ELECTRICITY OF CLOUD AND RAIN

IN his recent presidential address to the Royal Meteorological Society, under the above title, Sir George Simpson1 has given a full account of experimental results on the processes giving rise to electrical charges on raindrops and in clouds. Due largely to work in which Sir George has himself taken a big part, a fairly clear picture is now available of the electrical structure of a thunder-cloud, showing a positive charge above and a negative charge below, with a localized concentration of positive charge at a still lower level. There must be two processes of separation of charge: an upper one, in a region where the temperature is below freezing-point, and where the separation has been ascribed by Simpson and Serase2 to a process of friction between ice-particles, giving a negative charge to the larger particles and a positive charge to the air ; and a lower one, where the precipitation is entirely in liquid form, and where the separation of charge is ascribed to the breaking of drops, giving positive drops and a negative charge to the air.