Cumulus and stratocumulus clouds over Berkshire. Cumulus clouds form from water vapour in rising columns of air which condenses as it cools. They often form at cold fronts where the advancing, cool air forces warm air upwards. Stratocumulus clouds consist of large rounded masses that form lines, or waves. Both cloud types usually form below 2000 metres in altitude. Photographed mid-afternoon in summer, in southern Berkshire, UK.