Cloud Classification

Regular Clouds

Altocumulus (Ac):

They are white or grey puffs, or waves in patches or layers, from 1¼ to 5 miles high. They are usually composed of supercooled water droplets and associated with changeable weather, but without imminent rain. These are the most difficult clouds to identify.

Altostratus (As):

They are uniform white or grey sheets or layers, from 1¼ to 5 miles high. They are usually accompanied by the passage of a depression, and may totally obscure the sun. Predominantly a water-drop cloud, sometimes supercooled, they are produced by an upglide motion over a warm front. Ice crystals may be present. These clouds suggest that there will be rain in about two hours.

Cirrocumulus (Cc):

They are layers of small white puffs or ripples, frequently accompanied by virgo, from 5 to 10 miles high. They are caused by turbulent winds with a layer of unstable air at very high altitudes. Sometimes, they appear when a high-pressure system weakens. They create a beautiful spectacle at sunset, and may also make halo around the sun or moon.

Cirrostratus (Cs):

They are transparent, thin white sheets or veils, from 5 to 10 miles high. They are composed of a thin sheet of ice crystals, and tend to thicken and descend, turning into altostratus clouds on the approach of a warm front. A slight halo around the sun occurs. They have the same base temperatures cirrus clouds. They are caused by the widespread ascent of air ahead of a warm front.

Cirrus (Ci):

They are delicate streaks or patches, forming wispy, thin, white, “mare’s tails,” from 5 to 10 miles high. Composed of ice crystals, they are slow to evaoporate. The temperature at base level ranges is from -20°C to -60°C (-29°F to -76°F). They may be seen high in the sky before the passage of a warm front.

Cumulonimbus (Cb):

From 6,000 to 15,000 feet high, they are large puffy clouds of great vertical extent with smooth or flattened tops, frequently anvil-shaped, from which showers fall, often with thunder. The upper limits turn to ice. The dark base produces rain, hail, or snow. They are most common in the afternoons of spring and summer. They can reach the base of the stratosphere. They occur with the passage of a cold front.

Cumulus (Cu):

They are detached heaps or puff with sharp and flat base outlines, slight or moderate vertical extent, up to 1.9 miles high. They are caused by rising convection currents. The upper parts are dome-shaped. They are often seen in the cold sector of a depression after the passage of a cold front. Individual piecess of this fair weather cloud are short-lived.

Fog:

This is a form of stratus clouds. When ground loses heat during the night, air in contact with it is cooled. If there is enough moisture and little or no wind, the temperature is lowered to the dew point. The moisture condenses when it comes into contact with the ground. As more air is stirred, fog is formed. However, it dissipates when the sun heats the air.

Nimbostratus (Ns):

They are uniform grey layers with falling precipitation, up to 2½ miles high, blotting out the sun. Scud, or shreds of cloud, develop beneath them. They produce continuous rain, snow, or sleet. A halo suggests rain in six hours. They are seen at the passage of a warm front.

Stratocumulus (Sc):

They are grey or white patches or layers of large rolls or merged puffs, up to ¼ mile high. They may cover thousands of square kilometers around the flanks of a high-pressure system. Weather below them tends to be dry. They occur when free convection is limited. They are also formed when cumulus clouds meet a temperature inversion.

Stratus (St):

They are uniform grey layers, up to 1¼ miles high. They are identical to fog, and are formed when moist rising air is carried by a gentle breeze over a cold surface. The resulting precipitation is a drizzle or an insignificant fall of snow.

Unusual Clouds

Cap:

Also known as foehn walls they are long, low clouds just behind the peaks of a range of mountains. They tend to make the peaks appear larger.

Contrail:

They are created by jet airplanes flying in the high troposphere or low stratosphere. The temperature there ranges from -30°C to -65°C (-22°F to -85°F). Very little water is needed to produce saturation or supersaturation. There are uneven segments because moisture advances unevenly at this altitude.

Giant Convective:

These include thunderstorms, hailstorms, tornado clouds, and hurricanes.

Kelvin-Helmholtz:

These are a row of small, curving clouds resembling ocean waves. Cresting waves form as strong winds increase along the top of the clouds. They glow with brilliant colours at sunset.

Lenticular:

These are lens-shaped clouds, forming around mountain peaks because of a wave in the airstream as it passes over the peak. The air cools and condenses as it goes up, and warms and evaporates as it goes down. This leaves a cloud at the peak.

Mammatiform:

They occur when cloudy air comes into contact with a stable layer of cloud-free air. The falling air forms parcels called Bébard Cells. This mass of pendules form on the undersurface of the clouds. The rounded shapes make a spectacular sight, particularly at sunset.

Noctilucent:

They are usually seen after sunset or before sunrise at high altitudes (80 km or 50 miles) in sub-Polar regions. They are arare jet-stream, like wavelet clouds, containing very little water vapour.

Pileus:

Produced by the upward displacement of air that contains enough moisture so that the upward movement causes saturation; if colder, the Schaefer point, contain ice crystals; usually forms between 0°C and -30°C (+32°F and -22°F), often supercooled; forms above cumulus towers or at tops of standing waves; short lived.

Sporadic:

They include gaseous vapours, ashy residues from volcanoes, and water droplet clouds from geysers.

Standing Wave:

They are caused by mountain configurations, sometimes forming at a distance from the mountains causing them. They are carried on the jet stream. When the jet stream meets a mass of dense air, waves or billows are formed.

Traveling Wave:

They are caused by a complex interaction of stable and unstable regions of the air at the jet stream interface. They are also caused by a small hill that cannot control the position of the air driven by the jet stream. They appear as waves or billows.

If you are particularly interested in clouds, check the external site Cloud Appreciation Society.