Cloud Classifications

By convention, clouds are vertically divided into three étages (levels); low, middle, and high. Each étage is defined by the range of levels at which each type of clouds typically appears. The types of clouds are...

  • Cirrus (Ci), Cirrocumulus (Cc), and Cirrostratus (Cs) are high level clouds. They are typically thin and white in appearance, but can appear in a magnificent array of colors when the sun is low on the horizon.

  • Altocumulus (Ac), Altostratus (As), and Nimbostratus (Ns) are mid-level clouds. They are composed primarily of water droplets, however, they can also be composed of ice crystals when temperatures are low enough.

  • Cumulus (Cu), Stratocumulus (Sc), Stratus (St), and Cumulonimbus (Cb) are low clouds composed of water droplets.

Learning Lesson: A 'Hole' Lot of Clouds

Jet stream cross sectionIf you recall that the polar and subtropical jet streams are the dividing points between the polar, temperate, and tropical regions you will also notice the maximum height of the tropopause also decrease in each region as one moves from the equator to the poles.

This also has an impact on the elevation of the bases of clouds in the middle and high étage. The elevation of this change will vary by season. For example, the elevation where the change from the middle to high étage occurs will be lower in winter as compared to summer. Also, local atmospheric conditions will cause changes in the elevation of étages. The following table provides approximate heights of these limits.

Level Polar Region Temperate Region Tropical Region
High Clouds 10,000-25,000 feet (3-8 km) 16,500-40,000 Feet (5-13 km) 20,000-60,000 feet (6-18 km)
Middle Clouds 6,500-13,000 feet (2-4 km) 6,500-23,000 feet (2-7 km) 6,500-25,000 feet (2-8 km)
Low Clouds Surface-6,500 feet (0-2 km) Surface-6,500 feet (0-2 km) Surface-6,500 feet (0-2 km)

NWS Cloudchart High clouds - Filaments of Ci High clouds - Dense Ci in patches High clouds - Anvil shaped Dense Ci High clouds - Hooked shaped Ci High clouds - Cs not reaching 45° altitude High clouds - Cs exceeding 45° altitude High clouds - Veil of Cs High clouds - Cs not increasing or covering entire sky High clouds - Cc alone or main cirriform cloud Middle clouds - Thin As Middle clouds - Thick As covering the sun or moon, or Ns Middle clouds - Thin Ac at single level Middle clouds - Thin Ac in patches Middle clouds - Thin Ac in bands usually thickening Middle clouds - Ac from spreading Cu or Cb Middle clouds - Double layered or thick Ac or Ac with As and/or Ns Middle clouds - Ac in form of Cu-shaped turrets Middle clouds - Ac of a chaotic sky, usually at different levels Low clouds - Cu of fair weather Low clouds - Cu of considerable development Low clouds - Cb with tops lacking clear-cut outlines Low clouds - Sc formed from spreading Cu - bases at same level Low clouds - Sc NOT formed from Cu Low clouds - St or StFra (stratus fractus) but with no bad weather Low clouds - StFra and/or CuFra of bad weather Low clouds - Cu and Sc with bases at different levels Low clouds - Cb with cirriform top Cumulonimbus mamatus Tornades Wall Cloud Shelf Cloud Wave Clouds In meteorology, clouds are identified and divided into 27 categories; nine categories for each low, middle and high étage.

In each étage the clouds may be divided by type or they may be the same cloud type but differ on amount of skycover, whether the cloud is increasingly invading the sky, and/or and the number of layers in which the clouds appears.

The cloud chart (right) shows examples of each of these 27 categories. By clicking each image you will see various views of the cloud classification, the description from the World Meteorological Organization (WMO) International Cloud Atlas,, the symbol associated with that type of cloud, and more.

One thing to remember, clouds are identified based upon your observation point at your elevation. From sea-level, you might observe stratus clouds enveloping the top of a mountain. However, if you were on the mountain top and in that same cloud, you would observe and report fog.

The exception to this is cumulus or cumulonimbus clouds over a mountain. Even though these low étage clouds typically have bases under 6,500 feet (2,000 meters), if you were to observe them over a mountain top, their base might be 12,000 feet (3,600 meters) or higher relative to your location closer to sea-level. However, due their location over the mountain you would still call them cumulus or cumulonimbus clouds as appropriate.

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