DYNAMIC METEOROLOGY. 



401 



~Ta 



Fig. 6.— Adiabatics. 



In the rain stage the curve he of the change of condition is also less 

 inclined than the adiabatic hc^ ; at the same time it remains nearer to 

 the line of saturation. While traversing 

 the line be the condensed vapor is form- 

 ing clouds. 



If the addition of external heat con- 

 tinues, the air can even return to the 

 dry stage along the curve cd ; let c be 

 this point where this occurs ; the point 

 c will at the same time be the upper 

 limit of the lower clouds. 



From b to c the absorptive power of 

 the suspended water renders the warm- 

 ing by insolation relatively large. But 

 when one has entered into the dry stage, 

 beyond the point d, the absorptive power of the atmosphere diminishes; 

 however, the expansion continues, and with the expansion the cooling, 

 and at some point d, which corresponds to the height of the cirrus 

 clouds, we enter into the snow stage. The absorption of heat, as we 

 go from c to d, has become so feeble that the line cd, can be considered 

 as an adiabatic. From d to 2 we continue in the snow stage or 

 the cirrus-cloud stage. At 2 the air begins to descend in the anti- 

 cyclone 



During the first portion of the period of compression the air follows 

 a curve 2f, corresponding very nearly to the adiabatic 2/^ of the dry 

 stage, but departing from it always alittle toward the higher isotherms, 

 in proportion as we descend to the lower altitudes, where there exists 

 an energetic absorjition of heat. 



The final pressure at the point / at the base of the anti-cyclone is 

 greater than the initial pressure at the point a at the base of the cyclone. 

 Moreover, it generally happens that the point/ is situated to the right 

 of the point a, that is to say, that v is larger than t\„ or that the air in 

 an anticyclone is specifically lighter than in a cyclone. This results 

 from the fact that the addition of external heat compensates for the in- 

 fiuence of com])ression. 



If the air, descending in the anti-cyclone, encounters a new depres- 

 sion, this is represented in the diagram by the dotted line./W, supposing 

 evervthing else to be the same in the old and in the new cj'clone. The 

 line/a completes the cycle of the changes of condition. 



(C) Interchanges of air between a cyclone and anti-ci/clone, in icinter. — 

 AVe note at first that the curves of the changes of condition more nearly 

 approach the axes of co-ordinates in winter than in the summer, since 

 the temperatures remain relatively low and the higher isotherms are not 

 attained. At the initial point a the pressure is lower and the tempera- 

 ture higher ; at the final point d. the pressure is higher and the temi)era- 

 ture lower. The point d is therefore to the left and above the point a. 

 H. Mis. 142 26 



