Discussion of paper by W. H. Evans, ‘Electric fields and conductivity in thunderclouds’

A primary difficulty facing scientists attempting to solve the long-standing and perplexing mystery of thunderstorm electrification is that of obtaining good measurements of the electrical variables inside the cloud. Workers in this field are, therefore, indebted to Evans [1969] for initiating a new approach to this problem, a dropsonde device for measuring the electric field and conductivity. As Evans has noted, the tentative conclusions he draws from his experiments are at variance with commonly accepted ideas in that they show that the electric field is considerably smaller, and the conductivity considerably higher, than many investigators had believed. Conceivably, Evans' results may provide experimental verification of predictions of regions of high conductivity within the cloud, such as the predictions of Freier [1962], Doyle et al. [1964], Fuquay [1967], Vonnegut [1968], and Abbas and Latham [1969]. Such a finding would have implications of such importance to all ideas of thunderstorm charge generation that it appears appropriate to consider several questions that arise concerning Evans' measurements. In the discussion that follows it will be assumed that the instrument correctly measures both the electric field and conductivity in the vicinity of the field mill. It will be questioned, however, whether the values determined from these measurements are representative of the electric field and conductivity that would naturally exist elsewhere within the cloud.