92 THE MECHANICS OF THE EARTH's ATMOSPHERE. 



Therefore the zone of cyclones begins there, but these become more 

 frequent farther northward. We can certainlj- assume that the process 

 of mixture is not perfected immediately at the exact border of the 

 trade- wind zone, but that a part of the rapidly-rotating warm upper 

 stratum remains unchanged or half mixed, which will presently bring 

 about new mixtures farther on toward the pole. 



In general, in this zone of mixture, even below at the earth's surface, 

 the west wind must retain the upper hand because the increase of the 

 total moment of rotation which the mass of air, through friction, experi- 

 ences in the east wind of the trade zone must finally rise to such a irltch 

 that somewhere the west wind again touches the earth and experiences 

 sufficient friction to entirely give back the increase that it had. The 

 masses of air resting in the equilibrium of stratification can certainly 

 have no long-continued motion of rotation that differs essentially from 

 that of the earth beneath them. When therefore they are mixed with 

 the stronger west wind of the air from above, they receive a movement 

 toward the east. Moreover the falling rain that in great part comes 

 from the upper west winds, must transmit its motion to the lower strata 

 through which the rain falls. Eventually all zones that are pressed 

 polewards by intermixed masses moving equatorially and descending 

 from them will become west winds. 



Another permanent source of winds is the cooling of the earth at the 

 l^oles. The cold layers endeavor to flow outwards from each other at 

 the earth's surface and form east wind (or anti-cyclones). Above these 

 the warmer upper strata must fill the vacancy and continue as west 

 winds (or cyclones). Thus an equilibrium would come about, as is 

 shown in Sect, ii, if it were not that the lower cold stratum acquires, 

 through friction, a more rapid movement of rotation, and is therefore 

 competent for further advance. In doing this, according to the above 

 given views this lower stratum must remain on the earth's surface. 

 That in fact it does so is shown by frequent experiences during our 

 northeast winter winds whose low temperatures frequently enough do 

 not extend up to even the summit of the North German Mountains. 

 Moreover on the front border of these east winds advancing into the 

 warmer zone, the same circumstances are effective in order to bring 

 about a discontinuity between the movement of the upper and lower 

 currents, as in the advancing trade-winds, and there is therefore here 

 a new cause for the formation of vortex motions. 



The advance of the polar east wind, although recognizable in its 

 princii)al features, proceeds relatively very irregularly since the cold 

 pole does not agree with the pole of rotation of the earth, and also> 

 because low mountain ranges have a large influence. In addition to 

 this comes the consideration that in the cold zone fog causes only a mod- 

 erate cooling of the thicker stratum of air, but clear air brings about 

 a very intense cooling of the lower layer. By such irregularities, it is 

 brought about that the anti-cyclonic movement of the lower stratum 



