Cumulus humilis clouds seen from an aircraft over central France. 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. These clouds, of a type known as cumulus humilis radiatus, have formed rows called 'streets'. Photographed in spring.