METEOROLOGY. 371 



METEOROLOGY. 



Bjerknes, V., Bergen, Norway. Preparation of a work on the application of 

 the methods of hydrodynamics and thermodynamics in practical meteorology 

 and hydrography. (For previous reports see Year Books Nos. 5-20.) 



The importance of the atmospheric surfaces of discontinuity and their con- 

 nection with the cyclones have been emphasized in the reports for the last 

 years. The continued investigations have led to the following general view 

 of the meteorological phenomena of the temperate zone and their role in the 

 general atmospheric circulation. 1 



The cyclones of the temperate zone originate and propagate as waves in 

 atmospheric surfaces of discontinuity. These surfaces, or at least their most 

 conspicuous sections, extend from southwest to northeast. The cjrclones 

 generate in the region of the southwest extremity and die off in the region of 

 the northeast extremity, being, during their propagation, gradually transformed 

 from propagating waves to stationary vortices. In this way a somewhat 

 varying number of cyclones may coexist in the same surface of discontinuity. 

 The most generally occurring numbers seem to be two, three, or four. 



The entire system of the discontinuity with its propagating cyclones has 

 itself a motion from west to east. The cyclones are always moving through 

 this propagating system from its rear, where they originate, to its front, where 

 they die. Therefore the velocity of propagation of the individual cyclones is 

 considerably much greater than that of the discontinuity with its "cyclone 

 family." 



Such a family of cyclones is generally succeeded by a northerly current with 

 anticyclonic weather conditions. Then follows a new discontinuity with its 

 family of cyclones, again succeeded by the northerly current and the anti- 

 cyclone, and so on. This repeats itself with great regularity. The interval 

 between the arrival of one cyclone family and the next is 53^ days on the 

 average. This gives the explanation of a remarkable climatological period of 

 precisely this length, which has been found through statistics made upon the 

 different meteorological elements in a variety of different places of the 

 northern hemisphere. 



When we combine this length of period with the geographical extent of the 

 cyclone families or with their velocities of propagation as measured on the 

 charts, we arrive at the following view of the general atmospheric circulation 

 of the northern hemisphere: 



In general there exist four currents conveying polar air from the polar 

 regions to the trades. These four currents originate at right angles with each 

 other, but turning to the right they get a spiral form with a general direction 

 from northeast to southwest. Between and above these polar currents we 

 have a general poleward motion of tropical air, directed from southwest to 

 northeast. The system of these four polar currents goes round the earth from 

 west to east, performing an entire revolution in 22 days. At the left flank of 

 these revolving currents we have the moving discontinuities in which the 

 cyclones propagate. At the right flank we have the anticyclone, which 



1 Cf. J. Bjerknes and H. Solberg, Life cycle of cyclones and the- polar-front theory of atmos- 

 pheric circulation, Geofysiske Publikationer, vol. in, No. 1, Kristiania (1922). 



