A cumulus cloud is the puffy, cotton-ball type of cloud you often see on pleasant days, floating across a blue sky like giant white pillows. The word comes from Latin and means “heap” or “pile,” which makes sense when you look at how these clouds stack up in rounded, fluffy mounds.

Cumulus clouds form when warm air rises from the ground and cools as it climbs higher. The water vapor in that air condenses into visible droplets, creating these distinctive puffs. On a calm day, cumulus clouds might look small and peaceful, scattered across the sky like popcorn. But when conditions are right, they can grow taller and darker, piling higher and higher until they become cumulonimbus clouds: the towering thunderheads that bring storms.

Meteorologists (scientists who study weather) use the term cumulus to distinguish these puffy clouds from other types like stratus (flat, layered clouds) or cirrus (wispy, feathery clouds high in the atmosphere). If you've ever spent time watching clouds drift by, those cheerful-looking puffs you enjoyed were almost certainly cumulus clouds.