Precipitation╤in the form of rain, mist, and snow╤provides living creatures with the essential ingredient for their existence: water. The amount of precipitation in any part of the world has a direct effect on the types of habitats that develop, on the kinds of vegetation that can grow, and on the species of creatures that can populate that niche.
Areas that receive a lot of rain year-round usually teem with greenery. But where rainfall is seasonal╤as on the savannas of Africa╤plant growth is also seasonal, and animals must migrate to follow the growth of new food.
The world's equatorial regions receive the most rain, as the heat from the Sun draws a lot of moisture into the atmosphere. Such tropical regions are, logically, the site of the large rainforests. The combination of steady rainfall and warm temperatures provides an ideal environment for growth, and an innumerable variety of plants and animals populate such areas.
In other parts of the world, large wetlands or fertile grazing grounds develop during seasonal rainfalls. But rainy-season, dry-season fluctuations can lead, as in southwest Africa, to vast floods and then baking summers.
Coastal zones╤particularly the western shores of the continents in the northern hemisphere╤are usually buffeted by prevailing winds from the ocean, providing moderate rainfall that peaks in winter. The extensive temperate rainforests of northern Europe and northwestern America are the product of such precipitation patterns.
Wherever the air temperature is low enough, precipitation falls to the ground as snow rather than as rain. In many parts of the northern hemisphere, snow will lie on the ground for much of the winter.
Deserts have infrequent rainfall, of course. But surprisingly, the driest areas of the world are not the deserts, but the frozen wastes of Antarctica and the Arctic. At the poles, much less energy from the Sun is absorbed, so less water evaporates. Cold air also holds less water than warm air, so the low moisture content of the atmosphere keeps the Poles from receiving more than a few inches of rain each year.
