Chapter 15 — METEOROLOGICAL ELEMENTS 



actually attain a greater height than this. More 

 often the maximum height is from 40,000 to 

 45,000 feet. Air-mass thunderstorms in general 

 extend to greater heights than frontal types. 



DRAFTS AND GUSTS. —Rising and descending 

 drafts of air are, in effect, the structural bases 

 of the thunderstorm cell. A draft is a large-scale 

 vertical current of air that is continuous over 

 many thousands of feet of altitude. Speeds of the 

 drafts are either relatively constant or grad- 

 ually varying from one altitude to the next. 

 Gusts, on the other hand, are smaller scaled 

 discontinuities associated with the draft proper. 

 A draft may be compared to a great river flowing 

 at a fairly constant rate, whereas a gust is 

 comparable to an eddy or any other random 

 motion of water within the main current. 



Thunderstorm Weather 



RAIN. — Liquid water in a storm may be 

 ascending if encountered in a strong updraft; 

 it may be suspended, seemingly without motion, 

 yet in extremely heavy concentration; or it may 

 be falling to the ground. Rain, as normally meas- 

 ured by surface instruments, is associated with 

 the downdraft. This does not preclude the pos- 

 sibility of a pilot entering a cloud and being 

 swamped, so to speak, even though rain has not 

 been observed from surface positions. Rain is 

 found in almost every case below the freezing 

 level. In instances in which no rain is encoun- 

 tered, the storm probably has not developed into 

 the mature stage. 



Statistics show, although heavy rain is gen- 

 erally reported at all levels of a mature storm, 

 the greatest incidence of heavy rain occurs in 

 the middle and lower levels of the storms. 



HAIL. — Hail, if present, is most often found 

 in the mature stage. Very seldom is it found 

 at more than one or two levels within the same 

 storm. When it is observed, its duration is 

 very short. The maximum occurence is at mid- 

 dle levels for all intensities of hail. During 

 Project Thunderstorm, hail was encountered at 

 a maximum of 10 percent of the traverses at 

 any given altitude. However, the area from 

 which the data were taken is far removed from 

 the region of the greatest surface hail, the 

 Great Plains States. 



SNOW. — The maximum frequency of moderate 

 and heavy snow occurs several thousand feet 



above the freezing level. Snow, mixed in many 

 cases with supercooled rain, may be encoun- 

 tered in updraft areas at all altitudes above the 

 freezing level. This presents a unique icing 

 problem — wet snow packed on the leading edge 

 of the wing of the aircraft resulting in the for- 

 mation of rime ice. 



TURBULENCE. — There is a certain definite 

 correlation between turbulence and precipitation. 

 The intensity of associated turbulence, in most 

 cases, varies directly with the intensity of the 

 precipitation. 



ICING. —Icing may be encountered at any 

 level where the temperature is below freezing. 

 Both rime and clear ice occur, with rime pre- 

 dominating in the regions of snow and mixed 

 rain and snow. 



Since the freezing level is also the zone of 

 greatest frequency of heavy turbulence and gen- 

 erally heavy rainfall, this particular altitude 

 appears to be the most hazardous. 



SURFACE WIND. — A significant hazard asso- 

 ciated with thunderstorm activity is the rapid 

 change in surface wind direction and speed im- 

 mediately prior to storm passage. The strong 

 winds at the surface accompanying thunderstorm 

 passage are the result of the horizontal 

 spreading out of downdraft currents from within 

 the storm as they approach the surface of the 

 earth. 



The total wind speed is a result of the 

 downdraft divergence plus the forward velocity 

 of the storm cell. Thus, the speeds at the 

 leading edge, as the storm approaches, are 

 greater than those at the trailing edge. The 

 initial wind surge, as observed at the surface, 

 is known as the FIRST GUST. 



The speed of the first gust is normally the 

 highest recorded during storm passage, and the 

 direction may vary as much as 180 degrees 

 from the previously prevailing surface wind. 

 First-gust speeds increase to an average of 

 about 16 knots over prevailing speeds, although 

 gusts of over 78 knots (90 mph) have been re- 

 corded. The average change of wind direction 

 associated with the first gust is about 40 degrees. 



Classifications 



All thunderstorms are similar in physical 

 makeup; but for purposes of identification, they 

 may be divided into two general groups — frontal 

 thunderstorms and air-mass thunderstorms. 



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