Cloud Terminology & Images

Clouds are grouped into three main classes, based on where they are located in the atmosphere - low, middle or high. This triple division of clouds is based on the altitude or level on which a cloud formation normally occurs. The World Meteorological Organization (International Cloud Atlas, 1956) classifies 10 cloud types into three major groups (Cumulus or heap clouds, Stratus or sheet clouds, and Cirrus or fibrous clouds) by criteria essentially based on cloud formation.
Cloud Definition and Structures
Polar Mesopheric Class - Extremely High 264,000 to 280,000 feet (49 to 52 miles or 80 to 85km)
Noctilucent Clouds - A thin mostly cirriform-looking cloud seen most often after sunset and before sunrise.
Polar Stratospheric Class - Very High 60,000 to 100,000 feet (11 to 18 miles or 19 to 29km)
Nacreous Clouds (Mother of Pearl) - A thin usually cirriform-looking cloud seen most often after sunset and before sunrise. The clouds are peppered with tiny ice crystals that blaze with iridescent color when struck by light from the setting sun. It is these crystals that make nacreous clouds so rare: they require exceptionally low temperatures of minus 85 Celsius (-120 F) to form. They curl and uncurl hypnotically as they are modulated by atmospheric gravity waves.
Tropospheric Class - High 18,000 to 40,000 feet (3 to 7 miles or 5 to 12km)
Cirrus Clouds - are short, detached, hair-like wispy clouds found at high altitudes. In the day they are whiter than any other cloud in the sky. Whilst at sunrise or sunset they may taint with colours of the sunset.
Cirrocumulus Clouds - are lots of small white clouds - called cloudlets - grouped together at high levels. Composed almost entirely from ice crystals, the small cloudlets are regularly spaced, often arranged as ripples in the sky. They are relatively rare, and unlike altocumulus clouds, never have any shading.
Cirrostratus are transparent high clouds covering large areas of the sky. They sometimes produce white or coloured rings, spots or arcs of light around the sun or moon that are known as halo phenomena. Sometimes they are so thin that the halo is the only indication that a cirrostratus cloud is in the sky. Cirrostratus can span thousands of miles, and may be smooth or fibrous and are often fringed with cirrus clouds. Shadows will normally still be cast by the sun when shining through cirrostratus clouds, which can help distinguish them from similar nimbostratus clouds.
Tropospheric Class - Middle 6,500 to 23,000 feet (1 to 4 miles or 2 to 7km)
Altocumulus Clouds - are small mid-level layers or patches of clouds - called cloudlets - in the shape of rounded clumps. These are white or grey, and the sides away from the Sun are shaded. Mostly found in settled weather, altocumulus are usually composed of droplets, but may also contain ice crystals. The presence of shading can help tell the difference between altocumulus and cirrocumulus. Cirrocumulus are white but altocumulus can be white or grey, and the sides will be shaded.
Altostratus Clouds - are large mid level thin grey or blue coloured clouds. Usually composed of a mixture of water droplets and ice crystals, they are thin enough in parts to allow you to see the sun weakly through the cloud. The sun cannot cast shadows when shining through altostratus clouds, which is how you can differentiate between altostratus and nimbostratus. They are spread over a large area - up to thousands of square miles - and they are either featureless or can have parallel stripes.
Nimbostratus clouds - are dark grey or bluish grey featureless layers of clouds, thick enough to block out the sun. These mid level clouds are often accompanied by continuous heavy rain or snow and cover most of the sky. If there is hail, thunder or lightning it is a cumulonimbus cloud rather than nimbostratus.
Tropospheric Class - Low Base near surface to 6,500 feet (0 to 1 miles or 0 to 2km)
Stratus clouds - are very low level grey layers or patches of clouds with fuzzy edges. They are the lowest clouds and sometimes appear at ground level in the form of mist or fog. Stratus clouds are a fairly uniform grey or white colour and may be accompanied by drizzle, snow or snow grains. If there are no other clouds above the layer of stratus cloud, the sun or moon may shine through.
Cumulonimbus Clouds - are heavy and dense low level clouds, extending high into the sky in towers, plumes or mountain shaped peaks. Commonly known as thunderclouds, the base is often flat and very dark, and may only be a few hundred feed above the Earth's surface. Cumulonimbus clouds are associated with extreme weather such as heavy torrential downpours, hail storms, lightning and tornados. If there is thunder, lightning or hail, it's a cumulonimbus cloud rather than nimbostratus.
Cumulus clouds - are seperate cauliflower shaped clouds usually seen in fair weather. If they get bigger they can sometimes produce showers. The top of these clouds are mostly brilliant white when lit by the sun, their bases are usually relatively dark
Stratocumulus Clouds - are low level clumps or patches of cloud varying in colour from bright white to dark grey. They normally have well defined bases and some parts much darker than others. They can be joined together or have gaps between them. Stratocumulus clouds can be present in all types of weather conditions, from dry settled weather to light rain and snow.


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