WHEN the particles of water or ice which constitute a cloud or fog are all of the same size, and the air in which they are sustained is at rest or is moving uniformly in one direction, then these particles can have no motion relatively to each other. The weight of the particles will cause them to descend through the air with velocities which depend on their diameters, and since they are all of the same size, they will all move with the same velocity.
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Journal of the Meteorological Society of Japan. Ser. II (1985)
Quarterly Journal of the Royal Meteorological Society (1978)