The diurnal variation of wind over tropical Africa

Abstract In the Central Sudan wind speed decreases from morning to midday. Upper wind observations made twice daily at Khartoum during 1935 and 1936 provided data for the investigation of this unusual type of variation. It appears from mean monthly vector winds for morning and midday that the relationship of surface winds to geostrophic winds at a height above the surface layer of turbulence is similar to that in other latitudes and that a decrease in geostrophic wind occurs at midday. A corresponding decrease in pressure gradient can be shown only on the synoptic charts based on mean monthly sea level pressures for March, gradients for other months being indefinite and data scanty. Horizontal temperature gradients are worked out from the observed upper wind distribution, which agree with the temperature gradients obtained from surface temperatures and assumed lapse rates. These indicate a decrease from morning to midday, confirming a similar variation in pressure gradient. It is shown that this diurnal variation is typical of a wide belt of tropical Africa and, based on upper air temperature observations at Khartoum, the suggestion is made that it is due to the midday rise in temperature within the region of the thermal equator being less than that in regions north and south of it, at the height of the geostrophic wind level.