The primary types of clouds are Cirriform, Nimbus, Cumuliform, and Stratiform.
Cirriform clouds are usually composed of ice crystals and are above 20,000 feet. Cirriform clouds generally occur in fair weather and point in the direction of air movement at their elevation.
Nimbus is a rain cloud. These clouds typically form between 7,000 and 15,000 feet and bring steady precipitation.
Cumuliform clouds look like white, fluffy cotton balls or heaps and show the vertical motion or thermal uplift of air taking place in the atmosphere. The level at which condensation and cloud formation begins is indicated by a flat cloud base, and its height will depend upon the humidity of the rising air. The more humid the air, the lower the cloud base. The tops of cumulus clouds can reach over 60,000 feet. Cumulus clouds are formed when the air is unstable. The instability of cumulus clouds will often be turbulent. If there is precipitation, it is often showery. A cumulonimbus cloud is a rain cloud.
Stratiform clouds consist of a featureless low layer that can cover the entire sky, bringing generally gray and dull weather. The cloud bases are usually only a few hundred feet above the ground. Over hills and mountains, stratiform clouds can reach ground level, then referred to as fog. Also, as fog lifts off the ground due to daytime heating, the fog forms a layer of low stratus clouds. Stratiform clouds form in stable air.
Clouds can be further classified by their height. These cloud classifications include low, medium, high or those with vertical development. The levels overlap and their limits vary with latitude. Low clouds are from the surface to 6,500 feet AGL. Middle clouds are from 6,500 feet AGL to 20,000 feet AGL. High clouds are above 20,000 feet AGL. Certain clouds are generally found at specific altitudes. Clouds with vertical development can start at low levels and rise to the tropopause.
High clouds include Cirrus, Cirrostratus, and Cirrocumulus. Cirrocumulus and cirrostratus are sometimes informally referred to as “cirriform clouds” because of their frequent association with cirrus. They are given the prefix “cirro”, but this refers more to their altitude range than their physical structure. Cirrocumulus in its pure form is actually a high cumuliform genus, and cirrostratus is stratiform, like altostratus and lower based sheet clouds.
Medium clouds include Altocumulus, Altostratus, and Nimbostratus. Mid-level clouds are composed primarily of water droplets. However, they can also be composed of supercooled liquid water droplets and/or ice crystals when temperatures are below freezing. Altostratus is usually found in the middle level, but it often extends higher. Nimbostratus clouds can be defined as dark gray, middle to low level clouds. These Nimbostratus clouds, generally, form at 6,500 feet above ground level and extend to higher and lower altitudes.
Low clouds include Cumulus, Towering Cumulus, Cumulonimbus, Stratus and Stratocumulus.
Low clouds are composed of water droplets. However, they can also be composed of supercooled liquid water droplets and/or ice crystals when temperatures are below freezing. Cumulus and Cumulonimbus usually have bases in the low level, but their vertical extent is often so great that their tops may reach into the middle and high levels. As they are near the surface, low clouds impact VFR flight and are a significant factor for helicopter flight.
Clouds with vertical development include Towering Cumulus and Cumulonimbus. The bases of these clouds form in the low to middle cloud base region but can extend into high altitude cloud levels. Towering Cumulus clouds indicate areas of instability in the atmosphere, and the air around and inside them is turbulent. Towering cumulus of clouds often develop into cumulonimbus clouds or thunderstorms.
FAA AC 00-6B Aviation Weather pg. 13-1
FAA-H-8083-25B Pilot’s Handbook of Aeronautical Knowledge pg. 12-15