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High-Level Clouds
Clouds that form above 18,000 feet are known as cirrus clouds. These clouds are comprised of ice crystals, appearing as thin layers of white clouds. High-level clouds produce no precipitation and are generally an indicator of fair weather. There are three primary types of high level clouds: cirrus, cirrocumulus and cirrostratus. Cirrus clouds are the most common type. They are frequently referred to as mare's tail due to their wispy nature. Cirrocumulus clouds are rare. They typically cover the sky in a pattern similar to fish scales, earning their presence the nickname mackerel sky.
Mid-Level Clouds
Clouds that form between 6,000 and 18,000 feet are known as alto clouds. These clouds typically indicate the approach of a weather front. They rarely are associated with precipitation, though they can produce virga, light precipitation that does not reach the ground. There are two primary types of alto clouds: altostratus and altocumulus. Altostratus clouds appear as broad, flat layers of white clouds. Altocumulus clouds appear as puffy, white clouds.
Low-Level Clouds
Clouds that form below 6,000 feet are generally referred to as stratus clouds, though cumulus clouds are often grouped in this category. There are four primary types of low-level clouds: stratus, stratocumulus, nimbostratus and cumulus. Stratus clouds are featureless layers of clouds that typically span the entire sky. Cumulus clouds are isolated, puffy clouds with little vertical development. Stratocumulus clouds are a hybrid of stratus and cumulus features, produced by horizontal wind turbulence. Nimbostratus clouds are thick, dark layers of stratus clouds that are producing precipitation.
Vertical Clouds
Clouds that feature vertical, rather than horizontal, development cannot be categorized according to height. These clouds typically have low-level bases but rise to over 60,000 feet. There are two primary types of vertical clouds: towering cumulus and cumulonimbus. Towering cumulus, also known as cumulus congestus clouds, are developing thunderstorms. They are fueled by powerful updrafts in the unstable atmosphere ahead of a frontal boundary. Cumulonimbus clouds are mature thunderstorms, having developed powerful downdrafts that produce heavy precipitation.
Special Clouds
There are a number of special clouds that do not fit into any of these categories, such as lenticular clouds, nacreous clouds and noctilucent clouds. Lenticular clouds are found on the downwind side of mountains. Their height is dependent upon the height of the surrounding mountain peaks. Nacreous clouds, also known as polar stratospheric clouds, are found between 50,000 and 80,000 feet. These ice crystal clouds only occur over polar regions during the winter. Noctilucent clouds are found in the mesosphere, between 47 to 53 miles above the Earth's surface. These clouds are a recently discovered phenomena and little is known about them, though they have been potentially linked to climate change.