Convective storms, ice microphysics and charge separation

For lightning to occur, positive and negative charge must separate in the cloud. The key is deep convection and ice microphysics: strong updrafts carry moisture high into the freezing layers, where tiny ice crystals, larger graupel and supercooled water droplets exist simultaneously.

In collisions of these particles — especially between light ice crystals and heavier graupel — charge transfer occurs. The lighter, positively charged crystals are carried upward, while the heavier, negatively charged graupel descends. This causes the charge in the cloud to separate vertically and creates the voltage needed for a discharge.

From this follows a fundamental connection: lightning occurs only where there is enough deep convection and ice phase. The intensity and frequency of storms therefore depend very sensitively on temperature and humidity — and this very dependence is the physical basis of why the SR functions as a global thermometer Williams 1992.