AUTUMN RAIN 151 



applied. The difference between the shallow cyclonic de- 

 pressions which bring a day's light summer rain in England 

 and the destructive storms of the tropics is simply one of 

 degree. Cyclones are characterised by the thinness of the 

 air in their midst, and by the inrush of air from outside to 

 fill the vacuum ; a rough mental picture of their structure 

 and motion may be gained from the revolving eddies which 

 sweep downstream past the piers of a bridge in a flood or 

 a strong ebb-tide. The thin air within them exercises a 

 comparatively feeble pressure on the column of mercury in 

 a barometer ; it therefore sinks, and we speak of the 

 barometer being depressed, and of the system which causes 

 it as a depression. Additional appropriateness is given to 

 the phrase by the fact that an anticyclone consists of a great 

 pile or mound of air, while the centre of a cyclone has a 

 comparatively thin layer ; compared with the structure of an 

 anticyclone, a cyclone is a depression from this point of view 

 also. It is the agglomeration and compression of the air in 

 an anticyclone which causes the high pressure, as marked by 

 the barometer ; and the comparative rarity of the air in a 

 cyclone which brings the barometer down. The inevitable 

 tendency of the compressed air to flow into the eruptive 

 eddies causes the cyclonic winds ; and thus cyclones and 

 anticyclones are indissolubly linked. 



Cyclonic systems in our latitudes generally, though not 

 invariably, move from west to east ; and the on-coming of an 

 autumn gale is marked by definite and well-known features. 

 Often the first sign on a clear October morning is a shift of 

 the wind to the south-east, or a wind unexpectedly springing 

 up from that quarter out of a frosty calm. It is the first 

 suck of the air into the depression still many miles away ; 

 and as it is drawn from the Continent or the cold North Sea, 

 it is generally chilly. The sun gradually fades in a formless 



