Electrical Property of Precipitating Clouds -Raindrop Charge-size Measurements-

Abstract

Observational studies of precipitation electricity have been made in order to clarify the mechanical relation between cloud electrification and the growth of hydrometeors. In this paper, an instrument for the simultaneous measurement of charge and size of raindrops is described, and the results of the raindrop charge-size and the electric field measurements for three typical rainfalls are also described. The electric field disturbances accompanying the passage of precipitating convective clouds usually implied that the clouds were electrified in an ordinary polarity of dipoles. When a cloud was isolated and non-thundery, the field often carried out a well defined W-shaped time change. Mirror-image relation between the polarity of raindrop charge and that of electric field seldom held. The field disturbances associated with thunderclouds implied the existence of appreciable point discharge at the ground. Large raindrops showed the mirror-image relation, while smaller ones did not. In both cases of thundery and non-thundery, some of the raindrops carried excessive charges that couldn't be explained by the processes of polarization charging or selective ion capture. In the cases of continuous rain, weak electric field is not likely to affect selective ion capture. The positive charge on a drop and the negative one are nearly the same in magnitude and fluctuate almost in phase. The larger the drop diameter is, the earlier the fluctuation in the drop charge rises, which implies that the charging processes mainly occur aloft. The relation of raindrop charge and the rainfall intensity implied the important role of the interaction of hydrometeors on cloud electrification.