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CLOUD CLASSIFICATION.— The interna-tional classification of clouds adopted by most countries is a great help to both meteorological personnel and pilots. It tends to make cloud obser-vations standard throughout the world, and pilots that can identify cloud types will normally take the necessary steps to avoid those types dangerous to their aircraft.

Clouds have been divided into etages, genera, species, and varieties. This classification is based primarily on the process that produces the clouds. Although clouds are continually in a process of development and dissipation, they do have many distinctive features that make this classification possible.

Etages.— Observations have shown that clouds generally occur over a range of altitudes varying from sea level to about 60,000 feet in the tropics, to about 45,000 feet in middle latitudes, and to about 25,000 feet in polar regions. By convention, the part of the atmosphere in which clouds are usually present has been vertically divided into three etages—high, middle, and low. Each etage is defined by the range of levels at which clouds of certain genera occur most frequently.

Cirrus, cirrocumulus, and cirrostratus are always found in the high etage. Altocumulus and altostratus are found in the middle etage, but altostratus may often extend into the high etage. Nimbostratus are always found in the middle etage but may extend into the high, and especially the low etage. Cumulus, cumulonimbus, stratus, and stratocumulus are always associated with the low etage, but the tops of cumulus or cumulo-nimbus may extend into one or both of the two other etages.

The HIGH ETAGE extends from about 10,000 to 25,000 feet in polar regions, 16,500 to 45,000 feet in temperate regions, and 20,000 to 60,000 feet in tropical regions.

The MIDDLE ETAGE extends from about 6,500 to 13,000 feet in polar regions, 6,500 to 23,000 feet in temperate regions, and 6,500 to 25,000 feet in tropical regions.

The LOW ETAGE extends from near Earth’s surface to 6,500 feet in all regions of Earth.

Genera (Types).— As a weather analyst, inter-preter, and briefer, you will be viewing the state of the sky with distinctly different objectives in mind. 

A review of the various cloud types can help you to associate past observer experiences with synoptic conditions and trends. High clouds. High clouds are described as follows:

1. Cirrus (CI). Cirrus are detached clouds of delicate and fibrous appearance, are generally white (cirrus are the whitest clouds in the sky), and are without shading. They appear in the most varied forms, such as isolated tufts, lines drawn across the sky, branching featherlike plumes, and curved lines ending in tufts.

Since cirrus are composed of ice crystals, their transparent character depends upon the degree of separation of the crystals.

Before sunrise and after sunset, cirrus may still be colored bright yellow or red. Being high altitude clouds, they light up before lower clouds and fade out much later.

Cirrus often indicate the direction in which a storm lies.

2. Cirrocumulus (CC). Cirrocumulus, com-monly called mackerel sky, look like rippled sand or like cirrus containing globular masses of cot-ton, usually without shadows. Cirrocumulus are an indication that a storm is probably ap-proaching. The individual globules of cir-rocumulus are rarely larger than 1 degree as measured by an observer on the surface of Earth at or near sea level.

3. Cirrostratus (CS). Cirrostratus forma thin, whitish veil which does not blur the outlines of the Sun or the Moon but does give rise to halos. A milky veil of fog, thin stratus, and altostratus are distinguished from a veil of cirrostratus of similar appearance by the halo phenomenon, which the Sun or Moon nearly always produces in a layer of cirrostratus.

The appearance of cirrostratus is a good in-dication of rain. In the tropics, however, cir-rostratus is quite often observed with no rain following.

Middle clouds. Middle clouds are described as follows:

1. Altocumulus (AC). Altocumulus appear as a layer (or patches) of clouds composed of flat-tened globular masses, the smallest elements of the regularly arranged layer being fairly small and thin, with or without shading. The balls or patches usually are arranged in groups, lines, or waves. This cloud form differs from cirrocumulus by generally having larger masses, by casting shadows, and by having no connection with cir-rus forms. Corona and irisation are frequently associated with altocumulus.

2. Altostratus (AS). Altostratus look like thick cirrostratus, but without halo phenomena; altostratus form a fibrous veil or sheet, gray or bluish in color. Sometimes the Sun or Moon is completely obscured. Light rain or heavy snow may fall from an altostratus cloud layer. Altostratus can sometimes be observed at two different levels in the sky and sometimes in con-junction with altocumulus, which may also exist as two different layers in the sky.

3. Nimbostratus (NS). Nimbostratus appear as a low, amorphous, and rainy layer of clouds of a dark gray color. They are usually nearly uniform and feebly illuminated, seemingly from within. When precipitation occurs, it is in the form of continuous rain or snow. However, nim-bostratus may occur without rain or snow reaching the ground. In cases in which the precipitation does not reach the ground, the base of the cloud is usually diffuse and looks wet. In most cases, nimbostratus evolve from altostratus layers, which grow thicker and whose bases become lower until they become a layer of nimbostratus.

Low clouds. Low clouds are described as follows: 

1. Stratocumulus (SC). Stratocumulus appear as a layer (or patches) of clouds composed of globular masses or rolls. The smallest of the regularly arranged elements is fairly large. They are soft and gray with darker spots.

2. Stratus (ST). Stratus appear as a low, uniform layer of clouds, resembling fog, but not resting on the ground. When a layer of stratus is broken up into irregular shreds, it is designated as stratus fractus.

A veil of stratus gives the sky a characteristically hazy appearance. Usually, drizzle is the only precipitation associated with stratus. When there is no precipitation, the stratus cloud form appears drier than other similar forms, and it shows some contrasts and some lighter transparent parts.

3. Cumulus (CU). Cumulus are dense clouds with vertical development. Their upper surfaces are dome shaped and exhibit rounded protuber-ances, while their bases are nearly flat. Cumulus fractus or fractocumulus resemble ragged cumulus in which the different parts show constant change.

4. Cumulonimbus (CB). Cumulonimbus are heavy masses of cumulus-type clouds with great vertical development whose cumuliform summits resemble mountains or towers. Tops may extend higher than 60,000 feet. Their upper parts are composed of ice crystals and have a fibrous tex-ture; often they spread out in the shape of an anvil.

Cumulonimbus are the familiar thunder-clouds, and their precipitation is of a violent, intermittent, showery character. Hail often falls from well-developed cumulonimbus. On occasion, these clouds also display several readily apparent supplementary features, such as (1) mamma or hanging pouchlike protuberances on the under surface of the cloud; (2) tuba (commonly called the funnel cloud), resembling a cloud column or inverted cloud cone/pendant from the cloud base; and (3) virga, wisps or streaks of water or ice particles falling out of a cloud but evaporating before reaching Earth’s surface as precipitation.

The Aerographer’s Mate must learn to recognize the various cloud types and associated precipitation as seen from Earth’s surface. Figure 5-1-1 shows the various types of clouds in a tier with each cloud type at its average height. Although one never sees all cloud types at once, quite frequently two or three layers of clouds of different types may be present simultaneously.

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