Cirrus clouds are known for their wispy, feathery look. Cirrus clouds form only at high levels, above about 6 kilometers, where temperatures are very low. As a result, cirrus clouds are made of ice crystals.
Cirrocumulus clouds, which look like rows of cotton balls, often indicate that a storm is on its way. The rows of cirrocumulus clouds look like the scales of a fish. For this reason, the term "mackerel sky" is used to describe a sky full of cirrocumulus clouds.
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