WEATIIKR INFIXTP:NCES 177 



across the southern waters of the North Sea are 

 undertaken under the favourable meteorological condi- 

 tions which have just been explained — namely, the pre- 

 valence of fine weather at the area of embarkation, and 

 extending to the cast in spring and to the west in 

 autumn (see Plate V.). Some observations on the 

 autumn weather-intluences favourable and unfavourable 

 to passages along this interesting line of flight will 

 be found in Chapter XV'III., which is devoted to an 

 account of my bird - watchings on board the Kentish 

 Knock Lightship. 



Unfavourable Weather Conditions. — When we come 

 to inquire into the weather which prevails when the 

 winds range from west to north, which is the most 

 unfavourable type of all for the passage of migratory 

 birds to and from the British Isles, the reason becomes 

 at once apparent. It results from the presence of a 

 cyclonic or low-pressure area to the north-east or east 

 of the British Isles — a type of weather which is fatal 

 to intermigration between North - Western Europe 

 and Britain, because unfavourable meteorological con- 

 ditions then prevail over the area whence we derive 

 the majority of our migrants in the autumn, and 

 over our isles. It is also against the return move- 

 ments from the British shores in spring. On the 

 other hand, this type of weather may not be un- 

 favourable for migration from Iceland to Northern 

 Britain in autumn ; nor for arrivals in England 

 from countries to the south - west of us in spring (see 

 Plate VI.). 



During the prevalence of these westerly winds, if the 

 weather be not severely unsettled, some migration from 

 the Continent takes place, but no general movements 



I. M 



