Clouds that look like fluffy, rounded piles of cotton are called cumulus clouds. The word cumulus means "heap" or "mass" in Latin. Cumulus clouds form less than 2 kilometers above the ground, but they may grow in size and height until they extend upward as much as 18 kilometers. Cumulus clouds that are not very tall indicate fair weather.
Towering clouds with flat tops, called cumulonimbus clouds, often produce thunderstorms. The suffix -nimbus means "rain."
Part of a cloud's name may be based on its height. The names of clouds that form between 2 and 6 kilometers above Earth's surface have the prefix alto-, which means "high." Altocumulus is a "middle-level" cloud that is higher than regular cumulus clouds but lower than cirrus and other "high" clouds.
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