190 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



warm moisture-laden air up the mountain side and toward the con- 

 densation level. This chimney action of a mountain thus aids greatly 

 in thunderstorm formation by making it unnecessary to establish an 

 adiabatic temperature gradient through the free atmosphere up to the 

 cloud level. An example of a thunderstorm forming over a mountain 

 is shoAv^n in plate 2, figure 2. 



Mountains also aid in the formation of thunderstorms by deflecting 

 upward the air masses which are blown against them. The kinetic 

 energy of the moving air carries it up the slopes toward the condensa- 

 tion level. 



COLD-FRONT THUNDERSTORMS 



In many sections of tlic United Stales after a period of warm, 

 humid weather, thunderstorms frequently occur followed by clear, 



<•'*'' l.l'ill.'i'l, l.'n'i, '.,.!'. ,'i| 



Co/d /7/y 



ThUDder- 

 - storm '■- 



/?-Ca 



Warm /^/r> 



Thunder-' /7-S/:-- — 

 r Storm '-^js-Sw^^ 



^^nV. ► Co/c//7/r 



, — » '.'i'./.i'i 'far f/' n/r' ....>* t: i . ' f^ct/rr n 



(A) 



Figure 5.— Uorizontal fa) and vertical (6) cross-sections tlirough a low-pressure system, showing 

 the cold-front and the warm front. Ci., Ci-St., etc., refer to types of cloud. (After Bjerknes and 

 Solberg.) 



cool, dry weather. These may easily be confused with heat thunder- 

 storms. Since before these storms the air is warm and moist while 

 after the storms it is cool, dry, and clear, it is perhaps natural to 

 ascribe the clearing of the atmosphere to the thunderstorm. Actually 

 a large mass of cool dry air has moved into the region and replaced the 

 warm humid air which was there previous to the storm, and in doing 

 so has caused the thunderstorm. The cool air is usually of polar or 

 near-polar origin and is responsible for the bright clear days following 

 the storm. The warm humid air which it replaces is frequently of 

 tropical or near-tropical origin. 



