|MadSci Network: Earth Sciences|
The size of a raindrop when it falls to the ground is dependent on many factors, including the origination process of the rain itself, rainfall intensity, evaporation rates, air turbulence, and wind.
The processes that allow cloud droplets to become raindrops are very complex and not well understood. Droplets that reach 0.2 mm, the threshold for calling a drop rain, are usually heavy enough to overcome the force of ascending air that exists in every cloud. Rain can be either "cold rain", originating from melting snowflakes, or "warm rain", which evolves without the snowflakes. Cold rain has very large drops, but fewer drops overall. Warm rain contains very many, but small drops. Raindrops can range in size from 0.2 mm in diameter, to around 6 mm. Those larger than 6 mm are inevitably broken up in their fall to the earth. Raindrops are very rarely all the same size in any one rain. There are generally more small drops than large drops, but as the intensity of the rainfall increases, the number of larger drops grows. The very largest drops are found only in downpours with rainfall rates greater than 2 inches per hour.
Once a raindrop has formed and begun to fall, evaporation of water from the droplet may reduce its size, and turbulence in the air may induce further collisions and breakups either enlarging or reducing the raindrop's size. Wind can separate the drops according to size, with the larger ones falling to the ground faster, while the smaller ones are blown with the wind or the raindrops may get caught in updrafts or downdrafts within the cloud.
The path a droplet takes on its way to becoming a raindrop on the ground is a very complex one, with literally hundreds of possible outcomes. An interesting book about raindrops is From Raindrops to Volcanoes by Duncan Blanchard. It is no longer in print, but is available in many libraries.
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