AbstractThe structure and dynamics of tornadoes and the mechanisms leading to their formation are explored in the context of an axi‐symmetric numerical model of a tornado cyclone. The paper extends a recent study of tornadogenesis by the authors to include explicit representation of moisture effects.The calculations show that the distribution of buoyancy in the updraught of a severe ‘supercell’ thunderstorm can account for the generation and maintenance of an intense tornado when the level of rotation in the storm is within the observed range of values. The simulated vortex extends to high levels in the cloud but maximum swirling velocities occur within the lowest kilometre or two above the ground in concordance with recent Doppler radar observations.For a fixed vertical distribution of ambient temperature and moisture, the vortex strength is sensitive to the imposed circulation: if the latter is too small, the cloud updraught rotates weakly but there is no evidence of a vortex or a funnel cloud; if it is too strong, a vortex and its associated funnel cloud may form but terminate aloft. This explains why only a few apparently suitable thunderstorms spawn damaging tornadoes.The effects of surface friction on vortex strength and structure are studied in some detail and the results suggest that the terminating inflow layer at the ground may have a less important control on the vortex dynamics than has hitherto been sup