Generation of electric charge associated with the formation of soft hail in thunderclouds

Abstract

The electrical charging which results from collisions between ice crystals and a simulated hailstone is measured as a function of their temperature difference, and of the size and impact velocity of the crystals. It is found that the sign of the charging is governed by that of the temperature difference, the hailstone becoming negatively charged if it is warmer than the rebounding crystals. The magnitude of the charging is proportional to the temperature differ­ence but rather insensitive to the size and impact velocity of the crystals. With a temperature difference of 5°C, a rebounding crystal of diameter about 50 μ produces, on average, a charge of 5 x 10 -9 e. s. u. The electrification of an artificial pellet of soft hail growing by the accretion of supercooled water droplets (riming) is also investigated. Freezing of the droplets on the hailstone is accompanied by the ejection of positively charged ice splinters, the hailstone acquiring a negative charge. The manner in which the rates of charging and splinter produc­tion vary with the air temperature, drop diameter and impact velocity has been established. In a typical experiment, with the air temperature at -15°C, droplets of diameter 80 μ impacting at 10 m s -1 freeze to produce, on average, 12 splinters and a charge of 4 x 10 -6 e. s. u. per drop. Droplets of diameter less than 30 μ produce few splinters and little charging. The results of both sets of experiments are interpreted in terms of the authors’ theory of charge separation in ice under the influence of a temperature gradient, and are used to calculate probable rates of charge generation in thunderstorms. It appears that the electrification which accompanies the growth of pellets of soft hail through the freezing and splintering of supercooled droplets is capable of generating and separating charge at the required rate of about 1 C km -3 min -1 but, while rebounding ice crystals will usually charge the hailstones in the same (negative) sense, this mechanism will contribute only slightly to thunderstorm electrification.