METEOROLOGY 535 



and possibly abundant rainfall outside the original column. 

 All this can occur while the whole system is being developed 

 in the easterly wind, and it moves with the wind towards the 

 region where the surface is still warmer, and, consequently, 

 the surface air also becomes warmer ; the dynamical process 

 of scouring the central column is continued. But there will 

 come a time when the supply of hot moist air at the surface 

 is exhausted, and then the passage of the air through the 

 column by ascent from the bottom must cease ; the air can 

 only rise until its temperature is the same as that of the cooled 

 environment. When that stage has been reached, any hot 

 air remaining in the column will be ejected at the top by the 

 convergence from the sides, and we shall have obtained a 

 dynamical system consisting of a vortex with a ring of maximum 

 velocity of finite diameter and its interior protected from further 

 invasion, except at the bottom, by the velocity of rotation, 

 so that it can only be affected by the creeping of air or other 

 material into the interior along the bottom. The temperature 

 distribution will be that produced by convergence of the 

 environment towards the axis ; the whole effect of the con- 

 vection, originally due to the heated and saturated surface air, 

 will have been to cause the removal of the air from along the 

 axis, which Lord Rayleigh's exposition requires for the for- 

 mation of a vortex of revolving fluid. Thus the high tempera- 

 ture of the interior is merely a temporary incident in the 

 formation of a cyclonic vortex : by the time the vortex is 

 developed as a dynamical system the core is cold ; there is 

 no longer any convection in it ; it becomes a comparatively 

 small area protected from the ordinary vicissitudes of weather 

 by the enormous momentum of a vortex with a high rate of 

 spin, represented by the very violent winds of a certain ring, 

 but extending in less violent form over a vast area." 



Air can, however, enter the system near the surface, where 

 the rotary velocity and consequently the momentum is 

 reduced by friction : "So long as the surface air which is 

 dragged into the interior in this way is warm enough to cause 

 local convection in the region to which it is carried, additional 

 energy is conveyed to the dynamical system ; but when a 

 layer of cold air is attacked the convection only extends until 

 the temperature is reduced sufficiently for the air to remain 

 in dynamical equilibrium with its surroundings, and the 

 addition of mass to the interior, instead of helping to scour 

 the column, increases the pressure there. The ring of maximum 

 velocity is, therefore, enlarged, but its velocity is diminished. 

 The reverse of the process of formation is followed. As this 

 process is continued, the central pressure continually increases, 

 and the ring of maximum velocity is widened and depressed. 



