June 24, 1920] 



NATURE 



523 



results will be given here. These will be seen to 

 give, to some extent, both verifications and further 

 developments of ideas, which, although advanced 

 by leading theoretical meteorologists, have not 

 yet exerted any noticeable influence upon the 

 development of meteorology.^ 



Fig. 1.— Cyclone. 



Great changes in the weather in our latitudes 

 have been found to depend upon the passage of 

 a line of discontinuity which marks the frontier 

 between masses of air of different origin. A line 

 of this kind was first found to exist in every 



current of cold air (Fig. i). The whole system 

 moves with the east-bound current, and the 

 cyclonic centre with the lowest pressure is in the 

 region where the cyclonic track touches the border , 

 of the tongue. The front border, before this ; 

 point, is curved like a reversed '* S " ; the rear 

 border, behind this point, has a uniform concave 

 curvature. Along the front border warm air from 

 the tongue ascends the barrier formed by the cold 

 air, which, in return, passes round the tongue in 

 order to penetrate below the warm air along the 

 rear border. Two bands of rain are thus formed 

 — a broad one in front of the tongue, where the 

 warm air spontaneously surmounts the cold, and 

 a narrow one, generally called the squall line, 

 along the rear border, where the advancing cold 

 air violently lifts the warm air of the tongue.^ 



It has been found by use of the detailed maps 

 that the line of discontinuity exists even out- 

 side the cyclone, passing from one cyclone to the 

 other ; they follow each other along a common line 

 of discontinuity, like pearls on a string. 



When one has become acquainted with all the 

 signs — direct and indirect — which are seen to 

 indicate the position of a line of discontinuity on 

 the very detailed maps, it proves possible to dis- 

 cover them even on less detailed maps. Fig. 2 

 shows roughly the course of such a line, on 

 January i, 1907, as it may be drawn upon the 

 Hoffmeyer maps of the Atlantic Ocean for that 

 day. When similar charts are drawn from day to 

 day, as accurately as circumstances allow, a series 

 of large-scale results very distinctly presents itself. 



Though we have been able to draw the line only 

 half round the pole, there can be no doubt that 



-Line of demarcation between polar and equatorial air, January i, 1907. 



cyclone which is not perfectly stationary. It here 

 borders a tongue of warm air, which from an 

 east-bound current penetrates into a west-bound 



1 Dove : " Das Geselz der Sturme," Vierte Auflage (Berlin, 1873). 

 Helmhottz: "Ueber atmosphiirische Bewegungen," Sitzungsberichte der 

 K. Preuss. Akad. der Wissenschaften i888, Meteorologische Zeitschri/t, 

 1888. Brillouin : "Ventu Contigus et Nuages," Annales du Bureau 

 Cen'ral MMoro/o^igue, 1898. Margules : " EnergiederStfirme," Jahrbuch 

 der K. K, Centralanstalt fiir Meteorologie, 1003, Anhang. 



NO. 2643, VOL. 105] 



it surrounds the polar regions as a closed circuit. 

 On the northern side of this line all signs indicate 

 air of polar origin ; it has a low temperature for 

 the latitude, shows great dryness, distinguishes 

 itself by great visibility, and has a prevailing 



• Cf. W. N. Shaw : " Forecastinjt Weather," p. ai2 (London, 1911). 

 I. Bjerlcnes : " On the Structure of Moving Cyclones," Geofysiske Publi- 

 kationer (Kristiania, 1919). 



