The electrification of water drops on freezing or belting at terminal velocity in air

Abstract

Measurements on the electrification on freezing of individual water drops of diameter between 3 and 5 mm supported by an air stream have shown that the freezing behaviour of the drops is temperature-dependent. Above -10 c the freezing progressed uniformly throughout the drop from a single point. Below -10 c the outer surface of the drop froze rapidly with the exception of a small area at the top of the drop. Freezing then progressed uniformly from the base of the drop upwards. No shattering or electrification of the drops were observed during their freezing. These results are contracted with those of previous workers who observed the freezing of drops suspended on fibres. Measurements of the electrification of ice spheres supported by an air stream have indicated that the sign of the charge acquired by the spheres due to riming is temperature dependent. When the sphere is rimed by droplets at temperatures above -10 c it acquires a negative charge, and when the riming droplets are at temperatures below -10 c the ice sphere acquires a positive charge. An explanation is proposed for this effect in terms of the observed freezing behaviour of individual water drops. It is suggested that this effect could lead to thunderstorm electrification of the observed polarity. Measurements on the electrification of melting ice spheres supported by an air stream indicated that the sign of the charge acquired by the spheres is dependent on whether or not water is flung off from the sphere during melting. If no. water was flung off the charge was positive, while if water was flung off the charge was negative. This may explain the discrepancies between the laboratory and field measurements of MacCready and Proudfit.