Thunderstorm electrification of hail and graupel by polar dribble.
Hail and graupel falling through rain collect water that selectively dribbles upward from the upper surface of a hailstone. When the hailstones are polarized by nearly vertical electrostatic field these vertically discharged water drops carry away free charge of the same sign as that induced on the upper surfaces. The hail thereby accumulates an equilibrium charge of opposite sign, corresponding to the charges induced on the bottom surfaces. The equilibrium charges are large enough to be important in thunderstorms.
Gunn
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