A classification of cloud forms was first made (1801) by French naturalist Jean Lamarck. In 1803, Luke Howard, an English scientist, devised a classification that was adopted by the International Meteorological Commission (1929), designating three primary cloud types, cirrus, cumulus, and stratus, and their compound forms, which are still used today in modified form. Today's classification has four main divisions: high clouds, 20,000 to 40,000 ft (6,100–12,200 m); intermediate clouds, 6,500 to 20,000 ft (1,980–6,100 m); low clouds, near ground level to 6,500 ft (1,980 m); and clouds with vertical development, 1,600 ft to over 20,000 ft (490–6,100 m).
High cloud forms include cirrus, detached clouds of delicate and fibrous appearance, generally white in color, often resembling tufts or featherlike plumes, and composed entirely of ice crystals; cirrocumulus (mackerel sky), composed of small white flakes or very small globular masses, arranged in groups, lines, or ripples; and cirrostratus, a thin whitish veil, sometimes giving the entire sky a milky appearance, which does not blur the outline of the sun or moon but frequently produces a halo.
Intermediate clouds include altocumulus, patchy layer of flattened globular masses arranged in groups, lines, or waves, with individual clouds sometimes so close together that their edges join; and altostratus, resembling thick cirrostratus without halo phenomena, like a gray veil, through which the sun or the moon shows vaguely or is sometimes completely hidden.
Low clouds include stratocumulus, a cloud layer or patches composed of fairly large globular masses or flakes, soft and gray with darker parts, arranged in groups, lines, or rolls, often with the rolls so close together that their edges join; stratus, a uniform layer resembling fog but not resting on the ground; and nimbostratus, a nearly uniform, dark grey layer, amorphous in character and usually producing continuous rain or snow.
Clouds having vertical development include cumulus, a thick, detached cloud, generally associated with fair weather, usually with a horizontal base and a dome-shaped upper surface that frequently resembles a head of cauliflower and shows strong contrasts of light and shadow when the sun illuminates it from the side, and cumulonimbus, the thunderstorm cloud, heavy masses of great vertical development whose summits rise in the form of mountains or towers, the upper parts having a fibrous texture, often spreading out in the shape of an anvil, and sometimes reaching the stratosphere. Cumulonimbus generally produces showers of rain, snow, hailstorms, or thunderstorms.