TEMPERATURE OF THE AIR 233 



air. On being warmed, whether by conduction or radiation, con- 

 vection currents are started between the lower air and the air above. 



After the heat is received from the sun, therefore, it undergoes 

 redistribution. This is accomplished not only by radiation, con- 

 duction, and convection, but also by movements of the air 

 (winds) and movements of the water (especially ocean currents). 

 Without these movements of air and water, the average tempera- 

 ture of the equator would be much (perhaps 50 F.) higher than now, 

 and that of the poles much (100 F. or more, estimated) lower. 



The heating of land and water. Land is heated four or five 

 times as fast as water by insolation. The reasons are several: 



1. A given amount of heat raises the temperature of soil and 

 rock more than that of water. 



2. Water is a good reflector, while the land is not, and the latter, 

 therefore, absorbs a larger proportion of the heat of the sun's rays. 



3. Convection movements are established in water as soon as its 

 surface is heated locally. This prevents excessive heating at any 

 one point. The land, on the other hand, being solid, is without 

 movements of convection. 



4. There is more evaporation from a water surface than from 

 land surface, other conditions being the same, and evaporation 

 cools the surface from which it takes place. 



5. Light and heat rays penetrate water, but not soil and rock 

 to any considerable extent. The heat of insolation is therefore 

 distributed, at the outset, through a greater thickness of water than 

 of soil. Being confined to the surface of the latter, its temperature 

 is made higher. 



THE SEASONS 



We are now prepared to understand the seasons, and the rea- 

 sons for their differences of temperature. In most latitudes, the 

 seasons are usually said to be four spring, summer, autumn, and 

 winter. Each grades into the one which follows. 



In the United States, March, April, and May are commonly 

 called the spring months, June, July, and August the summer 

 months, September, October and November the autumn months, 

 and December, January, and February the winter months. In the 



