Precipitation FormsPrecipitation includes all forms of moisture that fallto the earth’s surface, such as rain, drizzle, snow, andhail. Precipitation is observed and classified by form,type, intensity, and character.PRECIPITATION FORM.—Precipitation formis the state that the moisture is in: liquid, freezing, orfrozen. Liquid precipitation is any precipitation thatfalls as a liquid and remains liquid after striking anobject, such as the earth’s surface or the skin of anaircraft. Rain and drizzle are the only two types ofliquid precipitation.Freezing precipitation is any precipitation that fallsas a liquid and freezes upon contact with an object, suchas freezing rain or freezing drizzle. In this form ofprecipitation, the liquid water may be a super-cooledliquid and freeze upon contact with an object, or thewater droplet may have an above freezing temperatureand freeze upon contact with an object that has atemperature below freezing. (Super-cooled liquidshave a temperature below their normal freezingtemperature, but still exist in the liquid state.) Smallfreezing drizzle particles form a milky white icecoating, typically referred to as rime ice, especially onaircraft in flight. Larger freezing drizzle and freezingrain drops form a transparent ice coating known as clearice on aircraft in flight or as glaze ice on the ground,power lines, or trees.Frozen precipitation is any precipitation of waterthat falls in its solid state, such as snow, hail, or icepellets. Different forms of precipitation may occurtogether, such as mixed rain and snow; but such anoccurrence is simply a mixture of forms, not a separateform of precipitation.PRECIPITATION TYPE.—Precipitation type isthe term used to identify various precipitation.Discussion of the types of precipitation follows:Rain—Liquid precipitation that has a waterdroplet diameter of 0.02 inch (0.5 mm) or larger. If thewater droplets freeze upon contact with a surface, thephenomenon is called freezing rain.Drizzle—Liquid precipitation that consists ofvery small and uniformly dispersed droplets of liquidwater that appear to "float" while following air currents.Drizzle usually falls from low stratus clouds and isfrequently accompanied by fog. A slow rate of fall andthe small size of the droplets (less than 0.02 inch)distinguish drizzle from rain. When drizzle freezes oncontact with the ground or other objects, it is referred toas freezing drizzle. Drizzle usually restricts visibility.Snow—Precipitation that consists of white ortranslucent ice crystals. In their pure form, the icecrystals are highly complex, hexagonal, branchedstructures. Snow falls as a combination of individualcrystals, fragments of crystals, or clusters of crystals.Warmer conditions tend to favor larger crystal sizes andclusters of crystals. Snow must form in cloudtemperatures below freezing, though it may fall throughair at above freezing temperatures for a short period oftime before melting.Snow Pellets/Small Hail—White, opaque, round(or occasionally conical) kernels of snowlikeconsistency, 0.08 to 0.2 inch in diameter. They arecrisp, easily compressible, and may rebound or burstupon striking a hard surface. Snow pellets occur almostexclusively in snow showers.Snow Grains—Very small, white, opaque grainsof ice similar in crystal structure to snow. Whereas thecrystal structure of snow has very fine, needlelikebranches, the crystal structure of snow grains hasthicker, denser elements, with the space betweenhexagonal branched commonly completely filled.Snow grains do not bounce or shatter on hard surfaces.They usually fall in small quantities, mostly fromstratus clouds and never as showers.Ice Pellets—Transparent or translucent particlesof ice that are either round or irregular (rarely conical)and have a diameter of 0.2 inch or less. They usuallyrebound upon striking hard surfaces and make a soundupon impact. The term ice pellets describes twodifferent types of similar looking solid precipitation.One type is composed of hard grains of ice formed fromfreezing rain or the refreezing of melted snowflakes. Itfalls as continuous precipitation and is sometimesreferred to as sleet. Another type is composed of pelletsof snow encased in a thin layer of ice. It is formed fromthe freezing of water droplets intercepted by snowpellets or by the refreezing of a partially melted snowpellet. This type falls as showery precipitation and isusually associated with thunderstorms.Hail—A clear to opaque ball of hard ice, rangingin diameter from 1/8 inch or so to 5 inches or larger.Hailstone size is measured and reported in inches, buthailstones are usually compared to common objectswhen reported to the public by television or radio, suchas pea size, walnut size, golf-ball size, baseball size, orsoftball size.Hail frequently displays a layeredappearance of alternate opaque and clear ice. It is1-33
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