266 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 5 1 



(a) Cyclonic Thunderstorms. — Cyclonic thunderstorms accom- 

 pany the central parts of the deeper, well-developed barometric 

 depressions. They are phenomena characteristic of a rapidly 

 ascending current of air such as is brought about in cyclones 

 by the greater disturbances of the atmospheric equilibrium. Hence 

 they occur during disturbed, cloudy weather and especially in the 

 neighborhood of the paths of barometric depressions and wherever 

 these develop into specially well-marked lows, or on the oceans 

 and on the coasts. During these cyclonic thunderstorms the 

 general motion of the atmosphere is cyclonic. This cyclonic mo- 

 tion itself goes on in a horizontal direction with a slightly upward 

 component around a vertical axis or at least one that is inclined so 

 as to intersect the earth's surface. The annual and diurnal periods 

 of these thunderstorms follow those of the cyclonic storms them- 

 selves. 



The cause of these thunderstorms is fundamentally the same 

 as that of the cyclones in general and can therefore at the present 

 time not be given with any more certainty than that of the cyclones 

 themselves, which according to the most recent researches are now 

 no longer to be explained as due to temperature and moisture con- 

 ditions alone, but to no little extent are consequences of the general 

 circulation of the atmosphere. 



The question whether there are other special circumstances on 

 which depend the presence or absence of thunderstorms as com- 

 panions of the cyclones must be cleared up by further investigations. 



(b) Heat Thunderstorms. — While, as just stated, the cyclonic 

 thunderstorms occur, during disturbed, stormy weather and 

 decided cyclonic movements of the atmosphere; on the other hand, 

 the heat thunderstorm demands quiet air for its formation without 

 decided cyclonic or anticyclonic movement and unrestricted power- 

 ful insolation. These occur neither in the central parts of the 

 barometric depressions nor in those of barometric highs, but in 

 the border regions between these two. 



The regions in which heat thunderstorms originate when there 

 is sufficient insolation are the areas of slight depression with 

 scarcely recognizable centers 16 extending in advance of a large 

 barometric depression such as are designated as thunderstorm 



16 These partial depressions or "pockets" are often so imperfectly devel- 

 oped that their shapes on the charts of isobars are entirely changed by slight 

 changes in the method of reduction of pressure to sea-level, so that one can 

 hardly speak of their centers. 



These slight depressions are most clearly revealed when we make use of 



