AEROGRAPHER'S MATE 3 & 2 



temperatures, the Asiatic Continent. In summer, 

 this high appears as an extension of the Pacific 

 high and is again displaced toward the area of 

 coolest temperature, which in this case is the 

 extensive water area of the Pacific. 



In winter over North America, the most 

 significant feature is the domination by high- 

 pressure cells. These cells are also due to 

 cooling but are not as intense as the Asiatic 

 cells. The Greenland high, for example, due 

 to the Greenland icecap, is a persistent feature, 

 but it is not a well-defined high during all 

 seasons of the year. It often appears to be an 

 extension of the polar high, or vice versa. 



Recent explorations into both the Arctic and 

 the Antarctic have revealed considerable 

 variations in pressure in these regions and the 

 presence of many traveling disturbances during 

 the summer months. 



Other continental regions show seasonal 

 variations from low pressure in summer, but 

 are generally of small size, and their location 

 is variable. Therefore, they are not considered 

 to be centers of action. 



In summer, the most significant feature is 

 the so-called heat low over the southwestern 

 part of the continent which is caused by extreme 

 heating in this region. 



counterclockwise in the Southern Hemisphere. 

 The windflow in an anticyclone is slightly across 

 the isobars and away from the center of the 

 anticyclone. (See fig. 13-9.) Anticyclones are 

 commonly called highs or high-pressure areas. 



The formation of an anticyclone or the 

 intensification of an existing one is called anti- 

 cyclogenesis. The decreasing of the intensity 

 of an anticyclone is called anticyclolysis. 



The vertical extent of pressure depends 

 greatly on the air temperature. Since density 

 increases with a decrease in temperature, 

 pressure decreases more rapidly along the 

 vertical in colder air than in warmer air. 



In a cold anticyclone (such as the Siberian 

 high), the vertical extent is shallow; while in a 

 warm anticyclone (such as the subtropical high) , 

 the vertical extent reaches high into the upper 

 atmosphere due to the slow decrease in tem- 

 perature with elevation. 



When thinking of highs, lows, anticyclones, 

 and cyclones, do not confuse the terms. A 

 cyclone is a low, and an anticyclone is a high. 

 The terms "cyclone," "anticyclone," "low," 

 and "high" refer to the wind circulation and 

 to the atmospheric pressure. 



MIGRATORY SYSTEMS 



General (primary) circulation of the atmos- 

 phere, based on an average of wind conditions, 

 is a more or less quasi-stationary circulation. 

 Likewise, much of the secondary circulation 

 depends on more or less static conditions which, 

 in turn, depend on permanent and semiperma- 

 nent high-pressure and low-pressure areas. 

 Changes in the circulation patterns thus far 

 discussed have been largely seasonal. However, 

 secondary circulation also includes wind systems 

 which migrate constantly, producing rapidly 

 changing weather conditions, especially in the 

 middle latitudes throughout all seasons. 



The migratory systems of circulation are 

 associated with air masses, fronts, cyclones, 

 and anticyclones. Air masses and fronts are 

 covered in detail in the next chapter. 



anti cyclonic 



circulatio 



209.40 



Figure 13-9. — Anticyclones. 



Cyclones 



Anticyclones 



An anticyclone is an area of relatively high 

 pressure having the wind circulation in a clock- 

 wise direction in the Northern Hemisphere and 



A cyclone is a circular or nearly circular 

 area of low atmospheric pressure around which 

 the winds blow counterclockwise, and slightly 

 across the isobars toward the center in the 

 Northern Hemisphere. The direction of rotation 



304 



