MOVEMENTS OF ATMOSPHERE GULDBERG AND MOHN 



Chapter VII 



W, 



VARIABLE SYSTEMS OF WIND 



§27. Variation of pressure in a stationary system of wind 



In permanent systems of wind the air that flows inward and pro- 

 duces the vertical current, is always homogeneous. In variable 

 systems of wind the flowing air is heterogeneous and 

 consequently the motion of the system varies with 

 the time. The intensity of the systems of wind de- 

 pends especially on the vertical current and we call 

 the air, which enters into a system of winds and has 

 such a physical state that it can produce or sustain a 

 vertical current either ascending or descending, the 

 alimentary air. We call the air which enters a sys- 

 tem of wind, but which cannot sustain a vertical cur- 

 rent the supplementary air. Let us consider a vertical 

 ascending current whose height is h and in the first 

 place suppose the motion to be permanent. Denoting the ver- 

 tical velocities by w and w x (see fig. 28) and the densities by p and 

 p v at the influx and the efflux of the current respectively, the equa- 

 tion of continuity assumes the form 



= p w — p r w t 



Suppose that after a certain time the inflowing air acquires the 

 density p' and the temperature x' and that the vertical velocities 

 remain unaltered in the first moments, the equation of continuity 

 takes the form 



d (ph) = hdp = (p' w — p x Wi) dt 



W 



FIG. 28 



Eliminating p x w v we find 



dp 

 dt 



w 



-jV-p) 



(1) 



The change of density produces a change of pressure and assum- 

 ing approximately 



we shall have 



dp dp p' 273 + x p 

 ~p ' ~p'~p = 273 - +7' ' p 



1 dp 



= a (273 + r ') 



dt 



w 

 a- h (r- O 



(2) 



