Abstract
In the Middle East Area (M.E.) troughs in the upper westerlies are found, generally, at levels over 2 km above M.S.L. intruding into low latitudes as far south as the Northern Sudan (lat. 20° N.). These troughs usually travel eastwards, but tend to trail over the NW. of the Abyssinian Plateau. They also frequently remain stationary for about three days (on the average) over the SE. Mediterranean, owing to the effect of the plateau of Asia Minor.
At the southern limits of these troughs, centres of convergence are usually set up within the westerlies, which have thus extended (at upper levels) into low latitudes, between lat. 22° N. and lat. 15° N. For the purpose of studying them the stream‐line map has been found the most satisfactory; the usual pressure map is of little assistance.
As a rule a cyclone (say of type “A”) tends to form at a centre of convergence, and travels towards the NE., NNE., or even N.; the northward movement of the cyclones being due to the tendency for them to move towards the “centres” of the quasi‐stationary troughs over the SE. Mediterranean. Cyclones may eventually appear on the surface synoptic charts as shallow areas of low pressure.
Another set (say of type “B”) of upper‐air cyclones, which move south‐westwards, tends to form farther south by the setting up of wave‐like perturbations in the usual E.–NE. upper current of the atmosphere of the Central Sudan. These perturbations appear at the time of re‐establishment of the E.–NE. current over the N. Sudan with the northward retreat of the westerlies, extend into low latitudes, and generally, when an upper‐air cyclone of type “A” has reached the M.E., another cyclone of type “B” develops over the East‐Central Sudan.
The analysis has some value in forecasting upper winds on the Cairo—Khartoum air‐route and the local weather associated with these cyclones, and also in studying the general circulation in these latitudes, more especially the characteristic features of these cyclones.