THE ENGINE OF THE AIR 155 



low-pressure instability and of high pressure, which is usually 

 cool and fair weather, seeking to fill the low pressure areas. 

 Now in the semipermanent movement of winds over the 

 North Atlantic there are certain areas that remain more or 

 less uniform. There is a central high between the Azores and 

 Bermuda. Opposed to this there is a northward area pre- 

 dominantly low, called the Icelandic low. Again over the 

 Arctic regions high pressure usually prevails. 



This gives us a general picture of the big circulations of 

 the engine of our climate. But within this are lesser engines 

 of varying intensity that control the local and more changing 

 aspects of our weather, but that may also have power to 

 influence the major movements. These are the cyclonic storms 

 revolving in a counterclockwise direction that in general move 

 along the paths of equal pressure, the isobars of the weather 

 maps. Cyclones are caused by the fact that any rising current 

 or convection of heat from the earth tends to spin the air 

 into a vortex just as any descending column of air — or water 

 in a drain — will do. Around the edges of the slow revolving 

 flywheel of our engine of the North Atlantic climate, cyclonic 

 weather has its course, and the general path of wind and 

 weather follows the path of the Ocean River coiling slowly 

 around the periphery of waters. This must be, because of the 

 location of pressure areas and because winds follow lines of 

 like pressure, the isobars, with low-pressure areas on their left. 



Seasonally the hurricanes — the smaller but more violent 

 cyclones of the tropics — twist up the pathway of the Gulf 

 Stream from the Caribbean and move oE across the ocean 

 on this same course. Above the roaring forties of the Atlantic, 

 between these invisible walls of pressure, a series of cyclonic 

 storms in a widely varying path move from the continent 

 northeasterly by the Grand Banks toward England and Eu- 

 rope, and form the invigorating and changeable climate of 

 the north temperate. This path marks what is called the 



