HEAT AND THE ATMOSPHERE 165 



Transfer of Heat at the Earth's Surface. Since 

 heat is motion, it is better to speak of the transfer of 

 it than of the motion of it. There are three ways in which 

 heat is transferred, and all of these are important in con- 

 nection with the natural distribution of it over the earth's 

 surface. These three methods of heat transfer are radia- 

 tion, conduction, and convection. Conduction and con- 

 vection you have already studied (see Chapters XVIII 

 and XIX) . Radiation we will now consider. 



Radiation. It is natural to place radiation first in this 

 list, because by this method heat travels from the sun to 

 the earth; our original supply of it comes to us in this 

 way. Radiation means movement outward equally in every 

 direction; it may be represented by the radii of a circle, 

 or, better, of a sphere. When we speak of the sun's "rays" 

 we are expressing the idea of radiation. A radiator radi- 

 ates heat out hi every direction in a room. 



Heat is not the only form of energy which travels by 

 radiation. Light also radiates. Heat and light are then 

 forms of radiant energy. (What is an example of energy 

 which is not radiant ?) Radiant energy travels in the form 

 of invisible "waves," and it is a kind of energy transfer 

 which does not require substance through which to move, as 

 is proved by the fact that light and heat come to us through 

 enormous spaces that are practically empty of molecules. 



Now the importance of radiation in connection with the 

 distribution of heat over the earth's surface is due to the 

 fact that the sun shines on only half of the earth at a time. 

 This is a very familiar fact, but we do not often stop to 

 think what all the consequences of it are. One of the 

 most important of them is that while half of the earth 



