consists of three distinct stages; they are the cumulus.stage, the mature stage, and the dissipating or anvilstage. (See fig. 5-15.)CUMULUS STAGE.—Although most cumulusclouds do not become thunderstorms, the initial stage ofa thunderstorm is always a cumulus cloud. The chiefdistinguishing feature of this cumulus or building stageis an updraft, which prevails throughout the entire cell.Such updrafts vary from a few feet per second in theearly cells to as much as 100 feet per second in maturecells.MATURE STAGE.—The beginning of surfacerain, with adjacent updrafts and downdrafts, initiatesthe mature stage. By this time the top of the average cellhas attained a height of 25,000 feet or more. As theraindrops begin to fall, the frictional drag between theraindrops and the surrounding air causes the air to begina downward motion. Since the lapse rate within athunderstorm cell is greater than the moist adiabaticrate, the descending saturated air soon reaches a levelwhere it is colder than its environment; consequently,its rate of downward motion is accelerated, resulting ina downdraft. (See fig. 5-16.)5-19AG5f051540,00035,00030,00025,00020,00015,00010,0005,000FREEZINGLEVEL45,000RAINSNOWICE CRYSTALCUMULUSSTAGEMATURESTAGEANVIL ORDISSIPATING STAGEFIRST RAIN AT SURFACERAIN DECREASINGAT SURFACE0 C0 C0 CFigure 5-15.—Life cycle of a thunderstorm cell.AG5f0516WIDTH UP TO ABOUT SIX MILESCOLD AIR SINKINGDIRECTIONOF STORMOUTWARD FLOW OFCOLD AIR THATPRECEDES THE RAIN5000 FT.SURFACEFigure 5-16.—Downdraft beneath a thunderstorm cell in the mature stage. Arrows represent wind flow. Dashed lines indicaterainfall.
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