METEOROLOGY. 301 



find the momentary direction of propagation of the cyclone. On 

 account of this property this hne of convergence may be termed the 

 steering-line. Simultaneously the velocity of the propagation may be 

 computed approximately by the differential properties of any one of 

 the two lines of convergence. 



The importance of these Unes is increased by their meteorological 

 properties. The warm air providing the cyclone with, a part or wdth 

 all its energy flows in the sector bordered by these two hnes. During 

 the propagation of the entire system the steering-line will form the 

 front of a propagating warm w^ave. In the same manner the other 

 line will form the front of a propagating cold wave. This cold wave is 

 much more marked than the warm one, and is followed by heavy 

 rain, falling from the warm air which is hfted by the propagating 

 cold air. In many cases this rain is associated with thunder and 

 violent wind. This line of convergence is therefore in reahty identical 

 with the squall-line, which has often been observed to accompany 

 cyclones. The steering-line is not accompanied by so violent mete- 

 orological phenomena as the squall-Une, though v/hen sufficiently near 

 the cyclone it gives much rain. This rain, which does not merely 

 follow after the hne, but may precede it up to 200 kilometers or more, 

 falls from the warm air sliding up the slightly inclined plane which 

 separates the upper warm air from the underlying cold air, which is 

 swept away during the propagation of the cyclonic system. 



The investigation of these two characteristic hnes of convergence 

 will evidently be important not only for the theory of cyclones but 

 even for practical weather forecasting. The success of the practical 

 methods will, however, depend upon the completeness and exactitude 

 of the meteorological observations. Great attention must be paid to 

 the observation of the wind, especially its direction, and the stations 

 for observing the wind must be much more numerous than meteoro- 

 logical stations of the first class now employing telegraphy. 



In order to utilize the new methods for the Norwegian weather 

 service during the most important summer months, a great number 

 of stations (observing only wind and w^eather) have been interpolated 

 between the first-class stations. The result has been encouraging. 

 This system has made it possible to give satisfactory weather fore- 

 casts in spite of the enormous difficulty arising from the lack of 

 weather telegrams from the surrounding countries involved in war. 



