The Science of the Weather 781 



deviated towards the right by the effect of the earth's rotation, 

 as already explained, so that a system of large eddies will be set up 

 in the atmosphere and there will be a tendency for the air to circu- 

 late round regions of high pressure in a direction similar to that 

 of the hands of a clock, while round low-pressure areas the direc- 

 tion will be counter-clockwise. All our changes of weather are 

 due to these circulating systems whose names are made familiar in 

 the daily weather forecasts: the "low-pressure system," which used 

 always to be termed a "cyclone," is now more commonly referred 

 to as a "low," or a "depression." The high-pressure system is 

 spoken of as an "anticyclone" or "high." The anticyclone may 

 be regarded in general as the normal settled weather, while the 

 depression bespeaks bad and usually stormy weather. 



The cyclonic depressions form and develop chiefly along a 

 belt which stretches from the great lakes of America across the 

 Atlantic to north-western Europe, and they are much more 

 numerous in winter than in summer. It is worthy of note that 

 at this season there is extreme cold in central Canada, while the 

 combined influences of the Gulf Stream Drift and the general 

 south-westerly current of wind across the warm ocean maintain 

 a temperature on the coast of Norway fully 25 F. warmer than 

 that found in the Gulf of St. Lawrence, about 1,000 miles farther 

 south. Then there is a cold ocean current streaming down past 

 Labrador from Baffin's Bay, and a steady flow of cold air is 

 sweeping from off the ice-clad plateau of Greenland. The 

 temperature conditions are therefore very complicated and very 

 favourable for the development of the depressions. The depres- 

 sions formed over America move generally north-eastward to- 

 wards Greenland, while those reaching our neighbourhood usually 

 commence out on the Atlantic, and either skirt our Islands in a 

 north-easterly direction towards northern Norway or move across 

 them on an easterly course to the Baltic. 



Let us see what happens when one of these depressions passes 

 over our Islands. For the purpose we will assume that for some 



