The salient facts concerning the dynamical, physical and electrical properties of athunderstorm, and of the detailed structure and associated electric field-changes oflightning flashes, are marshalled to deduce the criteria for a satisfactoryquantitative theory of charge generation and separation leading to thegrowth of electric fields strong enough to initiate and to sustain lightningactivity.A quantitative theory is presented of how charges are generatedand separated when supercooled cloud droplets make grazing contactwith the undersides of hail pellets (graupel) polarized initially by theEarth’s fine-weather electric field. The rebounding droplets acquire apositive charge and are carried by the convective updraught towards the topof the cloud, while the hail pellets carrying a net negative charge falltowards cloud base. This creates a vertical dipole field which increases thepolarizing charges on the hail pellets and so accelerates the rates of chargegeneration and separation, and so reinforces the vertical electrical field, whichgrows exponentially until insulation of the air breaks down and triggersa lightning flash.It is demonstrated that a thunderstorm cell, 2 km indiameter, producing small hail falling at 30 mm h−1can produce vertical electric fields of ∼5000 V cm−1in about 10 min involving the separation of ∼50 Cof charge, enough to initiate a lightning flash which, on average, neutralizes about20 C. As long as the hail persists, it continues to generate and separate sufficientcharge to produce a succession of lightning flashes at about 30 s intervals. Morefrequent discharges at say 10 s intervals would require high rates of hailproduction in larger cells but are more likely to be produced by largemulti-cellular storms sustained by strong convective currents for perhaps severalhours.