Cloud Classifications

NWS -  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.

Level Polar Region(PR), Temperate Region (TR), Tropical Region (TrR)

High Clouds - PR10,000-25,000 feet (3-8 km)  TR16,500-40,000 Feet (5-13 km)  TrR20,000-60,000 feet (6-18 km)
Middle Clouds PR 6,500-13,000 feet (2-4 km) TR 6,500-23,000 feet (2-7 km) TrR6,500-25,000 feet (2-8 km)
Low Clouds PRSurface 6,500 feet (0-2 km) TRSurface-6,500 feet (0-2 km) TrRSurface-6,500 feet (0-2 km)