When the water bodies get heated by solar radiations during the day, a large amount of water evaporates and goes in the air. The air carrying water vapours also gets heated. This hot air rises up carrying water vapours with it. As the air rises, it expands and cools. This cooling causes the water vapour in the air to condense as tiny droplets which slowly grow bigger by the condensation of more water droplets and forms clouds. When the droplets have grown big and heavy, they fall down in the form of rain.