The evaporation of raindrops and the role of collision with cloud droplets in rain of ice crystal origin

Following ananalysis of the occurrence of rain at Adelaide, S.A, during 1957-58-59, in relation to cloud characteristics and air mass trajectory, in which possible evaporation effects were indicated, a more detailed consideration is presented of the effects of evaporation in the sub-cloud layer, of accretion of water drops in the warm part of the cloud and of the intensity of rain originating as ice crystals in very thick stratified clouds. It is suggested that the intensity of rain of ice crystal origin, from very thick stratiform clouds, depends to a large extent on the mass increase of rain drops in the layer of cloud warmer than 0°C, and the decrease by evaporation effects in the layer from cloud ba se to ground. A “Dryness Index” is postulated, based on the empirical formula of Kinzer and Gunn (1951), to estimate the shrinkage of drops in the sub-cloud layer, and a “Coalescence Factor " is defined. It is suggested that moderate and heavy intensities of rain occur only when the Dryness Index is less than 5.0, while when the Dryness Index is high only weak intensities, say less than 4 points per hour, occur.