Heat and its Importance in Meteorology . 369 



(d.) Besides the horizontal movements and whirls, Helmholtz 

 has studied what may be called the vertical whirls or the 

 general vortex motion of which the ideal case is an atmospheric 

 vortex ring surrounding the earth at the equator and moving, 

 as a whole, poleward. If such a ring of incompressible air 

 is to retain its energy as it moves northward and is to contract 

 as the circles of latitude diminish, it must exhibit far greater 

 velocities than are observed, therefore some way must exist by 

 which it loses its energy. If, however, by reason of the irregu- 

 larities of the earth's surface such rings become broken up 

 each part will revolve as an independent ring having an incli- 

 nation to the earth's surface and by the interferences and mix- 

 tures of these rings the motions may become reduced so as to 

 agree with the observed winds. From this point of view all 

 movements from the equator carry rapidly revolving air into 

 regions where the rotation is less rapid and such air must rise 

 above the earth's surface, whereas for motions toward the 

 equator the air must descend and hug the earth's surface there- 

 by acquiring the greater speed of rotation that belongs to low 

 latitudes and forming local high pressure areas. 



(e.) We see therefore that whether we study horizontal or 

 inclined or even vertical whirls in the atmosphere, we are alike 

 brought into the presence of ascending and descending motions 

 which the air is forced to take for purely kinetic reasons. So 

 long as we, with the above-mentioned authors, consider only 

 the ideal case of a homogeneous, incompressible, dry atmosphere 

 having an average depth of about 8000 meters and whose tem- 

 perature at any point depends upon its absorption of solar 

 radiation and upon its warming up by conduction, we have 

 perhaps no need of a special study of the thermodynamics that 

 is so important in the real atmosphere of compressible air. 

 For such a problem it may therefore be sufficient to assume, 

 as seems to be implied by Ferrel, Hann and Helmholtz, that 

 the air loses heat by the simpler process of conduction to the 

 cold earth. But when we investigate the real atmosphere on 

 the basis of actual observation we find that the thermodynamics 

 of the compressible air acids a most important feature. 



(«/•) We have thus far concluded only that local areas having 

 a slight excess or deficiency of pressure would be maintained 

 if the atmosphere were perfectly dry and were an incompres- 

 sible fluid ; but if the air is moist and is compressible then the 

 regions of diminished pressure give rise, by Espy's thermody- 

 namic process, to the formation of clouds and rain which form 

 temporary and local episodes in the general motion. Such con- 

 densation of moisture especially tends to form in the rising 

 part of the circulating air, and thus cooperates with the general 

 mechanism to produce spiral, ascending currents that intensify 



