IX 



Climates of the World 



calm and clear, and by intense radiation the land cools 

 down greatly at night, and the temperature of the air falls. 

 This is the condition required for long spells of frost, and 

 in large towns and over lakes and estuaries it produces 

 dense, low-lying fogs. The low temperature tends to in- 

 crease the density of the lower air in an anticyclone, 

 and until very recently was viewed as the main cause of 

 the formation of this arrangement of pressure. Fig. 29 



FIG. 29. Isobars of an Anticyclone. (After the Hon. Ralph Abercromby.) 

 Direction of wind shown for the northern hemisphere. The prevailing 

 weather in winter is shown on the left side, that in summer on the right side 

 of the diagram. 



shows the form of isobars and the kind of weather in a 

 typical anticyclone, which may be summarised as a very 

 steady and nearly stationary descending eddy or whirl of 

 banked-up air crowded into one place by the converging 

 currents of the upper atmosphere as they flow toward the 

 poles. 



206. Cyclones. An area of low atmospheric pressure 

 which has the lowest pressure in the centre was called a 

 Cyclone, because the early observers believed that the wind 

 blew round it in circles. We now know that wind blows 

 in toward the centre of low pressure in a spiral curve with 



