Rain is liquid Precipitation as opposed to non-liquid kinds of precipitation such as hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface. On Earth, it is the atmospheric water vapor into drops of water heavy enough to fall, often making it to the surface. Two processes, possibly acting together, can lead to air becoming saturated leading to rainfall: cooling the air or adding water vapor to the air. Virga is precipitation that begins falling to the earth but evaporates before reaching the surface; it is one of the ways air can become saturated. Precipitation forms via collision with other rain drops or ice crystals within a cloud. Rain drops range in size from oblate, pancake-like shapes for larger drops, to small spheres for smaller drops.
Moisture moving along three-dimensional zones of temperature and moisture contrasts known as weather fronts is the major method of rain production. If enough moisture and upward motion is present, precipitation falls from convective clouds (those with strong upward vertical motion) such as cunnulonimbus (thunderstorms) 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 inter tropical convergence zone, brings rainy seasons to asvannah climes. Rain is the primary source of freshwater for most areas of the world, providing suitable conditions for diverse ecosystems, as well as water for hydroelectric power plantd and crop irrigation. Rainfall is measured through the use of rain gauges. Rainfall amounts are estimated actively by weather radar and passively by weather satellites.