Melting Electrification of Single Ice Particles in Simulated Free Fall

Abstract

Ice spheres of diameter 3 mm were allowed to melt while supported by horizontal fine wires in the vertical flow of a wind tunnel. The air speed and hence the drop ventilation were held close to the natural free fall values. The meltwater charge was positive, reaching 1.1 pC mg^-1 (= 1.1 μC kg^-1). The lower the previous freezing environment temperature, the higher was the melting charge. But for corresponding freezing temperatures the drops frozen in an airstream gave about seven times the charge of those frozen in still air, suggesting that the earlier freezing rate largely controls the charge of a ventilated ice particle on melting. As freezing temperatures were lowered the proportion of larger bubbles in the ice decreased (as reported by other workers), but at any one temperature those frozen in an airstream had mainly smaller bubbles than those frozen in still air. This association of smaller bubble diameter with large charge is in general agreement with the theory of Iribarne and Mason. We would expect these results to apply to the freezing and later melting of precipitation water in the atmosphere, especially in strongly convective conditions, the effect contributing significantly to the lower positive charge in the base of the thundercloud.

The Paper was presented by W. C. A. Hutchinson.