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The first scientific method of classifying clouds was developed in 1803 by English naturalist (a scientist specializing in the study of plants and animals in their natural surroundings) and pharmacist Luke Howard (1772-1864). In an article titled "On the Modifications of Clouds," Howard designated Latin names to four cloud categories, based on appearance: cumuliform ("piled") for puffy, heaped-up clouds; cirriform ("hair-like") for thin, wispy, feathery swirls of clouds; stratiform ("layered") for continuous, flat sheets or layers of clouds; and nimbus ("cloud") for dark rain clouds. Meteorologists again took up the topic of cloud classification in 1874, at the first meeting of the International Meteorological Congress. There a classification system was devised that used Howard's cloud names as a starting point. The new system included four categories of clouds based on their height in the sky. Those categories were subdivided into ten groupings of clouds based on appearance.
- High Clouds—composed almost entirely of ice crystals, the bases of these clouds start at 16,500 feet (5,005 meters) and reach 45,000 feet (13,650 meters).
Cirrus (from Latin "lock of hair")—are thin featherlike crystal clouds in patches or narrow bands. The large ice crystals that often trail downward in well-defined wisps are called "mares' tails."
Cirrostratus—is a thin, white cloud layer that resembles a veil or sheet. This layer can be striated or fibrous. Because of the ice content, these clouds are associated with the halos that surround the sun or moon.
Cirrocumulus—are thin clouds that appear as small white flakes or cotton patches and may contain supercooled water.
- Middle Clouds—composed of ice and water. The height of the cloud bases range from 6,500 to 23,000 feet (1,972 to 6,977 meters).
Altostratus—appears as a bluish or grayish veil or layer of clouds that can gradually merge into altocumulus clouds. The sun may be dimly visible through it, but flat, thick sheets of this cloud type can obscure the sun.
Altocumulus—is a white or gray layer or patches of solid clouds with rounded shapes.
- Low Clouds—composed almost entirely of water that may at times be supercooled (liquid below the freezing point); at subfreezing temperatures, snow and ice crystals may be present as well. The bases of these clouds start near the Earth's surface and climb to 6,500 feet (1,972 meters) in the middle latitudes.
Stratus—are gray uniform sheetlike clouds with a relatively low base or they can be patchy, shapeless, low gray clouds. Thin enough for the sun to shine through, these clouds bring drizzle and snow.
Stratocumulus—are globular rounded masses that form at the top of the cloud layer.
Nimbostratus—are seen as a gray or dark relatively shapeless massive cloud layer containing rain, snow, and ice pellets.
- Clouds with Vertical Development—contain supercooled water and grow to great heights. The cloud bases range from 1,000 feet (303 meters) to 10,000 feet (3,033 meters).
Cumulus—are detached, fair weather clouds with relatively flat bases and dome-shaped tops. These usually do not have extensive vertical development and do not produce precipitation.
Cumulonimbus—are unstable large vertical clouds with dense boiling tops that bring showers, hail, thunder, and lightning.
Sources: Ahrens, C. Donald. Meteorology Today: An Introduction to Weather, Climate, and the Environment, 5th ed., p. 164; Bair, Frank E. The Weather Almanac, 6th ed., pp. 198-200; Engelbert, Phillis. The Complete Weather Resource, vol.1, pp. 78-91.
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