Raindrops at the ground are surprisingly uniform in size, i.e. within 0.5-4 mm in diameter. The reason is that smaller drops fall slowly and evaporate before they reach the ground, whilst drops larger than about 4 mm or so tend to distort into an unsteady umbrella-like shape and then break up as they fall.
However, drops differ in heavy downpours from those in light falls. They are around 3 mm in diameter when the rate is 100 mm/h, but only 2 mm in 13 mm/h and 0.5 mm in drizzle. The relation between average drop size and rain rate is discussed further in Note 9.D.
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