MOVEMENTS OF ATMOSPHERE GULDBERG AND MOHN 225 



By the aid of this table we have constructed for every o.2 mm the 

 curves of equal variation shown in fig. 41. 



-0.2 



FIG. 41 



We can also determine the curves of equal variation by construc- 

 ting two systems of isobars for given dates. 



§33. Isotherms of systems of wind 



Permanent systems of wind demand a uniform temperature for the 

 air that enters into the barometric minimum. Assuming that the 

 temperature of the air varies with the pressure when nearest the 

 surface of the earth, it is evident that a permanent cyclone must 

 have circular isotherms around its center. Moreover, the system 

 of isotherms must itself be permanent. 



If we consider a variable and stationary cyclone, the isotherms 

 must be circular around the barometric minimum but they can vary 



