When water on the Earth's surface is heated by the Sun, it evaporates and turns into water vapour which rises into the air. When the air cools it condenses around some dust or other particles in the air, called condensation nuclei. These small droplets then become visible as clouds.
Some droplets fall through the cloud and coalesce into raindrops on their way down. As more and more droplets join together they become too heavy and fall from the cloud as rain.
Warm air can hold more moisture than cool air, so when the warmer air is cooled and the moisture condenses, it often rains more heavily.
The video and text below explain how rain forms, and the different types of rain that we get in the UK.
Transcript: Transcript - Why does it rain?