Precipitation occurs when water-laden air cools, causing the water to condense back into its liquid form and fall as rain. In order to acquire the water in the first place, warm air must flow over a body of water. When this happens, the molecules of water enter the gaseous state, forming clouds that float through the atmosphere.
Water is not created or destroyed in significant quantities on Earth; instead, it moves around as part of the hydrologic cycle. When water rains down out of the clouds, it flows across the surface of the Earth. Some of it collects in ponds, lakes or rivers, while other raindrops end up being absorbed by the ground. Eventually, all of it makes its way to the world’s oceans.
Most evaporation occurs over the oceans and the world’s largest lakes and reservoirs. However, as that air starts to rise, it begins to cool. When it cools enough, the water must leave the gaseous phase and become a liquid again. As the liquid water droplets are much denser than the gaseous form, the drops fall as rain.
In certain circumstances, the water that condenses may freeze. Depending on the specific details about its rate of fall and the altitude of the clouds, the water precipitates as snow, hail or sleet.