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A cloud droplet is very small compared to a raindrop. In fact, it takes about a million cloud droplets to make a single rain drop. Cloud droplets grow by the process of condensation. However this is a very slow process, and it would take days to grow a cloud drop to rain drop size due to just condensation. Raindrops grow in warm clouds due to collision and coalescence. Bigger drops fall faster and collide with smaller drops on their way down (collision) and sometimes those smaller drops will merge with the bigger drop helping it to grow (coalescence). In order for cloud drops to form there needs to be a high relative humidity and a condensation nuclei. In order for rain to form in a warm cloud there must be ample water content, a strong updraft, a thick cloud, varying size rain drops, and the correct electrical charges.