Cold air is more dense than warm air, so when a warm air mass meets a cold air mass, the cold air ends up below the warm air. Once the air has risen, it cools and clouds can form.
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Cloud Formation and Weather Fronts
Weather fronts can cause clouds to form. Fronts happen when two large masses of air collide into each other at the Earth's surface.
Warm fronts produce clouds when warm air replaces cold air by sliding above it. Many different cloud types can be created in this way: altocumulus, altostratus, cirrocumulus, cirrostratus, cirrus, cumulonimbus (and mammatus clouds), nimbostratus, stratus, and stratocumulus.
Cold fronts happen when heavy cold air moves the lighter warm air, pushing it upward. Cumulus clouds are the most common cloud types that are produced by cold fronts. They often grow into cumulonimbus clouds, which produce thunderstorms. Cold fronts can also produce nimbostratus, stratocumulus, and stratus clouds.
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