Rain is formed when wet and warm air rises and cools down.

When the air reaches a relative humidity of a hundred percent, also known as the dew point, the water vapor contained in it begins to condense into small cloud droplets. Finally, when these cloud droplets have achieved a great enough mass, they fall down in the form of raindrops. 

Rains can be divided into three main types depending on the factor that causes the rising air flow.