WEATHER INFLUENCES 181 



Hence it is worth while to consider the cloudiness of the 

 various barometric distributions. x'Xnticyclonic skies 

 are not necessarily free from cloud, and in the winter 

 there is a state of the sky which goes by the name of 

 "anticyclonic gloom " among certain experts; but as a 

 working rule in spring and autumn anticyclonic weather 

 is generally fine. 



Every migratory movement has its own particular 

 and peculiar meteorological associations, for the condi- 

 tions controlling them are often of a more or less 

 complex nature. IVIost of them admit of explanation, 

 when the official weather reports, which afford the key 

 for their solution, are consulted. 



The Weather and the Spring Ininiigratory Move- 

 ments. — The meteorological phenomena which are associ- 

 ated with and influence these, must be sought in the 

 weather conditions prevailing in those southern countries 

 whence the migrants set out to reach the British Islands, 



Birds often arrive in our islands at this season 

 when the meteorological conditions with us are dis- 

 tinctly unpropitious, but a reference to the charts of the 

 weather prevailing in the countries to the south of us 

 invariably indicates that favourable weather prevailed 

 there, and induced the birds to move northwards. The 

 chief factor in these favourable conditions is an increase 

 in warmth, the influence of which has already been 

 treated of under the heading of Temperature. The 

 southerly types of weather (see Plates V. and VIII.) 

 are the most favourable for these spring movements 

 across the Channel from the south. 



The first arrivals of the summer birds appear, as a 



I. M 2 



