On the relation of the terminal fall speed of hydrometeors to environmental conditions and particle size and density

Expressions are derived which relate the terminal fall speed of hydrometeors to their size and density and to the environmental parameters of the air, and for extrapolating terminal velocity measurements under known conditions to other conditions and altitudes. The formulation is an extension of previous work and is based on the assumption that on average the drag coefficient is related to the Reynolds number by a simple power law over a reasonable range of Reynolds numbers. This is shown to imply a power law dependence between the terminal fall speed and diameter of a particle, as is often observed, and also to be consistent with empirical power law relations between the Best and Reynolds numbers of falling particles. A set of terminal fall speed parameters is presented which is representative of a variety of precipitation types.