Ethiopia is in the tropical zone lying between the equator and the Tropic of Cancer.
There are three different climate zones in the country and weather varies substantially depending on altitude. The lowlands are generally hot and humid, while cooler temperatures characterise the Ethiopian Highlands. Although the low-lying areas are tropical in climate due to the proximity to the equator, the mountainous regions can get chilly and the climate is more alpine.
Ethiopia has five climatic zones, defined by altitude and temperature:
- The hot, arid zone covers the desert lowlands below 500 m, where average annual rainfall is less than 400 mm and average annual temperatures range between 28°C and 34°C or higher;
- The warm to hot, semi-arid zone includes those areas with an altitude of 500–1,500 m, average annual rainfall generally of around 600 mm (but as high as 1,600 mm in the western lowlands of Gambella), and an average annual temperature range of 20–28°C;
- The warm to cool, semi-humid zone covers the temperate highlands between 1,500 and 2,500. Average annual temperatures vary between 16°C and 20°C, and annual rainfall is generally around 1,200 mm, reaching 2,400 mm in the south-west;
- The cool to cold humid zone includes the temperate highlands between 2,500 and 3,200 m, where average temperatures range between 10°C and 16°C, with an annual rainfall of 1,000 mm and up to 2,000 mm in higher areas;
- The cold, moist temperate zone covers the Afro-alpine areas on the highest plateaus between 3,200 and 3,500 m; average temperatures are below 10°C and annual rainfall averages less than 800 mm.
According to the National Meteorological Services Agency, the highest mean annal rainfall of over 2,400 mm, is in the south-western highlands of the Oromia Region. The amount of rainfall gradually decreases to about 600 mm in the north in areas bordering Eritrea, and it drops to less than 100 mm in the north-east in the Afar Depression, and to around 200 mm in the south-east in the Ogaden Desert. The mountain areas over 3,500 m frequently receive snow and hail, but it usually melts within hours after it falls.
Based on this rainfall distribution pattern, the following four major rainfall regimes can be distinguished:
- Central, eastern and northern areas of the country experience a bimodal rainfall pattern, receiving the majority of their rainfall from the Atlantic, while some derives from the Indian Ocean. The big rains from June to September come mainly from the Atlantic, while the light spring rains between February and May come from the Indian Ocean. In each case, the amount of rainfall and the length of the rainy season decrease the further north one goes;
- Western and south-western parts of the country experience a unimodal rainfall pattern brought about by wind systems coming from the Indian Oceans and merge with those from the Atlantic to give continuous rain from March or April to October or November. The amount of rainfall and length of the rainy season decreases from south to north;
- Southern and south-eastern parts of the country experience a bimodal rainfall pattern brought about by the wind system coming from the Indian Ocean from September to November and from March to May. The most reliable rainy months are April and May. Ethiopians speak about the main and little rainy season;
- North-eastern parts of the country comprise part of the western escarpment of the Pitt Valley and the adjacent Afar depression. The lowlands have only one rainy season during whilch only a little rain falls. However, the escarpment, particularly in the north, can have a third rainy season brought by moist winds from Asia which have crossed the Arabian peninsula and cool as they rise over the Ethiopian escarpment. These can bring mist and rain anytime between November and February.
The highest mean maximum temperatures in the country, about 45°C from April to September and 40°C from October to March, are recorded from the Afar Depression in north-east Ethiopia. The other hot areas are the north-western lowlands, which experience a mean maximum temperature of 40°C in June, and the western and south-eastern lowlands with mean maximum temperatures of 35°C to 40°C during April.
The lowest mean temperatures, of 4°C or lower, are recorded at night in highland areas between November and February (National Meteorological Services Agency, 1989; Ethiopian Mapping Authority, 1988). Many of those areas, particularly in valley bottoms, have occasional ground frost.
Given the conditions of rainfall and temperatures, there can’t be true tropical evergreen rainforests in Ethiopia, as rainforests grow under conditions of 3,000 mm of rainfall per year and higher. During the their dry seasons, the lower vegetation of the forests of Ethiopia dry up.