JONATHAN Swift summed up contemporary ignorance of Africa in 1733.
So geographers, on
Afric maps,
With savage pictures fill their gaps,
And o'er uninhabitable downs
Place elephants for want of to wns.
Even nowadays, with the benefit of television, we tend to think of that continent as an ocean of trackless, burning, virtually lifeless sand, interspersed with islands of inhospitable bush, coarse grass and dried up river beds. It may have come as a surprise, therefore, to see recent pictures of Rwanda with rain teeming down on lush, thick vegetation.
As it happens, the total annual rainfall in Rwanda is very similar to Ireland's at some 900 to 1,200 mm per year, but the pattern, and indeed its origins, are very different. Frontal rain of the kind familiar to us here is quite unknown. The rain invariably falls in heavy, thundery showers, which occur almost exclusively in the country's "wet season" from October to May.
Rwanda is just barely south of the equator, on the edge of what has been called the "glittering equatorial slum where huge trees jostle with each other seeking room to live"
the tropical rain forest of the Congo basin. Rwanda's saving grace, however, is its elevation. It is a hilly, mountainous country, whose dominant, physical feature is a great mountain range running down its western flank from north to south, the so called "spine of Rwanda".
To the west there is a sharp drop from these mountains to Lake Kivu and the Zairian town of Goma; to the east the land slopes gently, allowing much of the country to exist on a plateau some 4 000 feet above sea level. This altitude gives Rwanda a climate that is, by equatorial standards, a very healthy one, and a temperature regime that is generally not unpleasant; it can also be a windy spot, the natural increase in wind with altitude being exacerbated by the squally effects of the seasonal thundery showers.
The wet and dry seasons come about because during the former period the equatorial pressure pattern provides a prevailing wind with a strong easterly bias, so that moisture collected by the air on its journey across the Indian Ocean is available to fuel the convective showers.
The plateau rising gently westwards, and the spine of Rwanda itself, provide the necessary lift to trigger such events. But during the dry season, from the end of May until September, there is hardly any rain at all, because the prevailing southerly or southeasterly winds have travelled over the parched surface of continental Africa, and have little moisture to surrender.