When upward convection occurs in a parcel of moist air, the rising air will cool. Further cooling will cause condensation of the water vapour in the air, and rain may result. If the air is very moist, the cooling results in condensation, and hence the release of latent heat. This causes the rising air to accelerate, and very tall cumulo-nimbus clouds form; see Sui et al. (1997) J. Atmos. Scis 54, 5.