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.

Last modified May 21, 2009 by Becca Hatheway.

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Altocumulus

Altocumulus clouds are part of the Middle Cloud group. They are grayish-white with one part of the cloud darker than the other. Altocumulus clouds usually form in groups. Altocumulus clouds are about...more

Altostratus

Altostratus clouds belong to the Middle Cloud group. An altostratus cloud usually covers the whole sky. The cloud looks gray or blue-gray. The sun or moon may shine through an altostratus cloud, but will...more

Cirrocumulus

Cirrocumulus clouds belong to the High Cloud group. They are small rounded puffs that usually appear in long rows. Cirrocumulus are usually white, but sometimes appear gray. Cirrocumulus clouds are the...more

Cirrostratus

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Cirrus

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