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Clouds are defined by their general appearance and level in the atmosphere. |  | Cirrus clouds are curly or fibrous | | 
| Stratus clouds are flat and/or layered | |  | Cumulus clouds are puffy and piled up. |
Additionally, a prefix is frequently given to the cloud name to indicate what level of the atmosphere it is in. Cirro is the prefix given to high clouds, those with bases above 20,000 feet.
Alto is the prefix given to mid-level clouds, those between 6,000 and 20,000 feet.
Nimbo added to the beginning, or nimbus added to the end of a cloud name means the cloud is producing precipiation. The system is by no means uniform. There is no term for low clouds, and there are some odd joinings, such as stratocumulus, which is a cloud with two different shapes. So here's how some cloud types stack up... | Cloud Type | Appearance | Altitude | | Cumulonimbus | Thunderheads | Near ground to above 50,000 feet | | Cirrostratus | Thin, wispy, above thunderheads | Above 18,000 feet | | Cirrus | Thin, often with "mare's tail" | Above 18,000 feet | | Cirrocumulus | Small puffy clouds | Above 18,000 feet | | Altostratus | Thin, uniform, sometimes with "wide wale corduroy" appearance | 6,000 - 20,000 feet | | Altocumulus | Medium-sized puffy clouds | 6,000 - 20,000 feet | | Stratocumulus | Broad and flat on the bottom, puffy on top | Below 6,000 feet | | Cumulus | Puffy clouds | Below 6,000 feet | | Stratus | Uniform, thick to thin layered clouds | Below 6,000 feet |
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