One of the primary elements in climate and a factor of tremendous importance in the distribution of plant and animal life, rainfall varies from less than an inch annually in a desert to more than 400 in. (1,000 cm) where the monsoons strike the Khasi hills in Assam, India, and on the windward slopes of Hawaiian mountains. In the United States the range is from less than 2 in. (5 cm) in Death Valley, Calif., to more than 100 in. (250 cm) on the coast of Washington state; in most of the country the average rainfall is between 15 and 45 in. (38 and 114 cm) annually.
Factors controlling the distribution of rainfall over the earth's surface are the belts of converging-ascending air flow (see doldrums; polar front), air temperature, moisture-bearing winds, ocean currents, distance inland from the coast, and mountain ranges. Ascending air is cooled by expansion, which results in the formation of clouds and the production of rain. Conversely, in the broad belts of descending air (see horse latitudes) are found the great desert regions of the earth, descending air being warmed by compression and consequently absorbing instead of releasing moisture. If the temperature is low, the air has a small moisture capacity and is able to produce little precipitation. When winds blow over the ocean, especially over areas of warm water (where evaporation of moisture into the air is active) toward a given coastal area, that area receives more rainfall than a similar area where the winds blow from the interior toward the oceans. Areas near the sea receive more rain than inland regions, since the winds constantly lose moisture and may be quite dry by the time they reach the interior of a continent.
The windward slopes of mountain ranges generally receive heavy rainfall; the leeward slopes receive almost no rain. The southwest coast of Chile, the west coast of Canada, and the northwest coast of the United States receive much rain because they are struck by the moisture-bearing westerlies from the Pacific and are backed by mountains that force the winds to rise and drop their moisture. The territories immediately east of the regions mentioned are notably dry. See weather.