LESSONS IN GEOLOGY. 333 



Thus the air serves the surface of the earth as a sun-shade by 

 day and as a blanket by night, tending to equalize its own tem- 

 perature. This effect is greatly increased by the presence of 

 clouds in the air. If it were not for this property of the air it 

 would be so hot in summer, and cold in winter as to destroy all 

 such life as exists on the earth at present. 



As the surface of the earth is the main source of heat for the 

 air, the temperature of the air must decrease from the general 

 surface upward. Mountains and high plateaus usually have a 

 lower mean temperature than the general surface of the earth in 

 the same latitude. 



The difference in temperature between the tropical and polar 

 regions is the chief cause of different pressures in the air, and the 

 unequal pressures cause currents of air or winds. 



Along the equator there is a belt of light pressure ; at about 

 32 north and 25 south there are belts of heavier pressure, and 

 at about 64 north and 70 south there are areas of light pres- 

 sure. The movement of the air is from the heavy pressure toward 

 the light pressure. Currents blow from the belts of light pressure 

 at 32 north and 25 south toward the equator; these form an 

 upward current, which, after rising some distance, spreads out 

 toward the light pressures north and south. 



Owing to the rotation of the earth these currents do not flow 

 directly north and south, but those going toward the equator 

 from 32 north, go southwest as northeast winds, and those 

 from 25 south go northwest as southeast winds. And those 

 toward the north pole go northeasterly as southwest winds; 

 and those toward the south pole, go southeast as northwest 

 winds. In general the winds of the tropics are easterly winds 

 and those of the temperate zones westerly winds. t 



These winds represent the general movements of the air subject 

 to many local exceptions, occurring in each grand division, es- 

 pecially in the coast regions. 



The air always contains water vapor, the amount depending 

 on the temperature of the air and on its distance from large bod- 

 ies of water. The air does not hold the water vapor as the 



