Effect of atmospheric space charge on initial electrification of cumulus clouds

During the summer of 1960, experiments were conducted in Illinois to determine how the electrical potential gradient and space charge above and beneath fair-weather cumulus clouds depend on the polarity and concentration of the space charge in the air from which they grow. Artificially produced space charge was released into the atmosphere near the ground during convection. Measurements showed that clouds that form in the electrically modified region exhibited significantly larger potential gradient perturbations than otherwise similar clouds forming nearby. When the charge released was negative, the fair-weather potential gradient over the cloud was enhanced; when it was positive, the potential gradient was decreased or of reversed polarity. Measurements just beneath the cloud base showed that space charge of the polarity released near the ground was being carried up into the center of the cloud and that space charge of the opposite sign was descending around the outer portions of the cloud. The experimental results show that the polarity and intensity of electrification of small nonprecipitating cumulus clouds is determined primarily by the polarity and concentration of the space charge in the air from which they grow. The agreement between the observations and predictions based on the convective electrification process proposed by Grenet and Vonnegut is consistent with the idea that electric charge is being carried by updrafts and by downdrafts as they have suggested.