The generation of electric charges and fields in thuderstorms

Abstract

The generation and separation of electric charge and the growth of electric fields in thunderstorms are accounted for by the rebound of a small fraction of the cloud droplets impacting on the undersides of small pellets of soft hail, thereby carrying away some of the polarized charge induced by the prevailing vertical electric field. It is shown that just those droplets making grazing incidence with the millimetre-sized hail pellets are able to separate sufficient charge to create large-scale fields of 4000 V cm^-1 within 10 min if, during this time, the precipitation intensity builds up from zero to 20 mm h^-1 to produce a total precipitation of about 2 mm. The growth rate of the electric field is determined largely by the precipitation intensity, which is related to the size, density and hence the falling speed of the hail pellets. The field, starting from the initial fine-weather value, grows exponentially but slowly at first, then more rapidly as the hail pellets grow and because the mechanism is self-accelerating, but eventually slows down as the electrical forces on the charged hail pellets and rebounding droplets counteract the gravitational forces and slow down their rate of separation. Calculation of the trajectories of the droplets that make grazing contact with the hail pellets of different sizes and densities, ρ_i, allow the collision cross sections and impact angles,θ, to be determined. Charge separation is proportional to cosθ and is most effective when 0.1 < ρ_i ≤ 0.5 g cm^-3. Very light hail pellets with ρ_i < 0.1 g cm^-3 become ‘suspended’ in the electric field before this reaches breakdown strength, whereas the faster-falling particles of ρ_i > 0.5 g m^-3 force the droplets to rebound very close to the electrical equator of the hail pellet where they carry away little charge. It is shown that thunderstorm cells of radius 2 km would be able to produce a succession of lightning flashes at intervals of about 30 s as long as the updraught and precipitation rate are maintained. Very intense lightning activity, with flashes at intervals of less than 10 s, would require storm cells exceeding 5 km in radius, but is more likely to be produced by large multicellular storms.