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Study Investigates Role of Electrostatic Attraction Between Drops in Rain Formation

Aerosol particles play a central role in cloud microphysics and hence in the formation of precipitation. In warm clouds, the process of precipitation formation begins with the nucleation of cloud droplets on aerosol particles. Many studies have explored the possibility of sending an electric charge into aerosol particles to induce precipitation.

Such methods can potentially help countries like the UAE reduce the risk of water stress given the high levels of dust and aerosol particles in its atmosphere due to its arid climate. In an attempt to understand the role of electrostatic attractions between drops in rain formation processes, Giles Harrison, Professor of Atmospheric Physics at the University of Reading in the United Kingdom and a second cycle awardee by the UAE Research Program for Rain Enhancement Science (UAEREP), worked on a study into the effects of charge on the behaviour of drops and many aerosols by modelling them electrically as conducting spheres.

The electrostatic interaction between two charged spheres is a remarkably complex process that requires summation over an infinite series of pairs of image charge interactions. Treating cloud drops as conducting spheres is a good approximation to reality as it simplifies the problem of their electrostatic interaction considerably when compared with the interaction between dielectric spheres.  

The study shows that the variability in charge across the drops leads to a net mean attraction between the drops, irrespective of the mean polarity of the charge of the drops. This has implications for our understanding of the role of electrical effects in processes such as warm rain formation or aerosol coagulation.

The study concludes that the ubiquitous presence of ion capture by drops in the atmosphere ensures that drops are attracted to each other at close ranges, irrespective of the presence of any mean drop charge. This is because the mean effect of drop charge fluctuations always yields an attractive interaction. The time scales for the evolution from cloud droplets to raindrops consequently become more rapid than for the neutral case.

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