From AMS Glossary
The downward transport of charge, from cloud region to earth, that occurs in a fall of electrically charged rain or other hydrometeors; a particular case of a convection current.
Observations of the charge on individual raindrops during thunderstorms have revealed a complex picture. On average, more positive than negative charge is brought to earth by precipitation currents, but wide deviations occur both within individual storms and from one storm to another. The reasons for these wide fluctuations are not understood. Precipitation currents in continuous rain generally vary from about 10-12 to 10-10 A m-2, while thunderstorm currents become as large as 10-8 A m-2.