CLOUD CLASSIFICATION.—The inter-national classification of clouds adopted by mostcountries is a great help to both meteorologicalpersonnel and pilots. It tends to make cloudobservations standard throughout the world, and pilotsthat can identify cloud types will normally take thenecessary steps to avoid those types dangerous to theiraircraft.Clouds have been divided into etages, genera,species, and varieties. This classification is basedprimarily on the process that produces the clouds.Although clouds are continually in a process ofdevelopment and dissipation, they do have manydistinctive features that make this classificationpossible.Etages.—Observations have shown that cloudsgenerally occur over a range of altitudes varying fromsea level to about 60,000 feet in the tropics, to about45,000 feet in middle latitudes, and to about 25,000 feetin Polar Regions. By convention, the part of theatmosphere in which clouds are usually present hasbeen vertically divided into three etages—high, middle,and low. The range of levels at which clouds of certaingenera occur most frequently defines each etage.Cirrus, cirrocumulus, and cirrostratus are alwaysfound in the high etage. Altocumulus and altostratus arefound in the middle etage, but altostratus may oftenextend into the high etage. Nimbostratus is alwaysfound in the middle etage but may extend into the high,and especially the low etage. Cumulus, cumulonimbus,stratus, and stratocumulus are always associated withthe low etage, but the tops of cumulus or cumulonimbusmay extend into one or both of the two other etages.The HIGH ETAGE extends from about 10,000 to25,000 feet in polar regions, 16,500 to 45,000 feet intemperate regions, and 20,000 to 60,000 feet in tropicalregions.The MIDDLE ETAGE extends from about 6,500 to13,000 feet in polar regions, 6,500 to 23,000 feet intemperate regions, and 6,500 to 25,000 feet in tropicalregions.The LOW ETAGE extends from near Earth’ssurface to 6,500 feet in all regions of Earth.Genera (Types).—Asaweatheranalyst,interpreter, and briefer, you will be viewing the state ofthe sky with distinctly different objectives in mind. Areview of the various cloud types can help you toassociate past observer experiences with synopticconditions and trends.High clouds. High clouds are described as follows:1.Cirrus (CI). Cirrus are detached clouds ofdelicate and fibrous appearance, are generally white(cirrus are the whitest clouds in the sky), and arewithout shading. They appear in the most varied forms,such as isolated tufts, lines drawn across the sky,branching feather-like plumes, and curved lines endingin tufts. Since cirrus is composed of ice crystals, theirtransparent character depends upon the degree ofseparation of the crystals.Before sunrise and aftersunset, cirrus may still be colored bright yellow or red.Being high altitude clouds, they light up before lowerclouds and fade out much later. Cirrus often indicatesthe direction in which a storm lies.2.Cirrocumulus (CC). Cirrocumulus, commonlycalled mackerel sky, looks like rippled sand or likecirrus containing globular masses of cotton, usuallywithout shadows. Cirrocumulus is an indication that astorm is probably approaching. The individual globulesof cirrocumulus are rarely larger than 1 degree asmeasured by an observer on the surface of Earth at ornear sea level.3.Cirrostratus (CS). Cirrostratus form a thin,whitish veil, which does not blur the outlines of theSun, or the Moon but does give rise to halos. A milkyveiloffog,thinstratus,andaltostratusaredistinguished from a veil of cirrostratus of similarappearance by the halo phenomenon, which the Sun orMoon nearly always produces in a layer of cirrostratus.The appearance of cirrostratus is a good indication ofrain. In the tropics, however, cirrostratus is quite oftenobserved with no rain following.Middle clouds. Middle clouds are described asfollows:1.Altocumulus (AC). Altocumulus appear as alayer (or patches) of clouds composed of flattenedglobular masses, the smallest elements of the regularlyarranged layer being fairly small and thin, with orwithout shading. The balls or patches usually arearranged in groups, lines, or waves. This cloud formdiffers from cirrocumulus by generally having largermasses, by casting shadows, and by having noconnection with cirrus forms. Corona and irisation arefrequently associated with altocumulus.2.Altostratus (AS). Altostratus looks like thickcirrostratus, but without halo phenomena; altostratusforms 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 altostratuscloud layer. Altostratus can sometimes be observed at5-4
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