RAINDROP ELECTRIFICATION BY THE ASSOCIATION OF RANDOMLY CHARGED CLOUD DROPLETS

Abstract

Light ions generated in the atmosphere by cosmic rays and radioactivity normally diffuse onto cloud droplets and establish a nearly symmetrical Gaussian distribution of positively and negatively charged droplets. When the cloud becomes unstable, these cloud elements grow by association and the charges accumulate to establish a new equilibrium consisting of nearly equal numbers of larger and highly charged positive and negative cloud droplets. Growing raindrops or graupel falling through such a cloud are accordingly bombarded by both positive and negative droplets, and this establishes a statistical accumulated charge on the various drops. The distribution of the number of drops in relation to the sign and magnitude of the free charge is worked out from basic principles and shown to agree remarkably well with the magnitude and distribution of drop charges measured inside precipitating clouds. The mean drop charge, irrespective of sign, depends on the square root of the drop size and kinetic energy of the small droplets relative to the larger moving drops. An equipartition is established between the electrical potential energy carried by the larger drops and the relative kinetic energy of the smaller drops. The estimated drop charges on warm rain or graupel are sufficiently large to account for thunderstorm phenomena, provided only that some unspecified process systematically separates the positively and negatively charged drops. The charges produced at the rain forming level commonly approximate 50 electrostatic units per gram. The observed electrification of quietly falling rain is primarily a manifestation of the ionization produced in the atmosphere by radioactivity and cosmic rays.