MOVEMENTS OF ATMOSPHERE GOLDBERG AND MOIIN 211 



The steady motion continues as long as the alimentary air remains 

 unaltered. Suppose that after a certain interval of time t, the air 

 flowing along the surface of the earth enters at the lower end of the 

 column with the density p', then the column will little by little be 

 filled with air at this density, at the same time that the velocity 

 decreases to zero, and the pressure p increases to p ' , and the motion 

 ceases altogether. 



The duration of the steady motion depends on the quantity of 

 air that can supply the current. If for example the system can be 

 regarded as a radial cyclone, then denoting by r the radius of the 

 vertical current, by r the radius of the alimentary air, by h its height, 

 and by t t the duration we shall have 



n r 2 w t t = n r 2 h 

 or 



u (S) 



If, on the contrary, the alimentary air can be regarded as a stratum 

 whose length is very great compared with the breadth, we can 

 imagine that the system of wind consists of a series of instantaneous 

 systems, such that the cyclone moves along the mean or central 

 line of the alimentary stratum (tornadoes, hailstorms). Let the 

 breadth be L and the velocity of propagation be W, we shall have 



and consequently 



L h W = tz r£ w 



TZ V W Q 



The time t that the cyclone consumes in passing any point, is 

 given by equation 



*-w (10) 



Let 



r = 200 m , 

 h = 100 m , 

 L = 1200 m , 



we find 



W = 14.9 m and 

 t = 27 seconds. 



