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.
If 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)|
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.