TEMPERATURE OF THE AIR 231 



the other end soon becomes hot. The heat seems to pass along the 

 iron rod from one end to the other. This method of passing heat 

 along is conduction. Any cold body in contact with a hot body is 

 warmed by conduction. The bottom of the air is warmed by con- 

 tact with the land (that is, by conduction) wherever the tempera- 

 ture of the land is higher than that of the air. The hand is warmed 

 by conduction when placed on metal or wood which feels warm; 

 it is cooled by conduction if the metal or wood feels cool. 



3. Convection. When a kettle of water is placed on a hot stove, 

 the water in the bottom is heated by conduction, that is, by contact 

 with the hot kettle. The heating of the water causes it to expand, 

 and when the water in the bottom of the kettle expands it becomes 

 lighter than the water above. The heavier water above then 

 sinks and pushes the lighter water below up to the top. This sort 

 of movement is convection. Another illustration of convection is 

 afforded by stoves, fireplaces, and furnaces. A thin sheet of light 

 paper may be held up for a moment by the rising air over a hot 

 stove, or even carried up if the convection current is strong enough. 

 Again, as the air in a chimney is heated, it expands and becomes 

 less dense than the air about it. The cooler, denser air about the 

 base of the chimney or stove crowds in below the expanded air 

 in the chimney, and pushes it up out of the chimney. Since 

 the air entering the chimney from below is being heated and ex- 

 panded all the time, the up-draught continues as long as there 

 is fire. Every draught from a chimney is an example of con- 

 vection. 



When the surface of the land is heated by the absorption of 

 heat from the sun, it warms the air above both by conduction and 

 by radiation. The lands of low latitudes are heated more than the 

 others. The heated air over the heated land expands and rises. 

 If the air in a given region were expanded as shown in Fig. 211, the 

 air at the top of the expanded column would flow away, much as 

 water would under similar conditions. After this takes place, the 

 amount of air at the base of the column h will be less than the 

 amount at the same level outside the heated area, and air from 

 outside the heated column will flow in. This inflow will push up the 

 column of expanded air, and further overflow above will cause 



