Clouds contain huge numbers of tiny droplets of moisture. Raindrops are formed when these tiny droplets are enlarged, first by
moisture from the surrounding air condensing on them and then by coalescing with other droplets during their descent. Raindrops
vary in size from about 0.02 in. (0.5 mm) to as much as 0.33 in. (8 mm) in thunderstorms. From the time they leave the bottom of the
cloud, evaporation takes place and, if the cloud is high, the air warm and dry, and the raindrops small, so that they fall slowly, they
may evaporate completely before they reach the earth. If they do so, the drops are called virga.
There are thousands of stations throughout the world where rainfall observations and records are made. Included in such records is
the fall of snow, reduced to its equivalent in rain. Rainfall is measured, in terms of inches or millimeters of depth, by means of a
simple receptacle-and-gauge apparatus or by more complex electrical or weighing devices placed where eddies of air will not
interfere with the normal fall of the raindrops. In addition to the daily, monthly, and annual totals, the depth of individual rainfalls and
their intensity (amount of rain falling during a specific period of hours or minutes) and other pertinent facts are recorded.
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.