METEOROLOGY 15 



but the ratio between the magnitudes of these vectors is a 

 function of the latitude. 



Class III. — " Antitriptic " winds, where the frictional 

 terms exceed the rotational and accelerational terms. The 

 equations are : 



'^ Bz' ~ pRW 



Bz^ pa) 80 



Bp 



Bz 



gp 



Here the wind is driven along by the pressure gradient, 

 and the friction prevents a steady increase of the velocity of 

 a particular sample of air throughout its journey, provided 

 that journey be long enough. 



The writer proceeds to consider how the various winds of 

 the Earth may be fitted into this scheme of classification. Con- 

 siderations of space permit only of a very short summary of 

 this second portion of the paper ; it should be observed that 

 whereas the work up to this point is mainly a recapitulation of 

 what has been achieved by mathematical meteorologists in the 

 past, this second section is entirely new. The conclusion is 

 reached that in the general circulation of wind over the globe, 

 the modified portions due to the presence of continents, together 

 with the cyclones of temperate latitudes, belong to the geostro- 

 phic class (Class II) ; the smaller wind system of the tropical 

 hurricane is Eulerian (Class I) ; while the land and sea breezes, 

 mountain and valley winds, etc., are probably more or less 

 antitriptic (Class III). Proceeding to a more detailed considera- 

 tion of a continental circulation of wind, it is pointed out that 

 to the geostrophic term must be added one or more of the 

 remaining terms, and reasons are advanced for selecting the 

 accelerational rather than the frictional terms. The ordinary 

 explanation of the Asiatic monsoon as an effect of the seasonal 

 variation of temperature over Asia appears to be substantially 

 correct, both qualitatively and quantitatively. Land and sea 

 breezes are considered next and shown not to be to any con- 

 siderable extent either geostrophic or Eulerian, so that they 

 must be antitriptic. A mathematical investigation of a sea- 

 breeze is next attempted, with results that confirm the supposi- 

 tion that these winds are " antitriptic." A similar investigation 

 of a valley wind leads to a similar conclusion. In both these 

 cases an important part is played by the departure of the " lapse 

 rate " of temperature from the adiabatic. 



Meteorological Conditions for the Formation of Rain, by 

 J. Bjerknes and H. Solberg— Geophys. Pub., vol. ii, No. 3 



