METEOROLOGY. 351 



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. 5-17.) 



The extended Norwegian Weather Service, alluded to in the previous 

 report, has furnished rich material for the investigation of the struc- 

 ture of cyclones and the conditions for the formation of rain. Pre- 

 liminary reports on the results obtained have been given in Professor 

 Bjerknes's address "Weather forecasting" and in J. Bjerknes's paper 

 "On the structure of moving cyclones," both of which have also been 

 reprinted in the Monthly Weather Review (February 1919). The 

 general results may perhaps best be summed up thus : 



The atmosphere is crossed and recrossed by surfaces of discontinuity, 

 separating from each other masses of air having more or less different 

 velocity and different physical properties, showing themselves by 

 differences of temperature and humidity, and, as pointed out by Mr. 

 Bergeron, also by marked differences of transparency. Almost every 

 change of weather is due to the passage of a surface of this kind. The 

 rain falls from the warm air when it is forced to mount the slanting 

 surface separating it from the underlying heavier and colder air. In 

 the cyclones this takes place on a gigantic scale along the "steering 

 surface" and the "squall-surface," as developed in the mentioned 

 paper by J. Bjerknes. But the same play may be recognized also in 

 the phenomena of smaller scale down even to the local showers. 



These results give a physically simple and intelligible view of the 

 phenomena of the weather chart and seem promising from a practical 

 point of view. For promoting weather-forecasting it will be of high 

 importance to arrange the observations so that the formation of the 

 discontinuities can be detected at an early state, and their propagation 

 followed as accurately as possible on the weather charts. 



