SUMMARYCollisions between vapour-grown ice crystals and a riming target, representing a graupel pellet falling in a thunderstorm, were shown by Reynolds, Brook and Gourley to transfer substantial charge, which they showed to be adequate to account for the development of charge centres leading to lightning in thunderstorms. Related experiments by Takahashi and Jayaratne et al. determined that the sign of charge transferred is dependent on the cloud liquid water content and on cloud temperature. There are marked differences between the results of Takahashi and Jayaratne in the details of the dependence they noted of the sign of graupel charging on cloud water and temperature. More recently, Pereyra et al. have shown that results somewhat similar in form to those of Takahashi are obtained by modifying the experimental technique used to prepare the clouds of ice crystals and supercooled water droplets used in the experiments.In order to help resolve the reason for the differences in charge transfer results in various studies, work has continued in the Manchester laboratory with a modified cloud chamber in which the cloud conditions of the crystals and droplets may be controlled independently. Results indicate a profound effect on the charge sign of the particle growth conditions in the two clouds involved. For example, by suitable adjustments to the water contents of the two clouds, graupel is charged negatively by rebounding ice crystal collisions at higher cloud water contents than have been noted previously. It is suggested that the most important influence on charge sign is the relative diffusional growth rate of the two ice surfaces at the moment of impact and that this is affected by an increase in cloud supersaturation experienced by the ice crystals during the cloud mixing process just prior to collision. A range of cloud conditions is used in the present work in order to help determine the reasons for the various results reported previously.Examination of some thunderstorm observations in the context of the present results points to the importance of mixing on the sign of the charge transferred during particle collisions when two cloud regions of different histories mix together.

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The terminal velocity of cloud particles may be particularly important to climate studies, because they may have a non-negligible effect on the radiation budget of the planet, and on the lifetime of clouds. In the present study, the terminal velocity of individual columnar ice crystals was measured under controlled laboratory conditions. The size and fall orientation were determined for ice crystals with maximum dimensions less than 160 μm. The results, which are in agreement with the fall velocity presented in previous studies, show a random orientation of the columnar ice crystals during free fall with a fall velocity which is best represented by their capacitance. A linear relationship between fall velocity and ice crystal capacitance is presented that could be used in cloud modelling. Although the mass of ice crystals was not measured, an estimation of this parameter was made and an empirical Best-Reynolds power-law relationship was found.