THERMODYNAMICS. OF ATMOSPHERE VON BEZOLD 267 



pockets on our charts of isobars, like shallow furrows or troughs 

 between two areas of maxima, ridges or tongues of high pressure 

 between two low areas and especially between shallow depressions 

 of wide extent. 



In other words, heat thunderstorms originate in regions above 

 which there is neither a decided ascending nor descending current, 

 so that at the earth's surface there is opportunity for such an over- 

 heating of the air as would bring about unstable equilibrium in this 

 part of the atmosphere. 



In this connection, in general, too much importance has, in my 

 opinion, been given to the depressions present in the distribution 

 of atmospheric pressure just described, and thus the difference 

 between the heat thunderstorm and cyclonic thunderstorm has 

 been effaced, whereas these depressions are so infrequently devel- 

 oped in the case of heat thunderstorms that they form a sort of 

 intermediary condition between barometric maxima and minima. 



We may therefore just as properly consider the protruding arm 

 of the area of maximum pressure which is necessary to the formation 

 of the so-called "thunderstorm pocket," or the ridge or tongue of 

 high pressure that most emphatically favors the formation of 

 thunderstorms, as the important feature and give less attention to 

 the accompanying barometric depression. 



The only important consideration is that there be the possibility 

 of an unusual rise in the temperature of the lowest layer of air 

 so that the potential temperature below may be higher than above, 

 i. e., so that unstable equilibrium may occur. 



But this is only possible when in the preparatory stage there is 

 neither a decided ascending current (such as occurs in areas of low 

 pressure where the lower air being warmed is carried along and 

 moreover by reason of the cloud necessarily formed by the ascent 

 checks the superheating) nor a strong descending current (such as 

 is present in the interior of an anticyclone causing a steady outflow 

 of the lower layer or the rapid dissolution of individual local ascend- 

 ing clouds). 



my method of partial isobars, which however assumes a very accurate reduc- 

 tion of atmospheric pressure to sea-level, 



Compare M. v. Rohr (Die gewitter, etc.) : ' ' The thunderstorm of December 

 11, 1 89 1, in connection with the simultaneous weather." Publications of 

 the Royal Preuss. Meteorological Institute. Results of thunderstorm ob- 

 servations in the year 1891. Berlin, 1895, pp. xi-xxxv. 



Also 



W. Wundt (Barometrische Theiledepression-en, etc.): "Barometric 

 pockets and their wave-like repetition." Memoirs of the Royal Prussian 

 Meteorological Institute, Vol. II, No. 4, Berlin, 1904. Note added 1905. 



