Rain in East Africa, with particular emphasis on rain in July and August

Abstract

The author's observations, statistical analyses and review of the previous works lead him to a “problem climate”, which still awaits a satisfactory explanation. That is the weather phenomena particularly of July and August. The author attempted to make a consistent explanation through a geographical approach.
Most parts of Uganda and the Kenya Highlands receive the most abundant rainfall in July and August, when the rest of East Africa records practically no rain or the least precipitation in a year (Fig. 3). The ITC theory fails to explain this copious rainfall in these regions, since the ITC is supposed to, be located over Sudan at this time. An attempt was made to explain this by cold fronts invading from the higher latitudes (Henderson 1949). But any kind of migrating system of rain bringing perturbation has not yet been-detected on synoptic weather charts nor by daily rainfall analyses (Thompson 1957 a, 1965, Johnson and Mörth 1960, Johnson 1962).
The weather of this time in the Nairobi district is characterized by high, cloudiness (Fig. 4) and frequent drizzles, and yet July and August are the driest months as far as precipitation is conserned. This, too, must be properly explained.
Rain in the tropical region is generally considered to be of local character. Indeed, the individual rain are areas small in size (Fig. 2), but there is not infrequently a simultaneous tendency toward increase or decrease of rain areas over an extensive region. The rain in July and Angust is frequently more widespread than local. And the rainy spell may be prolonged for more than a week, or even two weeks (Table 3).
All of these phenomena are expected to be associated with upper winds. Fig. 6 shows the seasonal change of the zonal and meridional components of the resultant winds over Nairobi. It is clearly evident that July and August are characteristically under the influence of the westerlies embedded between the SE monsoon and the upper easterlies. The westerlies are observed even over the southern border of Tanzania. Contrary to the commonly held idea that the westerlies come from the Congo region, they are least frequent over the western part of Uganda (Fig. 7) and of Tanzania.
The interrelation between the rainfall and the westerlies is further investigated by using the daily aerological and precipitation data for 1964. As a result, it is ascertained that the widespread rains are associated with the westerly incursions. Moreover. the most frequent and the most abundant rainfall during the westerly incursions are found on the west side of Aberdare Mountains (ca. 4, 000 meters), and, to a lesser extent, on the west side of Mt. Kenya (Fig. 8). This rainfall pattern is peculiar to the westerlies season, for the rainiest parts are found on the east facing slopes in other seasons.
The season of the westerlies abruptly ended in mid-October in 1964. Accompanied wish this abrupt change in the upper circulation, there occurred a drastic change also in the rainfall pattern. After the alternation of the upper winds, widespread rains became more frequent in the eastern parts of East Africa, that had been dry during the westerlies season. On the contrary, widespread rains gave way to local rains over the Kenya High-lands.
Kinematically as well as thermo-dynamically the westerlies favor the formation of clouds. But the existance of the stable upper easterlies over Nairobi suppresses the vertical development of clouds, resulting in stratified clouds and drizzles. On the other hand, the vertical develoment of clouds, which are probably explained by lack of the stable layers aloft, gives rise to heavy showers over the northwestern parts of East Africa. Winds with a component from the pole to the equator have a tendency toward uplift (Flohn 1960). This accounts for the dryness of Tanzania during the westerlies season. The dryness of Northeast Kenya is considered to be accentuated by the lee effect of the westerlies.