Why there is Rainfall from Cloud?

Inside a cloud, as there are continuously supply of water vapor, condensation and sublimation are also occurred continuously to the supersaturated water vapor causing growth of cloud droplets. The cloud droplets may then fall due to their size and weight. Larger cloud droplets of faster falling rate may collide with the smaller droplets of slower falling rate and thus grow larger in size. This process is called coalescence. As coalescence process continuously occurs inside a cloud, cloud droplets grow larger and larger until they are too heavy to remain in the cloud and thus fall toward the surface as rain. 

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