How does precipitation form?
Precipitation refers to the moisture that falls from the clouds or the air to the ground. The formation of precipitation depends on the cloud type and the temperature of the air below the clouds.
Rain is one type of precipitation. When a parcel of the air is heated, it becomes warmer and lighter than surrounding air. It expands, becomes unstable and then rises. As the warm air continues to rise, its temp. begins to drop, causing air to be saturated. After dew point is reached, condensation process happens, and clouds are formed. The water droplets in them combine to form bigger droplets, and when the clouds become too heavy, they fall or precipitate as rain.
When water evaporates, whether from a lake, ocean, river, what have you, the water vapor rises and collects in the atmosphere. The amount of water vapor any given section of atmosphere can hold until it is completely saturated depends on the temperature of the air, as warmer air can hold greater quantities of water vapor than cool air.
If the air should cool, the atmosphere can no longer hold the water vapor it has collected, and it condenses from a gas to a liquid and falls -- or precipitates -- back the surface.