Clouds are divided into two general categories: layered and convective.
These are named stratus clouds (or stratiform, the Latin stratus means
layer) and cumulus clouds (or cumiloform, cumulus means piled up). These
two cloud types are divided into four more groups that distinguish the
cloud's altitude. Clouds are classified by the cloud base height, not
the cloud top.
High clouds (Family A)
These form above 16,500 feet (5,000 m), in the cold region of the troposphere. They are denoted by the prefix cirro- or cirrus. At this altitude water almost always freezes so clouds are composed of ice crystals. The clouds tend to be wispy, and are often transparent.
Clouds in Family A include:
A contrail is a long thin cloud which develops as the result of the passage of a jet airplane at high altitudes.
Middle clouds (Family B)
These develop between 6,500 and 16,500 feet (between 2,000 and 5,000 m) and are denoted by the prefix alto-. They are made of water droplets, and are frequently supercooled.
Clouds in Family B include:
Low clouds (Family C)
These are found up to 6,500 feet (2,000 m) and include the stratus (dense and grey). When stratus clouds contact the ground they are called fog.
Clouds in Family C include:
Vertical clouds (Family D)
These clouds can have strong upcurrents, rise far above their bases and can form at many heights.
Clouds in Family D include:
* Cumulonimbus (associated with heavy precipitation and thunderstorms)
A few clouds can be found above the troposphere; these include nacreous and noctilucent clouds, which occur in the stratosphere and mesosphere respectively.