No abstract

The growth of waterdrops by inertial capture and coalescence induced by the size‐dependent terminal velocities in the earth's gravitational field is an important process in the formation of rain. The extensive experimental data on this fundamental process are reviewed and organized into a coherent summary of the collection efficiencies applicable to natural clouds, in the following circumstances: negligible effects of electric charges and fields, normal laboratory temperatures and pressures, and relative humidities between 50 and 100%. The judgments required to reduce the experimental data to the brief summary form are documented.

The charging of uncharged, equal or very nearly equal radius drops falling freely in an electric field has been observed to occur in two different modes. When the electric field exceeds a threshold value that is a function of drop size, the charge is transferred via a spark in the air between the near surfaces of approaching drops. Below this threshold, the rate of charge transfer is slower and varies with the conductivity of the water. The onset of the spark transfer mode occurs when the voltage and the separation distance between the surfaces of the drops reaches the minimum sparking potential required for breakdown between parallel plate electrodes. The ambient field strength at the onset of sparking varies with drop size according to the theoretical enhancement of the local field between the surfaces for a fixed separation. The information obtained in this way assists studies of the electrification of clouds and cloud particles and the growth of precipitation in thunderstorms. The previous work on radio emission from colliding charged drops can presumably be extended to include uncharged and weakly charged drops in electric fields.

Experimental measurements of the collision efficiencies of nearly equal cloud droplets falling freely in a vertical wind tunnel are presented. The efficiencies so measured increase with increasing droplet radius, exceeding unity when the droplets are larger than 40‐μ radius. The results are compared with earlier experiments on interactions between droplet pairs falling in still air chambers.

A droplet generator has been developed which can produce small droplets one at a time, in groups with adjustable time intervals between droplets or in a continuous stream with controllable production rates of up to 100 per second for the smallest size. Size, charge, and time of production as well as horizontal position at the time the droplet is made may be changed electronically from droplet to droplet. Droplets are produced with nine adjustable and overlapping size ranges covering radii from 4 to 130μ. An application is described where one droplet generator is used in a cloud physics study of collisions between pairs of droplets of unequal size and charge.