AUTUMN RAIN 155 



determined partly by study of the daily records of the 

 direction of the wind over the Atlantic and European region, 

 and partly by consideration of the causes which produce 

 rain. According to the present evidence, the rain in a 

 cyclonic depression is due to the meeting of the warm moist 

 southerly current with the cold and dry winds coming from 

 the east and west. When the depression has passed, so 

 that the southerly current on its front has retreated from 

 us, the rain quickly ceases, and the sky clears. The south- 

 west wind which usually blows during the wettest and 

 stormiest period of the cyclone, is due to the fusion of the 

 southerly and westerly currents ; and the north-west wind, 

 which is characteristic of the retreat of the depression, is 

 probably the easterly current on the north of the depres- 

 sion curving round to form the current from the west. 

 Where this cold current drawn from the north meets the 

 warmer and moister air to the south of it, it produces the 

 heavy ' clearing shower/ 



The same causes which produce rain in the different 

 quarters of a cyclonic depression, influence the rainfall in 

 the different parts of England. Rainfall is heaviest about 

 high ground ; and this is the chief reason why the west of 

 England is far wetter than the east. In its hills and high 

 moors gather the bulk of the air-borne moisture. Hills help 

 to precipitate moisture in two ways. Their surfaces are 

 colder than the warm air coming from the southern Atlantic, 

 so that they condense its invisible vapours ; and they throw 

 up the air-currents to a greater elevation, so that the 

 vapour condenses by the cooling consequent on reduction 

 of pressure. Both these processes are believed to help in 

 the production of a cyclonic rain-storm. The mingling of 

 the warm southerly current with the cold easterly and 

 westerly currents causes rain, in the same way as the cold 



