Ethiopia Fact

The daily measure of the mean surface wind speed and direction of ten minutes average at 1200 (noon) local time at the various stations are grouped in accordance with the seasonal wind prevailing directions.

The wind roses diagrams are drawn to obtain the complete picture of the seasonal wind for selected stations.

The wind speed is expressed in knots and the wind direction is also expressed according to the eight sectors of the wind roses from where the wind is blowing. That is:

North  (340-020)
North East (030-060)
East  (070-11)
South East (120-150)
South  (160-200)
South West (210-240)
West  (250-290)
North West (300-330)

The mapping of the prevailing wind direction is computed by expressing the recorded events of each of the main wind directions in percentage frequencies. Such mapping of the wind roses gives general information on the wind system at ground level for the specified stations.

From these wind rose diagrams, seasonal variations of the prevailing wind flow pattern is observed.

This seasonal variation is best explained by reference to the position of the Inter-Tropical Convergence Zone (ITCZ), a low pressure area of convergence along which equatorial wave disturbances take place.

The ITCZ oscillates seasonally within the tropics, its surface position is influenced by topography and local eddies. Thus this seasonal oscillation of the ITCZ causes a variation in the pattern of wind flows over Ethiopia.

Between June and September the ITCZ is located north of Ethiopia and pronounced cyclonic cells along the ITCZ are over North Africa and the Arabian Peninsula.

With the effect of topography on surface temperature the ITCZ meanders parallel to the Red Sea Coast. The rest of the country comes under the influence of the Atlantic equatorial westerlies and southerly winds from the equatorial Indian Ocean.

The south-west equatorial westerlies ascend over the south-western highlands and produce the main rainy season over most parts of highland Ethiopia.

The southerly winds from the Indian Ocean, despite the fact that they lose their moisture over the East African highlands, are blowing over the eastern lowlands of Ethiopia where their influence on rainfall is minimized due to the föhn effect.

From October to May is the longest period of the year during which the effect of the ITCZ is progressively located in central and southern Ethiopia both in spring (September, October and November) when it shifts northwards and in autumn (March, April and May) when it shifts southwards.

At these times there is a strong low pressure cell over central Sudan. The anticyclone system over north Africa and Arabian Peninsula weakens and a strong anticyclone cells develops over the Gulf of Aden and the Indian Ocean.

The Gulf of Aden-Indian Ocean high pressure system generates moist, easterly air current over south eastern Ethiopia and south easterly air currents along the Red Sea Coast.

In northern Ethiopia, the prevailing air currents at these seasons are the dry, subsiding continental air currents from the east and north. In south-western Ethiopia, the surface air current are the Atlantic equatorial westerlies.

The Gulf of Aden-Indian Ocean moist air currents form a zone of convergence with the northerly air currents along the northern half of the western escarpment of the Ethiopian rift system. Thus the ITCZ separates the Atlantic equatorial westerlies from the tropical easterlies from the north and east.

In December, January and February, the ITCZ is located in the southern hemisphere, south of Ethiopia.

High pressure systems develop over North Africa and Arabia. The main pressure systems which determine air circulation over Ethiopia are therefore, the anticyclones in northern Africa and Arabia and the low pressure systems in southern Africa.

The Arabian High also extends southwards to eastern Ethiopia, the Horn and the Indian Ocean. The dry, subsiding tropical north easterlies dominate the northern and most of the western half of the country while easterly air currents from the Indian High dominate the south-eastern part.

(Source: National Atlas of Ethiopia)

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