Due to heat of the sun in summer, water from lakes, oceans and seas becomes hot and changes into vapour. Since vapour is very light, it rises high. Fine dust particles in the air combine with vapour to form water droplets. This collection of water droplets is known as clouds.
Clouds move from one place to another due to wind. Clouds continue to cool and become large in the cooling temperatures. When the water droplets become big and heavy, they come down to earth in the form of rain.