A laboratory investigation of the electrification of snowstorms

Artificial snowstorms were created inside a large cold room by blowing snow crystals over a snow surface. The charges acquired by the visible blown particles and the residual snow surface and the electric fields and concentrations of positive and negative ions produced in the air above the snow surface were measured as a function of the wind velocity, U, the relative humidity, the amount of snow introduced into the air jet, and the temperature of the jet and the snow surface, Tj and Ts respectively; all of these variables were monitored and recorded continuously. In a typical experiment with Tj = − 20°C, Ts = −40°C, U = 430 cm sec−1, and snow crystals introduced into the air jet at a rate of 1 g sec−1 the snow surface acquired a charge of 5 × 103 e.s.u. in 10 sec, the charge residing on the visible blown particles was approximately equal and opposite to this amount, and the concentrations of positive and negative ions created in the air were 4,600 and 4,800 cm−3 respectively, corresponding to a net charge carried on the air within the cold room of less than 1 e.s.u. The charging was found to be a sensitive function of (Tj — Ts) over a very wide range but was independent of Ts except at high temperatures when large‐scale melting and erratic charging were observed. The charging was found to increase rapidly as U was increased from 1 to 5 m sec−1, and was slightly reduced as the relative humidity was raised. The measured electrification at temperatures below 0°C is shown to be entirely explicable, quantitatively and qualitatively, in terms of the temperature‐gradient effect. The charge acquired by the snow surface and the blown particles is a result of fragmentation of snow crystals which have temperature differences across them, asymmetric rubbing between snow particles and transient contact between crystals of different temperatures, all of which processes are manifestations of the temperature‐gradient effect. The primary source of the charge carried by the air is shown to be point discharge between highly charged particles. The contributions to the measured electrification of the Workman‐Reynolds effect, the Dinger‐Gunn effect and the evaporation of ice are shown to be minimal.