Melting ice and cloud electrification

Abstract

A vertical wind tunnel has been constructed inside a cold room to simulate the fall of a frozen water drop from the 0 C level in a thundercloud. It was possible to freely support an ice sphere clear of the sides but the particle crashed early in the melting process. The ice spheres were frozen onto a 120 pm diameter platinum wire and during the final stages of melting the particle hung from the wire and was free to rotate about all 3 degrees of freedom. The spheres melted on this type of support produced the same amount of electrification as those melted on the wire loop support used by DRAKE (1968).The charge on the meltwater was found to be always positive and to be highly dependent on the freezing rate, and water drops frozen in still air at between -10 and -15 C produced an order of magnitude less charging than drops frozen in an airstream flowing at 11 ms(^1) at similar temperatures. Examination of the ice particles under a microscope suggested that this effect was due to air escaping from the ice at low freezing rates and smaller air bubbles being formed at high freezing rates. Evidence was found for the enhancement of electrification at high melting rates which DRAKE attributed to the onset of vigorous convection in the meltwater. The effect of carbon dioxide on melting electification was also discussed. The electrification due to melting precipitation under ideal conditions in a thundercloud was estimated as 4 C km(^3) which can be compared to 8 C km(^3) found by SIMPSON and ROBINSON (l94l). It was suggested that the importance of melting ice in thundercloud electrification cannot be established until more information is available on the nature of the solid precipitation, the environment in which it melted and the location and magnitude of the lower positive charge.