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High level clouds

These generally form above 16,500 feet (5,000 m), in the cold region of the troposphere. However, in Polar regions, they may form as low as 10,000 ft (3,048 m). 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.

High level clouds include:

• Cirrus (Ci)
• Cirrus uncinus
• Cirrus Kelvin-Helmholtz Colombia
• Cirrostratus (Cs)
• Cirrocumulus (Cc)
• Pileus
• Contrail, a long thin cloud which develops as the result of the passage of an aircraft at high altitudes.

Medium level clouds

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.

Medium level clouds include:

• Altostratus (As)
• Altostratus undulatus
• Altocumulus (Ac)
• Altocumulus undulatus
• Altocumulus mackerel sky
• Altocumulus castellanus
• Altocumulus lenticularis

Low level clouds

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.

Low level clouds include:

• Stratus (St)
• Nimbostratus (Ns)
• Cumulus humilis (Cu)
• Cumulus mediocris (Cu)
• Stratocumulus (Sc)

Vertical clouds

These clouds can have strong up-currents, rise far above their bases and form at many heights.

Vertical clouds include:

• Cumulonimbus (associated with heavy precipitation and thunderstorms) (Cb)
• Cumulonimbus incus
• Cumulonimbus calvus
• Cumulonimbus with mammatus
• Cumulus congestus
• Pyrocumulus