 has Another facjtor which has been offered, as one solution of the problem, investigated is the lapse rate. Super- the possibility of the presence of a adiabatic lapse rates have been regu- permanent stationary frontal system larly observed, at least near the sur- in the Sudan low-pressure area. Ac- face, in soundings during August cording to the Margules formula and September at Khartoum. Meas- this front "will be of a type ured differences between maximum such that if its slope is increased, thermometers at the surface and in a thermal instability results in the Stevenson screen 4i feet higher lower air mass and conversely stabil- amounted to as much as 10 to 35F°. ity". Or in other words, according to It is probable that the buoyancy of the the previous data stated from Khar- parcels or "bubbles" of air due to Unauthenticated | Downloaded 09/26/21 03:05 PM UTC FIGURE 2. Sounding at Khartoum, April 21, 1936. their relatively high temperature near first sign of an impending "haboob" the ground, is a contributory cause of at Khartoum. Sometimes it appears their strong, upward velocity, which that only near or under cumulonimbus raises the dust, and it is suggested clouds would rising dust originate. that the brownish shade nearly always The following is a presentation of observed at the base of cumulus clouds the factors involved in the occurrence at Khartoum is evidence that even of a "haboob" at Khartoum on April on days of light winds the upward 20th, 1936. At 14:00 local time, April transport of dust occurs in this way 20th the temperature was 107.3 °F, "by a combination of eddy motion and 5.7° above the monthly mean. Four- buoyancy due to relatively high tem- tenths cumulus had developed. During perature." A characteristic whitish the forenoon there was a light SSE haze in the southeast under a large wind becoming calm from 12:00 to cumulonimbus cloud due presumably 15:00. The "haboob" occurred at to the reflection of sunlight from 19:20, when the wind veered sharply ascending dust particles is often the from N to SSE, increasing to 30 mph. Unauthenticated | Downloaded 09/26/21 03:05 PM UTC FIGURE 3. Hodograph of Upper Winds at Khartoum, April 20, 1936. The points in circles are at each 1000-ft. interval above sea-level, the ground being at 1,300 ft. (point 1). The radius of the diagram equals 30 mph. Arrows mark the generalized shear vectors. The temperature dropped from 99.6° plotted on the accompanying pseudo- at 19:15 to 94° at 19:20, to 88.5° at adiabatic diagrams with the com- 20:10. The pressure rose 4 mb beyond puted specific humidity, relative hu- the normal diurnal variation from midity, and potential temperature for 19:00 to 22:00. Distant lightning each level. An analysis of these was observed but no thunder heard at soundings shows: 23:15. Visibility decreased to 50 April 20, 1936, Figure 1 yards when "haboob" arrived, but lasted only 1 hour. 1. A very unstable lapse rate, prac- In Farquharson's report3 on "ha- tically the dry adiabatic with quite boobs" in the Sudan, were found two high specific humidity, especially at consecutive airplane soundings, for upper levels. April 20th, and 21st, 1936. These 2. Taking an average specific hu- soundings include pressure, and tem- midity throughout the first 1500 me- perature of wet and dry bulb readings ters of 16 parts per thousand, it was taken at approximately every 300 found that the convective condensa- meters. These soundings we have tion level was at 2800 meters, and Unauthenticated | Downloaded 09/26/21 03:05 PM UTC FIGURE 4. Hodograph of Upper \ nds at Khartoum, April 21, 1936. that a heating of 5C° at the surface lar condition aloft. However, the would be sufficient to start convection lower levels, that is up to 2200 m, and the formation of clouds. Any fur- show a relatively marked cooling, ther increase of temperature beyond which at the surface amounts to 6C°. 5C° should activate convection to At higher levels, on the other hand, higher levels and produce cumulo- there is an indication of past con- nimbus type clouds. vergence, the 316° A potential tem- 3. It was also determined from perature isotherm having risen 1000 this sounding that real latent instabil- m, likewise the 317° and 319° iso- ity existed which to be released needed therms. At all levels now the specific only some source of lifting. humidity is greatly reduced and 4. This sounding also shows a slight combined with the temperature indication of perhaps a frontal inver- changes gives positive indication of sion at about 3 kilometers, which is a change in air mass. The sounding more clearly verified by the pilot bal- as a whole is much more stable and loon ascents. only pseudo-latent instability is pres- April 21, 1936, Figure 2 ent, i.e. the negative area is now The second sounding shows a simi- much larger than the positive.

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