CHAPTER XIV 



CONVECTION 



85. Convection Currents. When air is heated, either by 

 conduction or radiation, it expands, becomes less dense, and is 

 pushed upward by the surrounding cooler air near it. This 

 upward current of air is called a convection current. It really is 

 but part of a circuit of larger or smaller diameter, for if the 

 current of air rises in one place a similar current must descend 

 in another to take the place of the 



air that has risen. A convection cir- 



cuit of this kind is always caused by 



a difference in the temperature of 



the air. A cake of ice which cools 



the air and makes it heavier will 



cause a convection current just as 



feadily as heat will, although in this 



case the initial movement is down- 



ward instead of upward. Convec- 



tion is sometimes described as a third 



way in which objects lose heat, but it 



is very evident that no new principle 



is involved, and that the heating or cooling of a body exposed 



to the air movement is primarily due to radiation or conducton. 



Convection currents may also be caused by variations in the 



amount of moisture in the air. Moist air is lighter than dry 



air and of course has a tendency to rise above it. 



86. Winds. Probably the most noticeable convection cur- 

 rents are those which occur as the result of the unequal heating 

 of the air over adjacent regions. When this occurs, a circuit is 



95 



FIO. 32. Convection cur- 

 in air - <- Black and 



