CHANGES IN THE DENSITY OF AIR 233 



263. Changes in the Density of Air. If we can imagine 

 that there is, near the earth's surface, a layer of air all of 

 about the same density, and if we also imagine that some 

 of this air becomes heated, we can understand how as- 

 cending air currents are produced (cf. 67). We say that 

 the heated air "rises"; what really happens is that the 

 surrounding, heavier air is pulled, by gravity, into the 

 place occupied by the heated, lighter air, and the heated 

 air is pushed up in the form of a current. The movement 

 of the air in currents must, of course, be more rapid than 

 the diffusion of gases (cf. 106), or the lighter air will be 

 mixed with the heavier air, and there will be no convection. 



Air is made lighter, not only when it is heated, but also 

 when it takes up moisture. Steam has a lower density 

 than air (cf. Appendix, Table IV) ; hence air that is mixed 

 with much of it will be decidedly lighter than dry air at 

 the same temperature. 



How high will an ascending air column go? It will rise 

 until it reaches a level at which its density is the same as 

 that of the air surrounding it. Then it will be flattened 

 out as horizontal currents (winds); in time these will 

 descend again to the earth. In this way we get "circu- 

 lation" in the atmosphere. 



Since the pressure of the atmosphere depends on the density of the 

 air, a barometer (cf. 40) placed in a region from which air is ascending 

 will show that the region is an "area of low pressure." The weather 

 observer reads the atmospheric pressure from his barometer, and uses 

 this pressure in ' * forecasting " the weather; but we must remember that 

 the barometer tells only the pressure of the atmosphere. If we wish 

 to draw any conclusions regarding the weather, we need other facts in 

 addition to the barometer reading. 



The aneroid barometer is more convenient to carry about than the 



