WEATHER INELUENCES 175 



tion of the weather charts affords a simple explanation 

 — namely, that though the weather in Scandinavia was 

 entirely favourable for emigration, the conditions prevail- 

 ing on the British coasts were adverse, owing to the too 

 close proximity of a western low-pressure centre. On 

 the location and character of this cyclonic centre depends 

 the nature of the weather in the immediate neighbour- 

 hood of our shores. If it be too close to Britain, or if the 

 depression be exceptionally deep, then strong winds are 

 experienced on our eastern shores, with the unfortunate 

 results to the bird-travellers just described. It must be 

 remembered, too, that the winds are not necessarily light 

 except in the central region of an anticyclone. On the 

 rims we sometimes get strong winds. If the south- 

 western rims were off the east coast of Britain similar 

 disasters would befall the approaching migrants. 



It has already been mentioned (p. 156) that under 

 certain weather conditions in the autumn, both arrival 

 and departure movements are in progress simultaneously 

 on our shores. On these occasions, it has been ascer- 

 tained that the anticyclone in North-Western Europe 

 covers a wide area and extends beyond the limits of the 

 British Isles, and hence weather is favourable for 

 departure movements from our islands. When such is 

 the case, there occurs, simultaneously, an inpouring of 

 birds on to our shores from Northern Europe, and an 

 outpouring from Britain of emigrants of many species. 



The duration of favourable spells for bird-migration, 

 in any area, is sooner or later broken by the setting in 

 of a cyclonic period, with more or less unfavourable 

 conditions, which may curb, or make impossible, the 

 progress of the seasonal movements. 



The following experience (one of many) at Fair Isle 



