AEROGRAPHER'S MATE 3 & 2 



Structure 



The fundamental structural element of the 

 thunderstorm is the unit of convective circula- 

 tion known as a convective cell. A mature 

 thunderstorm contains several of these cells, 

 which vary in diameter from 1 to 6 miles. By 

 radar analysis and measurement of drafts, it 

 has been determined that, generally, each cell 

 is independent of surrounding cells of the same 

 storm. Each cell progresses through a cycle 

 which lasts from 1 to 3 hours. In the initial 

 stage (cumulus development), the cloud consists 

 of a single cell; but as the development pro- 

 gresses, the new cells form and older cells 

 dissipate. 



The life cycle of the thunderstorm cell con- 

 sists of three distinct stages; they are the 

 cumulus stage, the mature stage, and the dis- 

 sipating or anvil stage. (See fig. 15-13.) 



RAIN DECREASING 

 FIRST RAIN AT SURFACE ' ' AT SURFACE V< 



CUMULUS 

 STAGE 



MATURE 

 STAGE 



ANVIL OR 

 DISSIPATING STAGE 



209.61 

 Figure 15-13. — Life cycle of a thunderstorm 

 cell. 



CUMULUS STAGE. — Although most cumulus 

 clouds do not become thunderstorms the initial 

 stage of a thunderstorm is always a cumulus 

 cloud. The chief distinguishing feature of this 

 cumulus or building stage is an updraft, which 

 prevails throughout the entire cell. Such up- 

 drafts vary from a few feet per second to as 

 much as 100 feet per second in mature cells. 



MATURE STAGE. — The beginning of surface 

 rain, with adjacent updrafts and downdrafts, ini- 

 tiates the mature stage. By this time the apex 

 of the average cell has attained a height of 25,000 

 feet or more. As the raindrops begin to fall, 

 the frictional drag between the raindrops and 

 the surrounding air causes the air to begin a 

 downward motion. Since the lapse rate within a 

 thunderstorm cell is more than the moist adia- 

 batic rate, the descending saturated air soon 

 reaches a level where it is colder than its 

 environment; consequently, its rate of downward 

 motion is accelerated . This is a downdraft. 



A short time after the rain starts its initial 

 fall, the updraft reaches its maximum speed. 

 The speed of the updraft increases with al- 

 titude. Downdrafts are usually strongest at the 

 middle and lower levels, although the variation 

 in speed from one altitude to another is less 

 than in the case of updrafts. Downdrafts are not 

 as strong as updrafts; downdrafts speed ranges 

 from a few feet per second to about 40 feet 

 per second. Significant downdrafts seldom extend 

 to the top of the cell because in most cases 

 only ice crystals and snowflakes are present, 

 and their rate of fall is insufficient to cause 

 appreciable downdrafts. 



The mature cell, then, generally extends far 

 above 25,000 feet, and the lower levels consist 

 of sharp updrafts and downdrafts adjacent to 

 each other. Large water droplets are encountered 

 suspended in the updrafts, and descending with 

 the downdrafts as rain. 



DISSIPATING (ANVIL) STAGE. — Throughout 

 the life span of the mature cell, more and more 

 air aloft is being dragged down by the falling 

 raindrops. Consequently, the downdraft spreads 

 out to take the place of the dissipating updraft. 

 As this process progresses, the entire lower 

 portion of the cell becomes an area of down- 

 draft. Since this is an unbalanced situation, and 

 since the descending motion in the downdraft 

 effects a drying process, the entire structure 

 begins to dissipate. The high winds aloft have 

 now carried the upper section of the cloud into 

 the anvil form, indicating that gradual dissipation 

 is overtaking the storm cell. 



Vertical Development 



MEASUREMENT. — Thunderstorms have been 

 accurately measured as high as 67,000 feet, and 

 it is believed that some severe thunderstorms 



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