積雲, or cumulus clouds, are fluffy, white clouds that often resemble cotton balls. They typically form in fair weather and indicate rising warm air. These clouds can develop into larger storm clouds if conditions are right, but they are generally associated with pleasant weather. These clouds usually appear in the afternoon when the sun heats the ground, causing warm air to rise. As the air rises, it cools and condenses, forming cumulus clouds. They can vary in size and shape, and while they are mostly harmless, larger versions can signal changing weather patterns.