Geology 



For this reason the Doldrums is a region of heavy 

 rains, which in any place are seasonal, occurring twice 

 a year, as the Sun passes and repasses on his northward 

 and southward journeys. 



Very different are the conditions in the regions of the 

 tropics of Cancer and Capricorn, where the cold, dry air 

 currents are descending and therefore undergoing com- 

 pression, and a rise of temperature. They reach the 

 surface as currents of warm, dry air, and cause excessive 

 evaporation in their respective regions. 



It is here that all the great desert regions of the 

 world are to be found. In the Northern Hemisphere 

 we have the Sahara, the deserts of Arabia and India, 

 and of Colorado, and the neighbouring parts of the 

 United States. 



In the Southern Hemisphere the deserts are not so 

 well marked owing to the preponderance of oceanic 

 over continental areas, but they are nevertheless to be 

 seen in the Great Central Desert of Australia, the South 

 African Desert, and the dry pampas of South America. 



The effect of a mountain chain upon rainfall is 

 similar in principle. The warm, moist air from an ocean 

 blows towards the land and encounters a mountain chain 

 which it must cross. To do this, the whole current must 

 rise, and will therefore reach a region of lower pressure. 

 Expansion, cooling, and precipitation of moisture are the 

 natural results, and thus we get heavy rainfall on the 

 windward slopes of our mountains. 



On the lee-side of the mountains, the air current, 

 having lost its excess of moisture, flows down the moun- 

 tain slopes into the regions of higher pressure below, 

 and it consequently becomes compressed and warmed, 



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