Rain is limited because cool air sinks into a warmer region.
As air warms its capacity to hold water vapor increases and rain cannot form. The great deserts of the world occur at about 30 degrees latitude on both sides of the equator and extend poleward in the interiors of the continents. These regions are areas of falling, warming air. Deserts can also be produced by the rain shadow effect of mountain ranges. As air masses approach from west to east in North America and meet a mountain range such as the Rockies, the air rises, cools, and loses much of its water vapor as rain or snow.