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SUMMER WEATHER OVER THE ANGLO-EGYPTIAN SUDAN By F./Lt. T. HARRY This article is reprinted by permission from the " Navigation Bulletin '' of Transport Command, R.A . F. The Sudan is dominated by the River Nile which flows from south to north across the country. A large percentage of the population live in the Nile valley, and the main air route is along the valley from Wadi Halfa in the north to Juba in the south. In the north-east the Red Sea Hills rise to about 6,000 feet and in the extreme west, between El Fasher and El Geneina, there is a limited area of high ground with isolated peaks to over 10,000feet. The north of the country is permanent desert and the south permanent pasture. This is of interest in so far as it affects the distribution of sandstorms. South of latitude 12'N. vegetation at all times of the year is sufficient to prevent sandstorms. Normally, sandstorms may occur anywhere north of this line, but in late summer sufficient vegetation has usually appeared between latitudes 12" N. and 14" N. to prevent sandstorms over this rather limited area. Weather charts, in contrast to those of temperate latitudes, show no marked day-to-day change in general outline. The inter-tropical front, lying east to west across the country in a trough of low pressure, is the main feature of every chart. This front is the surface of separation between the hot dry north-east trades and the warm moist south-west monsoon. The inter-tropical front slopes to the south and the moist air is usually 9,000 feet deep at a distance of 500 miles from the surface front. The mean position of the front advances north in early summer, reaches its most northerly position in August, and thereafter retreats slowly south. In May and October the mean position of the front is from just south of El Fasher to Kassala, and in August it follows the 18th parallel from the west to Kareima and then runs east-north-east to Port Sudan. The inter-tropical front is, however, subject to large-scale move- ments north and south of its mean position, and at any particular time may be as much as 200 miles from the mean position. On eight or nine days out of every ten the synoptic charts bear a very close resemblance to the chart of mean conditions for the particular time of the year, and the actual weather conditions conform to a fairly simple pattern. Under such conditions, the light winds in the hot dry north-east trades veer from north-north-east at the surface to east-north-east above 10,OOO feet and the light winds in the warm moist air to the south blow fairly steadily from the south-west, the direction changing rapidly to east-north-east above the frontal surface. In these circumstances, the weather is cloudless and visibility very good near and to the north of the inter-tropical front. South of the front scattered afternoon cumulus develops and by late afternoon the vertical develop- ment is usually sufficient to cause occasional showers and possibly isolated thunderstorms well to the south of the front. During the second half of the night and in the morning conditions are invariably good, the only cloud being 281

SUMMER WEATHER OVER THE ANGLO-EGYPTIAN SUDAN

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Page 1: SUMMER WEATHER OVER THE ANGLO-EGYPTIAN SUDAN

SUMMER WEATHER OVER THE ANGLO-EGYPTIAN SUDAN

By F./Lt. T. HARRY

This article is reprinted by permission from the " Navigation Bulletin '' of Transport Command, R.A . F.

The Sudan is dominated by the River Nile which flows from south to north across the country. A large percentage of the population live in the Nile valley, and the main air route is along the valley from Wadi Halfa in the north to Juba in the south. In the north-east the Red Sea Hills rise to about 6,000 feet and in the extreme west, between El Fasher and El Geneina, there is a limited area of high ground with isolated peaks to over 10,000 feet. The north of the country is permanent desert and the south permanent pasture. This is of interest in so far as it affects the distribution of sandstorms. South of latitude 12'N. vegetation at all times of the year is sufficient to prevent sandstorms. Normally, sandstorms may occur anywhere north of this line, but in late summer sufficient vegetation has usually appeared between latitudes 12" N. and 14" N. to prevent sandstorms over this rather limited area.

Weather charts, in contrast to those of temperate latitudes, show no marked day-to-day change in general outline. The inter-tropical front, lying east to west across the country in a trough of low pressure, is the main feature of every chart. This front is the surface of separation between the hot dry north-east trades and the warm moist south-west monsoon. The inter-tropical front slopes to the south and the moist air is usually 9,000 feet deep at a distance of 500 miles from the surface front. The mean position of the front advances north in early summer, reaches its most northerly position in August, and thereafter retreats slowly south. In May and October the mean position of the front is from just south of El Fasher to Kassala, and in August it follows the 18th parallel from the west to Kareima and then runs east-north-east to Port Sudan. The inter-tropical front is, however, subject to large-scale move- ments north and south of its mean position, and at any particular time may be as much as 200 miles from the mean position.

On eight or nine days out of every ten the synoptic charts bear a very close resemblance to the chart of mean conditions for the particular time of the year, and the actual weather conditions conform to a fairly simple pattern. Under such conditions, the light winds in the hot dry north-east trades veer from north-north-east at the surface to east-north-east above 10,OOO feet and the light winds in the warm moist air to the south blow fairly steadily from the south-west, the direction changing rapidly to east-north-east above the frontal surface. In these circumstances, the weather is cloudless and visibility very good near and to the north of the inter-tropical front. South of the front scattered afternoon cumulus develops and by late afternoon the vertical develop- ment is usually sufficient to cause occasional showers and possibly isolated thunderstorms well to the south of the front. During the second half of the night and in the morning conditions are invariably good, the only cloud being

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above 10,000 feet and confined to areas more than 300 miles south of the iront. In the north, afternoon temperatures reach 110" F. and in the south 90' F. After 08.00 to 09.00 G.M.T. bumpiness is usually moderate up to 8,000 or 10,000 feet, even in cloudless conditions,

Fig. 1. General map of the area with mean isobars and wind direction for August.

At times during the summer months these balanced conditions are upset by the flow of colder air over the area from the south and east ; and it is under these circumstances that the sandstorms appear and the thunderstorms become dangerously frequent and severe. Sandstorms over this area are of two types : the Haboob, which affects only a small area for a comparatively short time, and the widespread summer sandstorm, which affects a large area and may last for 10 or 12 hours. In addition to the normal thunderstorm rain, which is often heavy but of relatively short duration, outbreaks of rain covering a wide area and lasting up to 24 or 36 hours do sometimes occur.

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On occasional afternoons between April and October strong vertical currents, usually originating beneath nascent cumulonimbus clouds, develop and cause the sand over a limited area of desert to be lifted to considerable heights. The resultant sandstorms, known as Haboobs, though limited in area, constitute a serious flying hazard in itself, as sand is often carried up to six thousand or seven thousand feet and sometimes twelve thousand feet, while surface visibility within the storm falls to two hundred yards, and in exceptional cases to three yards. The storms always travel from an easterly quarter, and the first view of it is usually as a wall of sand which may appear at a great distance as a whitish haze under cumulonimbus cloud. Visibility is usually good right up to the edge of the storm which is accompanied by strong squally winds of rather variable direction, the strongest gusts, frequently well above gale force, nearly always occurring at the onset of the storm.

Haboobs are most frequent between 12.00 and 24.00 G.M.T. (14.00-02.00 Sudan time). They are usually from 10 to 50 miles wide and normally only affect one particular station for an hour, although they have been known to cause poor visibility in the same locality for three or four hours. These local sandstorms occur in all desert areas of the Sudan, but they are most common in the area between Port Sudan, Kareima, El Obeid and Kassala. It should be noted that there is no cyclonic wind circulation around the Haboob and that no marked change in barometric pressure accompanies the passage of such a sandstorm. The best weather conditions are encountered on the western side of the Haboob where visibility is very good and cloud confined to dense cirrus at a great height. Cloud in the rear of the storm is usually thicker and visibility is likely to be only moderate. Often in its later stage of development, and especially after midsummer, the cumulonimbus cloud overlying the Haboob produces a shower or thunderstorm which is sufficient to kill the sandstorm.

Widespread summer sandstorms are rare, occurring only once or twice a year, and sometimes not at all. Contrasting in every way with the Haboob, these sandstorms normally occur between 00.00 and 12.00 G.M.T. and affect a very uide area. The visibility in some cases falls to less than 500 yards over an area immediately behind the inter-tropical front, measuring 400 miles from east-north-east to west-south-west and 150 to 200 miles from north-north-west to south-south-east. The surface winds associated with this type of sandstorm blow steadily from the south-west with speeds varying from 20 to 30 knots : exceptional gusts are not experienced. The sand in such cases is rarely carried above 3,000 to 4,000 feet, and there is usually a good lane of clear air between the top of the sand and the base of the abundant high and medium cloud which accompanies this type of sandstorm. Widespread summer sandstorms usually occur in early and mid summer and are most severe in an area between Port Sudan, Kareima, El Obeid and Kassala. Such sandstorms are rarely of long duration in the extreme north and never penetrate into Upper Egypt.

Thunderstorms occur very rarely north of latitude 18" N., occasionally between 18"N. and 14"N., and frequently south of 14"N. and over hlgh ground. There is no essential difference in structure between cumulonimbus

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clouds in this area and those encountered in temperate latitudes. However, k the tropics the percentage of cumulonimbus clouds which could be classed as very intense would be higher and the area affected at any one time would be greater than in temperate regions. Thundery precipitation is mostly in the form of rain, although hail is not unknown, especially over high ground. Even in heavy thunderstorms the main base of the cumulonimbus cloud is rarely below 6,000 feet in the Northern Sudan 01 below 2,500 feet in the Southern Sudan. Occasionally, these clouds touch the surface over the high ground in the extreme east and west of the country. In very heavy rain, scud cloud forms below the main base of the cumulonimbus cloud and visibility in the rain may fall as low as 200 yards. Cumulonimbus clouds are always associated with violent vertical currents and should, whenever possible, be avoided. If it is impossible to avoid a cumulobimbus cloud, then the least dangerous flying height is 300-500 feet above the ground. Over most of the Sudan, thunderstorms affect only a limited area at any one time and move from between north-east and south-east a t speeds of 15 to 30 knots. In the extreme west, however, a series of thunderstorms sometimes join together to form a line squall. These line squalls are invariably orientated in a north to south direction and may be 200 miles long. They always move from the east and are, of course, much more difficult to avoid than the ordinary air-mass thunderstorm.

In summer the Sudan is subject to occasional outbreaks of widespread con- tinuous rain. These periods of rain are most common at night, but sometimes last for 24 or 36 hours. Although the rain is of a thundery nature actual thunderstorms rarely occur. In such cases, the cloud forms a continuous sheet over a very wide area, the base being 8,000-10,000 feet. The cloud top consists of a series of cumulus heads at 15,000-20,000 feet. A certain amount of scud cloud does form below the main base in periods of heavier rain, but visibility is usually moderate to good.

The freezing level over the area falls from about 17,000 feet in the north to about 13,000 feet in the south. Normally there is little day-to-day change in the freezing’*level, but in periods of disturbed weather it usually falls about 2,000 feet. Pressure changes are at all times small and even in the vicinity of a thunderstorm the pressure is unlikelyto deviate by more than three millibars from the forecast pressure.

WEATHER BACK NUMBERS REQUIRED

The Editors are desirous of obtaining copies of No. 1, the May 1946 issue, .of Weather and invite readers who do not intend preserving their copies to be good enough to return them. Copies should be sent to the Assistant Secretary, Royal Meteorological Society, 49 Cromwell Road, London S.W. 7.

The reeponsibility for opinions expressed in articles and correupondence rests in all cases with the respective authors.

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