Cumulus humilis clouds on a boundary layer beneath an intensifying anticyclonic subsidence inversion. Cumulus clouds typically occur under 2000 metres altitude. They form from water vapour in rising columns of air which condenses as it cools. Cumulus clouds often form at cold fronts where the advancing, cool air forces warm air upwards. The humilis variety occurs either as the first stage of the formation of other cumulus types, or when heat transfer (convection) in the atmosphere is weak. Due to its shallow depth, cumulus humilis does not produce any significant weather. Photographed in the early afternoon in summer, from Heligan, Cornwall, UK.