Question

how is rainfall happen

Answer 1

Hey mate, here is your answer :

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.
Hope this answer helps you !!!

Answer 2

Due to the heat of the sun, the water of the oceans evaporates in the form of water vapour. These water vapours condenses and become heavy in course of time. They become so heavy that the clouds become incapable to handle and thus, they fall down in the form of rain or snow.  Hope this helps you.