Cumulus clouds are fluffy, white clouds that often resemble cotton balls. They typically form in fair weather and indicate that the atmosphere is stable. These clouds develop when warm air rises and cools, causing moisture to condense into visible water droplets. Cumulus clouds can vary in size and shape, and they often grow taller as they develop. When they become larger and darker, they may indicate the potential for thunderstorms or rain. Overall, cumulus clouds are a common sight in the sky and are a key part of the water cycle.