Rain is water droplets that have condensed from atmospheric water vapor and then
fall under gravity. Rain is a major component of the water cycle and is responsible
for depositing most of the fresh water on the Earth. It provides water for
hydroelectric power plants, crop irrigation, and suitable conditions for many types
of ecosystems.
The major cause of rain production is moisture moving along three-dimensional zones
of temperature and moisture contrasts known as weather fronts. If enough moisture
and upward motion is present, precipitation falls from convective clouds (those
with strong upward vertical motion) such as cumulonimbus (thunder clouds) which can
organize into narrow rainbands. In mountainous areas, heavy precipitation is
possible where upslope flow is maximized within windward sides of the terrain at
elevation which forces moist air to condense and fall out as rainfall along the
sides of mountains. On the leeward side of mountains, desert climates can exist due
to the dry air caused by downslope flow which causes heating and drying of the air
mass. The movement of the monsoon trough, or Intertropical Convergence Zone, brings
rainy seasons to savannah climes.
The urban heat island effect leads to increased rainfall, both in amounts and
intensity, downwind of cities. Global warming is also causing changes in the
precipitation pattern, including wetter conditions across eastern North America and
drier conditions in the tropics. Antarctica is the driest continent. The globally
averaged annual precipitation over land is 715 mm (28.1 in), but over the whole
Earth, it is much higher at 990 mm (39 in).[1] Climate classification systems such
as the Köppen classification system use average annual rainfall to help
differentiate between differing climate regimes. Rainfall is measured using rain
gauges. Rainfall amounts can be estimated by weather radar.
