Latitude Effects

Some deserts occur as a result of wind patterns and latitude (distance north or south of the equator). Warm, wet air rises from the equator. As it cools, the water vapor in the humid air falls as precipitation, leaving dry air. The dry air flows north and south away from the equator, cooling as it goes. Cooler air is denser, so it tends to drop down towards the earth´s surface at about 30o North (or South) of the equator. The dry air, now falling down towards earth, once again becomes warmer, which means it can hold more water vapor - so it causes lots of evaporation at about 30o latitude. The two factors combine - dry air is constantly coming into an area, not moisture-laden air that could bring rain; plus, the dry air is actually "sucking" up any moisture it finds already there. Result? Desert.