542 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 5 1 



It is stated of tropical whirlwinds that ordinarily within their 

 areas there is observed at the earth's suface no great difference of 

 temperature and that the cyclonic distribution of pressure only 

 extends upward to altitudes of a few kilometers. Hence it may 

 be concluded that near the ground the central part of the storm 

 area is the warmest. Helmholtz, in his address on "Whirlwinds 

 and Thunderstorms," assumes that the storm begins with an 

 unstable condition above a small area; according to the computa- 

 tions of Reye, the vertical distribution of temperature around this 

 central region may be stable; the lower strata are pushed upward 

 through this central hole; the layers that at first were above then 

 sink lower. This process is dependent materially on the vapor 

 content ; and on another occasion we will investigate how the avail- 

 able energy is to be computed for this case. In order to judge 

 whether this idea will apply to tropical cyclones we must know 

 accurately the conditions, especially the temperature both inside 

 and outside the relatively small area of the storm. 



W. M: Davis has shown 7 that tornadoes originate only in the 

 neighborhood of the boundary between cold and warm masses of 

 air, a circumstance that had scarcely been considered before and 

 the knowledge of which can be very useful in finding the source 

 of energy of these enigmatical storms. 



ADOPTED NOTATION 



§(9) The following notation and the numerical values of the more 

 important constants will be used : 

 t, the time. 

 x y z, the rectangular coordinates of a point referred to a 

 system of axes that rotate with the earth, z being ver- 

 tical and positive upwards 

 c, the velocity at this point relative to the origin of these 

 axes. 

 uvw, the corresponding components of the velocity c. 

 k, the volume. 

 m, the mass. 



— , the symbol for the total differential coefficient of a 

 dt 



function with regard to the time or 

 d d d d d 



-r = — -4- W — -V V — + W-— 



dt dt dx ' dy dz 



7 Hann: Lehrbuch der meteorologie, Braunschweig, 1901, p. 705. 



