Cumulus clouds are most noticeable for their fluffy white appearance. These low-level clouds typically form on sunny days as the sun heats the ground and warms the air. As the warm air rises and meets cold air, water vapor cools and condenses forming these cotton-like clouds.
Cumulus clouds commonly have rounded tops and flat darker bottoms. Those with little vertical development indicate that the weather will be fair. Cumulus clouds can also grow vertically forming cumulonimbus clouds. These clouds indicate heavy rain and severe weather.