It has been believed that the climate of East Africa is basically controlled by the two “monsoons” which are prevalent over the region. But, in addition, frequent appearances of westerlies and their rain bringing character have long been known empirically. The hitherto reports, however, contain some contradictory remarks regarding the season of frequent appearance, the spatial distribution, their origin and their association with weather. The author has attempted to make a full use of all the available data of meteorological observations to investigate the time and spatial distribution of the westerlies, and to discuss their origins and the relation with weather.
The pilot balloon and radio-sonde observations clearly show that there is a distinct seasonality in the frequency of appearance. With the two minima in April and in November, which are common to most of the stations in East Africa, the westerlies during January-March are distinguished from those during May-October (Fig. 2).
The horizontal and vertical distributions of the westerlies are entirely different between the two seasons. The westerlies during January-March are most frequent over southern Tanzania, steadily decreasing toward the north, and they are equally frequent at least up to 5 kilometers in altitude. By contrast, the westerlies during May to October, particularly in July and August, are most frequent over the eastern half of East Africa, with a large meridional extention from southern Tanzania across the equator at least to central Kenya. The wester-lies of this season characteristically appear at the 3-4 kilometer levels overlying the SE mon-soon (Fig. 3).
Emphasis has been put in this paper upon the May-October westerlies, because of the insufficient data concerning the January-March westerlies.
The May-October westerlies over Nairobi are by no means cool nor unstable compared with the easterlies. Rather, it must be noticed that a stable layer is repeatedly recognized between the westerlies and the underlying monsoon (Fig. 4). The moisture content is signifi-cantly greater in the westerlies than in the easterlies (Table 1). In addition, the time section analysis suggests the possibility of wave-like perturbations in the westerlies (Fig. 5), A drastic change demarcates the termination of the westerlies season (Fig. 6).
According to the daily 500mb charts, a low pressure, which is different from the equatorial trough, is formed over the continent at about 15- S in the period January-March, and the westerlies are observed on the northern side of the low (Fig. 7). On the contrary, the May-October westerlies seems to be associated with the passages of troughs extending from the higher latitudes in the southern hemisphere, which are likely to develop over the eastern coast (Fig. 8).
It is considered that the relationship between the westerlies and weather differs from region to region, for the reason that there is a great discrepancy between the spatial distributions of the westerlies and the weather. Even under the probable control of the westerlies, the Kenya Highlands experience frequent afternoon showers, while the Nairobi district is characterized by extensive stratified clouds but with only insignificant rainfall. An extreme dry condition prevails just east of the East African Highlands in northern Kenya, and also in the most parts of Tanzania.
In spite of the common belief that the equatorial westerlies tend to produce rain, the author has shown that all of those regional differences of weather are related to the westerlies, and that they can be explained without much difficulty. The dryness in the east of the High-lands is possibly due to the subsidence of the westerlies on the lee side. The difference between the Nairobi district and northern Kenya may be attributed to the regional differences of character of the SE monsoon.