Diverse rainfall and temperature patterns are largely the result of Ethiopia's location in Africa's tropical zone and the country's varied topography. Altitude-induced climatic conditions form the basis for three environmental zones-- cool, temperate, and hot--which have been known to Ethiopians since antiquity as the dega, the weina dega, and the kolla, respectively.
The cool zone consists of the central parts of the western and eastern sections of the northwestern plateau and a small area around Harer. The terrain in these areas is generally above 2,400 meters in elevation; average daily highs range from near freezing to 16°C, with March, April, and May the warmest months. Throughout the year, the midday warmth diminishes quickly by afternoon, and nights are usually cold. During most months, light frost often forms at night and snow occurs at the highest elevations.
Lower areas of the plateau, between 1,500 and 2,400 meters in elevation, constitute the temperate zone. Daily highs there range from 16°C to 30°C.
The hot zone consists of areas where the elevation is lower than 1,500 meters. This area encompasses the Denakil Depression, the Eritrean lowlands, the eastern Ogaden, the deep tropical valleys of the Blue Nile and Tekezé rivers, and the peripheral areas along the Sudanese and Kenyan borders. Daytime conditions are torrid, and daily temperatures vary more widely here than in the other two regions. Although the hot zone's average annual daytime temperature is about 27°C, midyear readings in the arid and semiarid areas along the Red Sea coast often soar to 50°C and to more than 40°C in the arid Ogaden. Humidity is usually high in the tropical valleys and along the seacoast.
Variations in precipitation throughout the country are the result of differences in elevation and seasonal changes in the atmospheric pressure systems that control the prevailing winds. Because of these factors, several regions receive rainfall throughout most of the year, but in other areas precipitation is seasonal. In the more arid lowlands, rainfall is always meager.
In January the high pressure system that produces monsoons in Asia crosses the Red Sea. Although these northeast trade winds bring rain to the coastal plains and the eastern escarpment in Eritrea, they are essentially cool and dry and provide little moisture to the country's interior. Their effect on the coastal region, however, is to create a Mediterranean-like climate. Winds that originate over the Atlantic Ocean and blow across Equatorial Africa have a marked seasonal effect on much of Ethiopia. The resulting weather pattern provides the highlands with most of its rainfall during a period that generally lasts from mid-June to mid-September.
The main rainy season is usually preceded in April and May by converging northeast and southeast winds that produce a brief period of light rains, known as balg. These rains are followed by a short period of hot dry weather, and toward the middle of June violent thunderstorms occur almost daily. In the southwest, precipitation is more evenly distributed and also more abundant. The relative humidity and rainfall decrease generally from south to north and also in the eastern lowlands. Annual precipitation is heaviest in the southwest, scant in the Great Rift Valley and the Ogaden, and negligible in the Denakil Depression.
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