Ask Mr Brain...all will be explained

Why is it that many of the world's deserts are located on the west sides of continents? You find them in the western United States and along the west coasts of South America and Africa.

It's certainly true when you look at the world map that a number of deserts lie on the west coasts of continents. But they exist in other places as well.

A more consistent trend finds them straddling the tropics of Cancer and Capricorn. This is due in part to warm, water-laden air that rises from equatorial regions and releases rains in the tropics as it moves towards the Poles. Later it descends, minus its moisture, in the sub-tropical latitudes - perpetuating the aridity.

In one of nature's paradoxes, deserts can exist next to oceans. Examples would be the fog-banked deserts of South America and Africa, which owe their existence largely to cold ocean currents that flow from Antarctic regions towards the Equator.

The Peru Current streams along South America's west coast off the Atacama Peruvian deserts. The Benguela Current sweeps the Namib desert on the southwestern edge of Africa.

The cool water of these currents brushing against warm coastal air throws off blankets of fog, which trap moisture at the surface level and prevent the moist air from rising to form rain clouds.

Such fog can quench aridity, but winds along the western fringes of South America and Africa flow parallel to the coast. These southerly winds hold fog near the shore so that little moisture reaches the desert realms.