On the eastern edge of the Great Plains, prairie consists of tall grasses and a tremendous diversity of other herbaceous plants. In a savanna the tallgrass prairie is either inter-digitated with patches of woods, or trees may occur as scattered individuals in the grassland. Fire is a natural part of their environment and controlled burning can be an important conservation measure. Annual rainfall declines as one goes westward in the Great Plains. Grasses decrease in size and the number of herbaceous plants declines in numbers and diversity.
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.