The salient facts concerning the dynamical, physical and electrical properties of a thunderstorm, and of the detailed structure and associated electric field-changes of lightning flashes, are marshalled to deduce the criteria for a satisfactory quantitative theory of charge generation and separation leading to the growth of electric fields strong enough to initiate and to sustain lightning activity.

A quantitative theory is presented of how charges are generated and separated when supercooled cloud droplets make grazing contact with the undersides of hail pellets (graupel) polarized initially by the Earth's fine-weather electric field. The rebounding droplets acquire a positive charge and are carried by the convective updraught towards the top of the cloud, while the hail pellets carrying a net negative charge fall towards cloud base. This creates a vertical dipole field which increases the polarizing charges on the hail pellets and so accelerates the rates of charge generation and separation, and so reinforces the vertical electrical field, which grows exponentially until insulation of the air breaks down and triggers a lightning flash.

It is demonstrated that a thunderstorm cell, 2 km in diameter, producing small hail falling at 30 mm h⁻¹ can produce vertical electric fields of ~5000 V cm⁻¹ in about 10 min involving the separation of ~50 C of charge, enough to initiate a lightning flash which, on average, neutralizes about 20 C. As long as the hail persists, it continues to generate and separate sufficient charge to produce a succession of lightning flashes at about 30 s intervals. More frequent discharges at say 10 s intervals would require high rates of hail production in larger cells but are more likely to be produced by large multi-cellular storms sustained by strong convective currents for perhaps several hours.