From AMS Glossary
A theory of thunderstorm charge separation based upon the suggested occurrence of the Lenard effect in thunderclouds, that is, the separation of electric charge due to the breakup of water drops.
This theory, advanced by Sir George C. Simpson (1927), was initially intended to account for a bipolar charge distribution within a thundercloud having the main positive charge center near the base of the cloud and the main negative charge center higher up. Simpson's theory, however, does not explain this phenomenon well because temperatures are below the freezing point in this region of the cloud. Evidence does remain to support a weak positive charge center that lies slightly below the lower main negative charge concentration in many, if not all, thunderclouds where the temperature is above the freezing point. Hence, the breaking-drop theory is best ascribed to this localized secondary positive charge center.
Simpson, G. C. 1927. The mechanism of a thunderstorm. Proc. Roy. Soc. London A. 114. 376–401.