THERMODYNAMICS OF ATMOSPHERE VON BEZOLD 265 



tively, so that in this region the secondary maximum for January 

 in the annual curve of thunder storms attains a very great signifi- 

 cance. 



This comparison plainly shows that we have to do with two 

 fundamentally different groups of thunderstorm phenomena which 

 certainly must owe their origin to very different causes. Indeed it 

 is the unequal relative frequency of occurrence of cyclonic and heat 

 thunderstorms in the two regions to which we would ascribe these 

 peculiarities in the diurnal and annual periods of the coastal and 

 interior regions, as Hellmann has already clearly stated 14 in the year 

 1885, when he wrote: 



The cyclonic thunderstorms occur most frequently in the colder parts of the 

 year and the day; the heat thunderstorms occur most frequently in the 

 warmer parts of the year and the day. 



In the same place, Hellmann also emphasizes the following: 



That the winter thunderstorms, or those of the cold season from October to 

 March, occur always in connection with cyclonic storms and frequently at 

 night time; that they often occur rapidly over long stretches of country, but 

 individually in rather more interrupted succession and in rather narrower 

 extent of territory than the average thunderstorm of the warm season; that 

 they are of shorter duration but generally accompanied by some strokes of 

 lightning which on account of the low altitude of the clouds from which they 

 emanate produce conflagrations more frequently than in the summer season. 



Whereas in these sentences, which I heartily indorse, it is expressly 

 stated that the cyclonic thunderstorms, even when their paths are 

 very long, have only a small extent, we also find elsewhere 15 the 

 remark that the cyclonic thunderstorms are the larger while the 

 heat thunderstorms, by contrast, seem as rather local phenomena. 



On the other hand, it seems to me that most of the thunder- 

 storms occurring in Central Europe, many of whose fronts extend 

 from the German coasts to the Alps, must be classed as heat thunder- 

 storms. 



The one feature common to all thunderstorms is the presence 

 of a strong ascending current of air as the fundamental condition 

 for the formation of the great clouds that never fail in any thunder- 

 storm, but the process by which this ascending current comes about 

 is quite different in the two kinds of thunderstorms. 



I will now attempt to give such definitions of the two groups as 

 will, as far as possible, prevent confusion : 



14 G. Hellmann, Meteorologische Zeitschrift, II, 1885, p. 445. 



15 Sohneke: Meteorologische Zeitschrift, V, 1888, p. 413. 



