Precipitation

In meteorology, precipitation is water in either solid or liquid form that falls in Earth's atmosphere. Major forms of precipitation include rain, snow, and hail. When air is lifted in the atmosphere, it expands and cools. Cool air cannot hold as much water in vapor form as warm air, and the condensation of vapor into droplets or ice crystals may eventually occur. If these droplets or crystals continue to grow to large sizes, they will eventually be heavy enough to fall to Earth's surface.

Precipitation is obviously important to humankind as a source of drinking water and for agriculture. It cleanses the air and maintains the levels of lakes, rivers, and oceans, which are sources of food and recreation. Interestingly, human activity may influence precipitation in a number of ways, some of which are intentional, and some of which are quite unintentional. These are discussed below.