43 



have usually been guided by the mean cyclone track, as compiled by 

 the statistician, and the barometric tendency gained by simultaneous 

 observations from scattered meteorological stations. Probably some 

 of the most valuable recent contributions to the forecasting art are 

 the investigations of Bjerknes into the structure of cyclones. De- 

 tailed analysis of individual cyclones revealed the following two main 

 types of classification: 



(a) Cyclones which have a definite warm sector separated 

 from the cold part by definite surfaces of discontinuity. 



(&) Cyclones exhibiting no such individual parts at the 

 surface of the earth. 



The former are young intensifying storm centers while the latter 

 are old ones which tend toward retardation in their paths. When 

 they are treated separately a real discovery was made that class A 

 cyclones move in the direction of the air current in the warm sector 

 and very nearl}'^ with the same speed as the velocity of the air 

 in that sector. Since the direction of the wind is taken along the 

 isobars, the direction of travel of the storm center shown in Figure 8, 

 page 44, is AB. The isobars in the warm sector are drawn nearly 

 straight because it is found in general practice that they are quite 

 flat. The speed of the cyclone is found by multiplying the distance 

 between the isobars by the sine of the latitude. The whole wind 

 system is in motion and as a rule the direction of the isobar AB in the 

 northern hemisphere will swing anticlockwise and the path of the 

 center O will gradually curve to the left. Sometimes, however, 

 when a small cyclone moves along the edge of a warm anticyclone 

 the change is in the opposite direction. Bjerknes at the Geophysi- 

 cal Institute, Bergen, has found that class B cyclones although not 

 having distinct discontinuity surfaces such as class A exhibit on 

 the earth's surface, do have weather characteristics which correspond 

 to these latter and which do furnish similar information on the 

 career of class B cyclones. 



The cyclone is said to be born when two air masses of differing 

 densities come within proximity of each other; the thermal character 

 of the two bodies is the usually accepted index. There follows a 

 period of growth with a corresponding increase in intensity so long 

 as the structure is fed by a sufficient supply of cold and warm cur- 

 rents. Class A cyclones eventually begin to fill up or occlude as the 

 lower limits of the warm sector lift off the earth's surface and shallow 

 out. They are then known as class B cyclones, and the discon- 

 tinuity surfaces are only to be found at increased heights in the 

 troposphere. 



A great number of the storms which aft'ect the ice regions in early 

 season are class A cyclones and it is quite often the case that we are 

 able to observe the passage of the surface of discontinuity in many well- 



