Cumulus and stratus clouds. Cumulus cloud with layers of low grey stratus clouds in the distance. Cumulus clouds form at low altitudes (under 2000 metres) from water droplets that condense out of rising columns of moist air. They develop into vertical heaps with flat bases and cauliflower- shaped heads, measuring 500-1500 metres in height. Although dark when seen from below their tops are brilliant white (as seen here). Cumulus clouds often form at cold fronts, where advancing cold air pushes warmer air upwards, causing moisture to condense very quickly. These clouds sometimes grow into towering storm clouds (called cumulonimbus), which can grow up to 15 kilometres tall.