A PRELIMINARY QUANTITATIVE ANALYSIS OF PRECIPITATION MECHANISMS

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Abstract
The two principal physical mechanisms for the formation of precipitation are considered to be the Bergeron-Findeisen ice-crystal process and the collision of particles of unlike size in the gravitational field. Quantitative estimates of the growth of precipitation elements by the two processes are made. It is found that the ice-crystal effect is much more rapid than the collision process in the initial stages of growth. When the particles are of a mass comparable to drizzle drops, the two processes are equally effective; for larger particles the accretion process becomes more rapid than the ice-crystal effect. For typical mid-latitude conditions it is concluded that neither process, acting alone, is capable of producing rain drops of the customarily observed size. It appears that there is required some combination of the two processes such as the clumping of ice crystals to form snow flakes, the growth of ice crystals which melt and then grow by accretion, or the growth of graupel by the simultaneous effects of accretion and sublimation. Particularly in low latitudes there is reason to believe that the accretion process alone can explain the formation of sizeable rain drops. Lack of adequate data makes some of the numerical results uncertain, but it is not believed that this affects the validity of the principal conclusions.

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