Clouds are masses of water droplets and ice crystals that result from surface evaporation and subsequent condensation. The location of the cloud relative to the ground determines what type of cloud it is. Some clouds are near the surface, while others are extremely high in the atmosphere. Depending on its elevation, clouds are classified as either high level, mid-level, or low level clouds. Each classification contains 9 cloud varieties. In total, there are 27 different types of clouds.
High Level Clouds
Clouds that are 10,000 feet above the surface or higher are deemed high level clouds. In the polar regions of Earth, high level clouds reach an elevation of 25,000 feet. In temperate regions, the upper limit exceeds 40,000 feet. In tropical regions, high level clouds exist up to 60,000 feet above the surface. These clouds are thin and usually white, but can appear colorful when the sun is low. Here are the main types of high level clouds:
Cirrus Cirrocumulus Cirrostratus
Mid-Level Clouds
Mid-level clouds reside at a specific elevation depending on the region. In polar regions, mid-level clouds are 6,500 to 13,000 feet above the surface. In temperate regions, these clouds can rise to an elevation of 23,000 feet. In tropical regions, mid-level clouds form as high as 25,000 feet. These clouds contain water droplets or ice crystals. Here are the main types of mid-level clouds:
Altocumulus Altostratus Nimbostratus
Low Level Clouds
Low level clouds exist below 6,500 feet in all regions on Earth. It can even exist at the surface in the form of fog. These clouds are composed entirely of water droplets. Here are the main types of low level clouds:
Cumulus
Stratocumulus Stratus Cumulonimbus
Reference:
Cloud Classification – National Weather Service Jetstream MAX
How Clouds Form – HowStuffWorks