However, this study indicates that attempts to measure vertical speeds accurately by use of the continuity equation may not be too successful. Before the meteorologist can attempt sagaciously to modify rainfall from clouds or before he can predict with any degree of certainty the distribution of unmodified rain over a geographical area, he must gain an understanding of the thermodynamic and kinematic principles that describe the formation and subsequent evolution of this rain. In the study of the physical processes of precipitation, an important consideration is the delineation of the spatial distribution of water in the cloud and the variations of this distribution with time. The primary purpose of this study was to investigate the feasibility of using a weather radar to obtain precise measurements of the formation of rain in cumulus clouds. A secondary objective was the accurate measurement of the vertical velocities in a cloud that are related to the process of rain formation.
Convective clouds form as the result of the expansion and consequent cooling of ascending air.
Cloud droplets will form by the condensation or sublimation once a sufficiently high relative humidity is reached. In the case of clouds which extend to heights in the atmosphere where temperatures are much less than OC, an important mechanism for the formation of rain is one in which drops begin their life as ice crystals and then grow rapidly at the expense of surrounding water drops. To account for rain which falls from clouds that do not reach the height of the OC-isothermal surface, another mechanism appears to be operative. Growth of the droplets to raindrops will occur if there are adequate collisions and coalescence between the smaller cloud droplets. The probability of a collision is enhanced by vertical velocities that are strong enough to support the droplets and maintain a relatively high concentration in the cloud.