Electrification processes in warm rain clouds

Abstract

The problem of electrical charges on raindrops from non-thunderstorm clouds is investigated. Measurements of raindrop charges, precipitation currents and space charges made in various parts of the world indicate that on the average positive charges prevail on raindrops. To explain rain charges, a well known hypothesis requires the melting of snow. In Hawaii, however, rain often forms by condensation and coalescence without ice or snow in the upper portions of the cloud. Yet raindrop charges do not differ from charges measured in other type s of rain. Measurements of charges on Hawaiian rain are discussed together with observations of electric field and condensation nuclei profiles. It was found that positive charges are of the order of 10^-4 esu. A correlation of these charges with wind speed was found. Electric fields in rain clouds are generally positive except at the lower cloud boundary where negative fields of up to -1000 V/m exist. Several thunderstorm theories are discussed and their applicability to the warm rain mechanism is examined. A new hypothesis is necessary to explain the observed electrical features of warm rain. A theory based on influence is presented and formulated. Due to low electrical conductivity inside the cloud, positive space charges form in the upper:. part of the cloud and negative in the lower, influenced by an impressed conduction current from above the cloud. These space charges diffuse to the raindrops. Considering a coalescence mechanism for drop growth, the residence time of raindrops in the negative space charge area is much smaller than the corresponding time in the positive area, and so the drops usually leave the cloud with a positive charge. Numerical solutions to drop charges and electric field profiles using a realistic cloud model were obtained through the use of digital computer techniques. The theoretical results for charges on raindrops and electric field patterns agree well with observations.