From AMS Glossary
(Also called spray electrification, waterfall effect.) The separation of electric charges accompanying the aerodynamic breakup of water drops, first studied systematically by the German physicist P. Lenard (1892).
Experiments have shown that the degree of charge separation in spray processes depends upon the drop temperature, presence of dissolved impurities, speed of the impinging air blast, and contact with foreign surfaces. The largest fragments of the broken drops are observed to carry positive charges and the fine spray of drops carried off in the impinging air current carries a net negative charge. Distilled water drops of 4-mm diameter, broken after a 5-cm free fall into an updraft of 1 m s-1, were found by Chapman (1953) to yield about 10-10 C of separated charge per drop. The Lenard effect was incorporated by Simpson (1927) into his breaking-drop theory of thunderstorm charge generation, but many critical details are but poorly understood.
Chapman, S. 1953. Thunderstorm Electricity. Byers, H. R., ed., . 207–213.
Simpson, G. C. 1927. The mechanism of a thunderstorm. Proc. Roy. Soc. A. 114. 376–401.
Lenard, P. 1892. Über die Elektrizität der Wasserfälle. Ann. Phys., Lpz. 46. 584–636.