248 THE MECHANICS OF THE EARTH'S ATMOSPHERE. 



between cyclone aud anticyclone in summer and winter was investi- 

 gated, at least in its principal features. 



But in this study it is not necessary to limit oneself to the summer 

 or the winter, but rather one can apply the scheme for the summer 

 generally to all cases where the radiation is in excess, that is to say, 

 not only to the summer time in general, but to the day-time and the hot 

 zone; the scheme for the winter, on the other hand, is ai)plicable not 

 only to tlie winter season, but to the night-time and the cold zones of 

 the earth. This normal scheme for the ascending and descending cur- 

 rents will therefore appear as shown in Fig. 36. The portion a b has 

 reference to the ascending current in the dry stage, h c is its continua- 

 tion in the condensation stage, finally c d is the portion of the curve 

 that corresponds to the descending current. 



This scheme differs only a little from that communicated in the first 

 memoir. (For the case of the foehn, see page 240.) We can not expect 

 it to be otherwise, since in the foeun one has also to do with an ascend- 

 ing and descending current of air in which the velocity with which the 

 whole process goes on affords only a small opportunity for the gain aud 

 loss of heat. However, the diagram given in figure 30 as the " normal 

 scheme" differs from that which obtains for the foehn in this respect, 

 that the branch cd is longer. This is due to the fact that in the ordi- 

 nary interchange between cyclone ;md anticyclone there always pre- 

 vails a higher pressure at the base of the latter than at the base of the 

 former; that is to say, the ending point d in the normal scheme must 

 always lie higher than the starting point a, which is not the case in 

 the foehn diagram. In general, one has to consider the process in the 

 foehn as only a feature inserted into the normal interchange between 

 anticyclone and cyclone. In the foehn the passage over the mountain 

 chain Jorces the air in its normal interchange to describe an antecedent 

 ascent and a subsequent descent which is only then followed by the 

 definitive ascent in the cyclone. This being premised, the processes in 

 the interchange, according to the normal scheme, will now be more pre- 

 cisely considered. 



If w'e introduce the conception of the potential temperature, we at- 

 tain the foUowing theorems without any difficulty : 



(a) In the ascending branch* the potenlial temperature increases 

 steadily from the beginning of the condensation ; in the descending' 

 branch it remains constant at the maximum value attained in the whole 

 process. This maximum value corresponds also to the highest point 

 to which the air has risen in its path. 



(b) The potential temperature of the upper strata of the atmosphere 

 is in general higher than that of the lower. 



The first of these two theorems results directly from the diagram ; the 

 second follows from the fact that in the lower stratum the potential 



* By the asceudin<; brauch is meant the portion ah which corresponds to the ascent 

 in the atmosphere ; the portion cd is considered as the descending branch. 



