Charge transfer accompanying individual collisions between ice particles and its role in thunderstorm electrification

Experiments were conducted with a wind tunnel in a cold-room in order to investigate the individual charges transferred when ice spheres collided with an artificial hailstone. The charge transferred to a sublimating hailstone was negative and had a magnitude proportional to the velocity of impact and to the diameter of the ice sphere to the power 1.7. The charge transferred when 100μm diameter ice spheres collided with the hailstone at a velocity of 8ms−1 was typically –15 fC. No variation of charging could be detected over the temperature range –5 to –25 °C, or when the ice was doped with impurities, or when hailstones of different surface roughness were prepared. Hailstones which were cooled below the ambient cold-room temperature and subsequently grew by deposition, charged positively by typically 100 fC per collision. However, a hailstone which was cooled by the same amount but was maintained in a sublimating condition charged negatively. These experiments show that thermal effects did not play a direct role in the charge transfer process. Experiments were performed with a riming hailstone in order to simulate the conditions found in a natural cloud. The hailstone charged positively at temperatures of –5°C and –10°C and negatively at –15°C and –20°C with liquid water contents in the range 0.05 gm−3 to 0.85gm−3, the magnitudes of the charges being typically 30 fC. It is suggested that the charge transfer occurs from one ice surface to the other at the contact interface and that the driving force for the charge transfer at the contact interfaces is the different surface potentials and/or different charge carrier densities of ice surfaces formed in different ways. It is possible that the charge carriers may be surface ions of a liquid like layer on the ice surface. The results with the riming hailstone are consistent with thunderstorm observations that the negative charge centre is above the –10°C isotherm and that, although precipitation particles are predominantly negatively charged, a mixture of signs is usually present.

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