GENERAL CIRCULATION OF THE ATMOSPHERE 277 



atmospheric moisture, and some to other causes. These irregular 

 winds will be referred to in the next chapter. 



Summary. We may now recall the chief points thus far studied 

 in connection with atmospheric circulation. These are as follows : 



(1) Above the lower part of the atmosphere there is a pole- 

 ward movement of the air from low latitudes. 



(2) There must be a return movement of air from high lati- 

 tudes to low; but outside the extra-tropical belts of high pressure, 

 this movement is not well defined in the lower part of the air. 



(3) The extra-tropical high-pressure belts are the zones from 

 which the dominant planetary winds at the bottom of the atmos- 

 phere start. 



(a) These planetary winds tend to blow poleward and equator- 

 ward in each hemisphere, from the belts of high pressure. They 

 (and all other winds) are deflected to the right in the northern 

 hemisphere, and to the left in the southern hemisphere, by the 

 rotation of the earth, thus establishing the double trade-wind 

 zone, and two zones of westerly winds. 



(4) The simplicity of the system of planetary winds is inter- 

 fered with by the inequalities of temperature between land and sea 

 in the same latitude. 



Velocity of wind. In general, the average velocity of winds is 

 greatest in latitude 50 or thereabouts. The average velocity for 

 the United States has been estimated at about 9.5 miles per hour, 

 and for Europe 10.3. The velocity is greater over the sea than over 

 the land, largely because the moving air is checked on land by 

 friction with the uneven surface. 1 It is also greater in the upper 

 air than in the lower, for the same reason. 



THE GENERAL CIRCULATION AND PRECIPITATION 



Rainfall is of great importance to all plants and animals which 

 live on the land. Few plants live in desert regions; and few animals 

 live where plant life is scanty. Human industries, too, are much 

 affected by rainfall, for no arid region supports a dense population. 



1 Helmholtz has calculated that if the whole body of air were set in motion 

 at the uniform rate of 20 miles per hour, it would take nearly 43,000 years to 

 slow it down to 10 miles, as a result of friction. 



