Electrical charging of hail pellets in a polarizing electric held

Abstract
Earlier experiments in which the authors investigated the electrical charging of a simulated hailstone by collisions with ice crystals, and by the accretion, freezing and bursting of supercooled droplets, have now been repeated in the presence of polarizing electric fields of up to about 1000 Vcm -1 , which are typical of large-scale fields in thunderstorms. It is found that such fields have no detectable influence on the charging produced by rebounding ice crystals, apparently because the times of contact are less than the relaxation times for the conduction of charge between the particles. After much longer times the charge A q transferred between two ice spheres of radius R, r in a uniform polarizing field E agrees with the theoretical equation Δ q = {γ± E cos θ ={γ2( q/R 2 )} r 2 , where γ 1 and γ 2 are calculable functions of r/R , q is the charge on the larger sphere and θ is the angle between the line of centres and the field. The dependence upon time of contact, particle shape and surface temperature is also investigated. The charging which accompanies the impaction and freezing of supercooled droplets on the hailstone is altered by only about 10 % by the application of fields of order 1000Vcm -1 . The conclusion is that the charging of hailstones by either process will not be greatly accelerated by the build-up of polarizing fields in thunderstorms.

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