Electric potential gradient and current during steady precipitation

Abstract

An outdoor site adjoining Durham Observatory was instrumented to measure the atmospheric potential gradient and precipitation current at the ground during periods of steady, quiet precipitation. A system was constructed to automatically record these quantities and to present the data in a form suitable for input to a computer. Examination of most periods of quiet precipitation between January and June 1972 shows that rain is usually positively charged, with the potential gradient being negative, while during snow these signs are reversed. There is nearly always significant correlation between the two electrical quantities, with during rain variations in potential gradient most often leading those in precipitation current by several minutes; during snow the precipitation current leads by a similar amount. These two effects correspond to the so-called "inverse relation" and "mirror-image- effect" often quoted in previous work. These results are shown to be consistent with the precipitation charge being due to two charging processes, one acting on solid precipitation within the cloud, and the second occurring during the melting of snow to rain. Examination of aerological data shows that the time lags between corresponding variations of the electrical quantities can be explained during rain by the effects of the wind shear between the cloud and the ground on the falling precipitation. The different electrical behaviour of a few periods of precipitation can be explained by ,the effects of a slow-moving cloud where electrical development is taking place, rather than by a passing cloud with constant electrical activity. Comparison of periods of quiet precipitation with "disturbed" periods, when the electrical activity is much greater, suggests that the transition to disturbed precipitation occurs when the rate of electrical sign reversals exceeds 2 per hour and the precipitation rate exceeds 1-0 mm hr ,^ The connection between the degrees of electrical and meteorological activity agrees with the suggestion that the electrical activity reflects the degree of atmospheric stability within the cloud.