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Winds and rainfall

We have seen that the temperature belts move north and south with the sun. Likewise, the pressure, wind, and rainfall belts also move north and south with the apparent movements of the sun.

In Africa, the high-pressure belts, with their out flowing winds, stretch across the north and south of the continent, while the low-pressure belt lies about the Equator. All these belts move north and south with the sun. The northeast and south-east trades always blow from the high-pressure belts towards the low-pressure belt, and as they are blowing from cooler to warmer regions their capacity for absorbing moisture is increased, and they are dry winds gathering moisture from the land over which they pass.

So in Northern Africa the north-east trade winds blowing across a great land mass are dry, and their dryness becomes greater as they pass over the land hence in the trade-wind zone there is the great desert belt of the Sahara extending from the Atlantic to the shore of the Red Sea. In the high-pressure belt in the south of Africa, the south-east trades blow over the Indian Ocean and on reaching the eastern side of the continent bring rain to the windward slopes of the plateau. The Kalahari arid region is much smaller, both on account of the lesser breath of the southern part of the continent, and because of the rainy region to the east. Though often termed a desert, the Kalahari is by no means a waterless area, such as the Sahara.

When the trade winds, blowing towards the equatorial low-pressure belt, meet, then the currents of heated moisture-laden air rise, and being cooled by expansion cause heavy convectional rains in this region.

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