Abstract The development of electricity in a shallow wintertime cumulus was studied using an axisymmetric cloud model containing both microphysical and electrical charge separation processes during graupel formation. The charge separation mechanisms considered included ion induction, ion diffusion, polarization and riming electrification. An unexpected result was that polarization did not intensify cloud electrification. Instead, riming electrification appears to be the principal charge separation process acting to intensify cloud electricity. The cloud is electrified during graupel formation, and a relatively large positive potential gradient forms initially near the cloud top along the cloud boundary between negative graupel and positive ions. Later, graupel particles, electrified positively through riming electrification, produce a relatively strong negative potential gradient between the positive space charge and the upper-level negative space charge produced by snow crystals. As these positively char...