Clouds form when moist air rises, cools, and can no longer hold all of its water vapor. The excess condenses onto tiny particles of dust, salt, or smoke, producing droplets so small that they float. Their shapes tell a great deal about the air around them. Fluffy cumulus clouds mark rising columns of warm air on a fair day, while flat layers of stratus bring gray skies and drizzle. Wispy cirrus, made of ice crystals high in the atmosphere, often signal a change in the weather within a day or two.
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