Abstract
I have analysed the climate of Ethiopia on the basis of daily weather records. Ethiopian seasons can be divided into 6 periods from the viewpoint of precipitation and daily maximum temperature. They are Dry Period I, Rain Period I, Dry Period II, Rain Period II, Dry Period III and Rain Period III.
Dry Period I (from the beginning of December** to middle March) is the longest dry season, when dry east air comes from the Asian Continent, bringing the most pleasant sky to the Ethiopian Plateau. Cold fronts extended from the Mediterranean cyclones, which move from the Mediterranean to the Arabian Sea, bring rains but rarely.
Rain Period I (from middle March to middle May**) is caused by the penetration of humid air from the Indian Ocean. This rain is usually referred to as “small rain” in the Ethiopian literature.
Dry Period II (from middle May** to middle May _??_ end of June***). The prevailing wind direction changes clockwise and now it is south. It does not bring sufficient moisture and causes a short but evident dry period.
Rain Period II (from middle May_??_ end of June*** to middle September end October***) This rain, which is often called “big rain”, is caused by the shifting of Northern Intertropical Convergence Zone (NITC). Therefore, it starts earlier and ends later in south. There are two types of rainfall in this period. Both at the beginning and at the end of this period, frontal rain falls continuously for several days, which is known to farmers as egngn bella (without stopping). Afternoon shower is the other type and recognized commonly. Normally sky is completely clear before noon, when this type prevails. Some old Ethiopian farmers know several dry days in the midst of this rain period (see the diagram of Jima, Fig. 1) and these are called derke hawaria (dry apostel). A small anticyclone at the foot of the Ethiopian Plateau is seen on the surface chart (Fig. 4), to which derke hawaria might be attributed. Dry Period III (from middle September _??_ end of October*** to beginning of November**). After the retreat of the Equatorial Westeries to the south wind blows from south and create dry weather, which is similar to Dry Period II.
Rain Period III (from beginning of November** to beginning of December**) is caused by the penetration of humid air again from the Indian Ocean. Since the counterclockwise change of prevailing wind direction is swifter, this rain period lasts shorter than Rain Period I, sometimes bringing no precipitation at all. The existance of this season has not been well recognized in the literature.
Not all stations of Ethiopia, however, have these 3 peaks of rainfalls. In the south, Rain Period II is longer, resulting in almost one peak. On the contrary, those stations on the leeside of the Plateau to the Equatorial Westeries have no main peak, but with two peaks.
The gradual shifting of the Equatorial Westeries is shown by the daily location maps of NITC (Fig. 2) and by the cross sections of wind (Fig. 3). Fig. 4 shows typical examples of the surface charts for each period.
