This article reviews those mechanisms of cloud electrification which are based on gravitational separation of precipitation particles and examines at some length the inductive charging mechanism. It is concluded that through the inductive mechanism, glaciated clouds can develop a maximum current density Jmax of 0.1 µA m−2 with a maximum field Emax of about 3.5 × 105 V m−1 (which values are necessary for the occurrence of lightning flashes in them) within a time of 1000 s provided that the precipitation intensity in them is greater than 1.39 × 10−5 m s−1 (50 mm hr−1). For precipitation intensities less than 8.33 × 10−6 m s−1 (30 mm hr−1), Jmax and Emax cannot develop to such high values. By its inherent nature this mechanism does not work in warm clouds.

A time dependent model for the rate of growth of electric field at various points at the ground surface due to a thundercloud of finite dimensions has been presented. Calculations have been made for various sets of paramters viz., precipitation intensity *0, vertical cloud motion U, rebound mean angle probability <*>, cloud radius W and the distance of the surface observation point D. The charge within the charging zone of the thundercloud is computed by the combination of both precipitative and convective charging mechanisms. We have obtained: (Eout)max*-9*103Vm-1 within 1200s at the ground surface (D=2km) due to a thundercloud (W=1.0km), if *0=10mm h-1 and U=3 ms-1 (taking H=0.5km, L=1.5km, h=5.0km, *1= *2=0.25, *=100s, <*>=1), where H is the vertical length of both lower main negative and upper main positive charge regions; L is the length of charging zone on each side from the center of the cloud; h is the vertical height of the cloud base from the ground; *1, *2 are the fractional constants for screening charge density and electric field, respectively; and * is the relaxation time. The obtained results are consistent with the observations of electric field due to a thundercloud at the ground surface.

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Considering the growth of hydrometeors along with electrical state and interaction among all categories of particles in thunderclouds, the effect of screening charge transport on electric field, precipitation intensity, charge, size and fall velocity of hydrometeors, has been studied. The study reveals that screening charge transport increases the growth rates of electric fields and decreases the precipitation intensities. The calculations show that thunderclouds containing low precipitation intensities, can also generate high electric fields within appropriate time intervals.