The Climate of Zimbabwe
1.a What is climate?
Climate is the average state of the atmosphere over a particular place or region of the earth's surface, related to a particular period of time and taking into consideration the average and extreme variations to which the atmospheric state is subject. In other words, both average and extreme values and frequencies of weather elements constitute climate. Climatic data are usually expressed in terms of an individual calendar month and/or season and are determined over a period of 30 years or more.
The main climatic elements are: precipitation, temperature, humidity, sunshine, wind velocity, fog, thunder, hail and soil temperature.
The climate of a place is mainly determined by its latitude, position relative to continents and oceans, position relative to large scale atmospheric circulation patterns, altitude and orography.
The broad classifications of climate derived from some of the above factors are: tropical continental, tropical maritime, polar continental and polar maritime.
Season refers to any of distinct periods into which the year may be divided, particularly in terms of climatic conditions, as a result of changes in duration and intensity of solar radiation. the beginning and end of a season usually coincide with or are accompanied by biological and phenomenological changes which are caused by changes in climatic conditions.
In temperate regions the year is divided into four seasons, essentially reflecting the life cycle of cultivated plants: winter (dormancy), spring (sowing), summer (growth) and autumn (harvest). In the tropics the division of the year into seasons is usually made in terms of rainfall and temperature.
1.c Factors governing the climate of Zimbabwe
Zimbabwe lies wholly in the tropics - stretching from 15.5 to 22.5 degrees south latitude. The sun is overhead twice a year. The least angle of elevation of the sun is 45 deg. over the southmost parts of the country.
b) Position relative to the oceans and lands
Zimbabwe is completely surrounded by land. At its closest to the nearest ocean (the Indian ocean), Zimbabwe is at least 200 km away. The country does not experience direct effect of the ocean.
An important geographic landmark to the climate of Zimbabwe is the presence and position of Madagascar. The SE trades originating from the subtropical anticyclone are not afforded a thorough flow to mainland Africa. They deviate northward over Madagascar but curve southward over the Mozambique Channel, then turn northward over mainland Africa. The island also acts as a deviator of a large number of tropical cyclones which might directly change the climate of Zimbabwe an indeed, that of the whole subcontinent.
c) Position relative to the general circulation features
The southern hemisphere subtropical anticyclone and the ITCZ are the main features which influence the surface circulation and hence the climate of Zimbabwe. These two features normally oscillate at zero phase and in sympathy with the movement of the sun on the earth's surface.
The subtropical anticyclone gradually shifts northward over South Africa from April through to June and is usually northmost in July. The axis of this zonal high pressure belt lies along the 27-deg south latitude. During the same period the ITCZ will be also moving northward. Although it does not move en masse as a belt, the ITCZ's winter position, latitudinally opposite Zimbabwe, is over southern Sudan in July and/or August. Zimbabwe's position relative to the subtropical anticyclone ensures that the surface flow has an easterly component throughout the year. In winter (mid-May to mid-September) the SE trade winds from the subtropical anticyclone predominate over the country. However the Eastern Highlands act as a strong mechanical deviator of the trades as they track northward and westward. On reaching the Eastern Highlands, the trades are deflected northward along the slopes and curve westward after the northern edge of the Highlands. Thus the trades reach Harare and surrounding areas as an easterly flow.
The subtropical anticyclone recedes southeastward and southwestward from Southern Africa in August through to October and is replaced by a trough. The trough normally stretches from Zaire through Angola, Namibia and western Botswana and has its southern limit over the Orange Free State. It is locally referred to as the leader of the ITCZ over Southern Africa. It has no remarkable weather associated with it. As the anticyclone recedes and the equatorial trough replaces it the airflow across Zimbabwe gradually backs and becomes NE'ly over the northern areas and easterly in the south. The mean position of the ITCZ proper at this time is along the 5-deg south latitude. Between November and December, a series of cold fronts pass over South Africa stretching from the Southern Sea to as far north as the 30-deg south latitude.These fronts usually link with the equatorial trough to form what is locally referred to as westerly waves. Behind the southern limit of the waves there is always a high pressure system. As this combination continues moving eastward the wind flow over Zimbabwe first becomes wholly NE'ly when the part of the wave nearest to the country is over east Botswana. The flow changes to SE'ly in the southern districts as the southend of the wave passes South Africa and the high pressure system covers the country (i.e. South Africa). The SE'ly flow of course starts from eastern South Africa and works its way to southern and then northern Zimbabwe and sometimes as far as Zambia. The westerly waves have a period of 5 to 7 days.
In January and February, the wind flow is predominantly NE'ly over the whole of Zimbabwe as the equatorial trough dilates over central Southern Africa, to cover Zimbabwe, Botswana and Transvaal as well. Then it is the ITCZ. As the ITCZ tracks northward beginning from mid-February, the wind flow over Zimbabwe veers to E'ly until mid-May and the cycle starts again.
Occasionally, however, the equatorial trough and the ITCZ move further south than the above described position and the subtropical anticyclone recedes further poleward than is normal. The wind flow over Zimbabwe becomes more northerly or even NW'ly which is very moist. On the other hand, sometimes the subtropical anticyclone remains anchored over Southern Africa thereby blocking the southward shift of the equatorial trough. In this case the airflow across Zimbabwe remains predominantly E'ly which is relatively dry.