Thunderstorm in the Australian Outback in spring. This storm produced frequent and severe cloud-to-ground lightning and hailstones the size of marbles. This storm developed from cumulus congestus clouds. Cumulus clouds often form at cold fronts where the advancing, cool air forces warm air upwards. The congestus variety (also known as towering cumulus) can rise to great heights and can develop into cumulonimbus clouds (thunderheads). Photographed near Uluru, in the late afternoon in October.