Rain and other forms of precipitation fall from the clouds. When warm air passes over a body of water, it causes the water to evaporate. Once the water rises high enough, it clumps with other water vapor and becomes a cloud. The air cannot hold an infinite quantity of water, so when the amount of water present in the cloud exceeds the air’s ability to hold it, the water falls in droplets as rain.
On earth, water flows in a constant cycle and none is ever truly lost. The water that falls out of the clouds as rain and strikes the earth flows downhill until it reaches an ocean or lake. Warm air moving over the water will absorb the water, carrying it high into the atmosphere. Because some forms of pollution bond with water, the pollutants can end up in the clouds where it will eventually rain down as polluted water. This can exacerbate the problem of pollution as the rain spreads it to a large area.
Not all clouds are the same, and some types are more likely to produce rain than others. Usually, clouds that produce rain have the term “nimbus” in their name, such as cumulonimbus clouds, which produce thunderstorms.