284 EARTH FEATURES AND THEIR MEANING 



tions of a very pronounced type. Each great ice mass with its 

 atmospheric cover constitutes a sort of refrigerating air engine 

 and plays an important part in the wind system of the globe. 

 (See Fig. 291, p. 263). Both the domed surface and the low tem- 

 perature of the glacier are essential to the continuation of this 

 pulsating movement within the atmosphere (Fig. 312). The air 

 layer in contact with the ice is during a period of calm cooled, con- 

 tracted, and rendered heavier, so that it begins to slide downward 

 and outward upon the domed surface in all directions. The ex- 

 treme flatness of the greater portion of the glacier surface a 



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CONTINENTAL. GLACIER 



FIG. 312. Diagram to show the nature of the fixed glacial anticyclone above 

 continental glaciers and the process by which their surface is shaped. 



fraction only of one degree makes the engine extremely slow 

 in starting, but like all bodies which slide upon inclined planes, 

 the velocity of its movement is rapidly accelerated, until a blizzard 

 is developed whose vigor is unsurpassed by any elsewhere experi- 

 enced. 



The effect of such centrifugal air currents above the glacier is 

 to suck down the air of the upper currents in order to supply the 

 void which soon tends to develop over the central portion of the 

 glacier dome. This downward vortex, fed as it is by inward-blow- 

 ing, high-level currents, and drained by outwardly directed sur- 

 face currents, is what is known as an anticyclone, here fixed in 

 position by the central embossment of the dome. 



The air which descends in the central column is warmed by 

 compression, or adiabatically, just as air is warmed which is forced 

 into a rubber tire by the use of a pump. The moisture congealed 

 in the cirrus clouds floating in the uppermost layer of the convec- 

 tive zone, is carried down in this vortex and first melted and in 

 turn evaporated, due to the adiabatic effect. This fusion and 

 evaporation of the ice by its transformation of latent, to sensible, 

 heat, in a measure counteracts, and so retards, the adiabatic ele- 



