i6 SCIENCE PROGRESS 



(May 1 921). This paper may be regarded as the joint pro- 

 duction of the forecast service at Bergen, supplemented by 

 researches carried out there and in Stockholm by T. Bergeron, 

 in 1919, and summarises the results of three years' research 

 into the mode of rain formation in Norway. 



It has been the practice at Bergen for forecasters to attempt 

 an explanation of all rain reported at their observing stations, 

 and this system has done much to stimulate enquiry, A scheme 

 of classification has been adopted, which is outlined at the end 

 of the paper, as follows : 



(I) Cyclonic Rain. 



(a) " Warm-front " rain, formed by warm air pushing 



upwards over a retreating wedge of cold air, 



(b) " Cold-front " rain formed in warm air displaced by 



an advancing wedge of cold air. 



(II) Instability Showers. 



{a) Produced by heating from warm sea surfaces. 

 {b) By insolation over land (local showers), 



(III) Fog and Rain (drizzle). 



Slight rain formed in the lower layers of the atmosphere 

 by cooling of air against relatively cold land or sea 

 surfaces. 



(IV) Orographical Rain. 



Formed in air currents when ascending mountains. 



Maps showing lines of flow of the surface air are freely used 

 throughout these researches, and in the case of the large-scale 

 maps upon which the greatest amount of detail is depicted, 

 isobars are omitted because the number of observing stations 

 reporting wind is far in excess of those for which reliable 

 readings of the barometer are available. In drawing these 

 lines of flow the peculiarities of individual stations have been 

 allowed for to some extent. The different classes of rain are 

 taken one at a time, beginning with the orographical type. 



Orographical Rain. — ^The synoptic charts of August 7-9, 

 1920, furnish instructive examples of this kind of rain. On 

 these days a W. to N.W. wind-current crossed the Norwegian 

 mountains. Whereas the western slopes of the mountains 

 experienced cloudy to rainy weather, on the eastern sides it 

 was fair and dry. The greatest falls of rain occurred along a 

 strip of country about 50 kilometres from the coast. [This 

 occurs sufficiently often to be apparent on maps of annual 

 rainfall.] Betw^een the 7th and 8th rain fell at all stations 

 north of 61°, even at those on outlying islands of low level, but 

 farther south a strip of coast escaped altogether. It was con- 

 cluded, therefore, that the rain in the north was partly orographical 

 and partly due to showers carried along in the westerly current 



