THE WORK OF THE ATMOSPHERE 87 



and semiarid regions. When it is noted that desert regions 

 cover about 11,500,000 square miles, or over one fifth of the 

 land of the world, it is evident that the areas where wind work 

 is of prime importance are by no means restricted. It is not 

 apparent, furthermore, that desert areas are relatively more 

 extensive now than at various times in the past, so that the 

 wind has been a very important agent of change through long 

 ages. Although it is most important in dry regions, the work 

 of the wind is by no means confined to them. Rather is it 

 world wide. 



Material gets into the air in many ways. It is picked up 

 by ascending air currents, given out by chimneys, stirred up 

 along dusty roads by animals and vehicles, discharged by 

 volcanoes, and delivered to the air in a variety of other ways. 

 Once in the air, gravity tends to pull it back to the surface, 

 but its fall is retarded by friction with the atmosphere. If 

 the material is fine (dust), the surface it exposes to the friction 

 of the air is great in proportion to its weight, and it settles 

 very slowly. Before reaching the ground, it may encounter 

 ascending currents and be carried up with them. When it 

 falls again, it may meet and be lifted by other up-going cur- 

 rents, or may settle to the ground. Fine dust carried up to 

 great elevations has sometimes remained in the air for many 

 weeks at a time. After it reaches the surface, it may be 

 moved repeatedly by the wind. 



The amount of dust in the air at one time, even in moist 

 regions, is often very great. In addition to vast numbers of 

 larger dust particles, the air in many places (e.g. great cities) 

 contains hundreds of thousands of invisible dust motes per 

 cubic centimeter (about f of a cubic inch). It has been 

 estimated that in violent dust storms the air may contain as 

 much as 126,000 tons of dust and sand per cubic mile. 



Dust is transported great distances by the wind. It settles 

 on ship deck in mid-ocean, has been carried from volcanoes 

 to snow-capped mountains in distant parts of the world, 

 where it was found subsequently, and in the exceptional case 



