Cumulus fractus clouds at sunset over southern England. 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 fractus variety of cumulus clouds is ragged in appearance and can form in clear air as a precursor to large cumulus clouds. Photographed in the late afternoon in early spring in southern England.