METEOROLOGY 17 



from the sea, observations of showers by a steamer between 

 Norway and the Faroe confirming this conclusion. At the 

 southern stations where the rain was presumably orographical 

 only, 2 mm. in 24 hours was the largest amount. Again, in 

 the succeeding 24 hours to 7 h. on the 9th, a strip of coast escaped 

 rain, on this occasion up to latitude 62°. The purely orographi- 

 cal rain farther inland in this case nowhere reached 5 mm. 

 Three years of study of such records as these have led to the 

 conclusion that purely orographical rain never exceeds this 

 amount in Norway, although a westerly current in which only 

 scattered showers are falling may yield as much as 30 mm., 

 most of which must be orographical. In this connection it 

 should be observed that stable air-currents when forced to 

 ascend the mountains tend to become heavier than the surround- 

 ing air at the same level ; they will therefore generally move 

 round an obstacle rather than over it. With unstable currents, 

 however, there is more likelihood of direct ascent of the 

 obstacle. It is for this reason no doubt that the orographical 

 rain from unstable currents is so greatly in excess of that from 

 stable currents. Most of the orographical rain of Norway 

 occurs together with rain of different origin. 



Cyclonic Rain. — ^To illustrate the production of cold- 

 front rain, an example which occurred on July 24, 191 8, is 

 examined in great detail. A mass of cold air which arrived 

 from the west gave rise to a well-developed " squall-line " with 

 a narrow strip of continuous rain immediately behind it and 

 in its rear scattered showers only (this type of rain has received 

 so much attention in the earlier papers of V. and J. Bjerknes 

 that a detailed description is unnecessary). Several observa- 

 tions of the temperature of the upper air are available for the 

 region between North-Eastern France and Southern Denmark, 

 and enable the slope of the incoming wedge of cold air to be 

 determined as having been roughly i in 275. The upper 

 surface of the cold wedge showed in some cases as a pronounced 

 inversion of temperature and in others as a diminution of the 

 " lapse-rate." The explanation offered of the narrowness of 

 the " rain band " is that the winds above the boundary surface 

 were westerly and mostly stronger than those below, conse- 

 quently the warm air did not have to ascend here as it must 

 have done in front of the squall line. 



A type of rain of the cold-front variety that may be 

 called " prefrontal " rain, due to an opposing barrier of moun- 

 tains, is also discussed. 



As regards warm-front rain, due to the ascent of a retreat- 

 ing wedge of cold air by advancing warm air, an example in 

 January 1921 is studied in great detail, but little is added 

 to the general conclusions arrived at in the earlier papers of 



