A puffy, white cloud with a flat base, often seen on sunny days, is called a cumulus cloud.
Cumulus clouds form when sunlight warms the ground and causes pockets of moist air to rise. As the air climbs, it expands and cools; water vapor then condenses into visible droplets. The cloud’s flat bottom often marks the altitude where rising air reaches its condensation level, while its rounded top reflects continued convection.
The name comes from the Latin word cumulus, meaning “heap” or “pile.” Cumulus is a cloud genus with several species. Cumulus humilis is the classic small “fair-weather” cloud, while cumulus congestus grows into tall towers and may develop into cumulonimbus, the thunderstorm cloud.
A common mix-up is confusing cumulus with stratus. Stratus forms broad, layered sheets, whereas cumulus has detached, lumpy shapes. Not every cumulus cloud guarantees sunshine: growing cumulus can signal increasing instability and later showers or storms.