Differential charge transport in thunderstorm clouds

Abstract

Measurements of charge versus size of drops within warm thunderstorm clouds indicate that the mean specific charge per gram decreases dramatically as the size increases. Because coalescence between smaller and larger drops occurs with an approximately geometric cross section, the charge neutralization length (the distance in which the smaller drops of one charge sign could neutralize the charge of the larger drops by coalescence) becomes small in comparison with cloud dimensions and hence ensures that any effective charge separation mechanism rapidly approaches steady state. The downward, steady-state, large drop mass flux averaged over the cloud is usually less than the upward, convected, small drop mass flux. At the interior of the cloud, quasi steady-state charge separation requires an equal and opposite charge flux carried by smaller and larger drops. The two conditions are inconsistent with current drop charge versus size measurements by several orders of magnitude. Currently, the assumption of equal and opposite charge flux, is not substantiated and hence charge separation by differential ‘falling’ of larger and smaller drops within the cloud should be a small contribution to the total charge transport.