|MadSci Network: Earth Sciences|
A very good question. Raindrops grow by collision and coalescence...a process of drops bumping into one another and sticking together, or by deposition.
Typically, larger raindrops grow so large because a strong updraft exists within the cloud. This is typical in thunderstorms. In that case, the drop is held in the air and has more time to bump into other drops and grow. When it reaches a certain size and is too heavy to remain suspended, it falls. Remember, this is dependent on the strength of the updraft.
In the case of light rain, updrafts are generally weak and the drops don't have to grow as large to fall to the ground.
In the case of mist, it is possible that the air at the ground is completely saturated (like in the case of a dense fog). In that case, it is basically the same as being inside a cloud. The mist can actually be suspended cloud droplets rather than raindrops.
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