156 GENERAL SCIENCE 



show that* the isobars usually run in concentric lines 

 around areas marked in some cases high pressure, and 

 in other cases low pressure. The atmosphere tends to 

 move from high to low pressure, and the greater the 

 difference between the high and the low pressure the faster 

 will be the movement of the air or the higher will be the 

 wind velocity. This may be tested by comparing the 

 weather maps on a windy day with the weather maps on 

 a calm day. 



Atmospheric Temperature. A very small amount of 

 heat comes from the stars, but the sun is the real source 

 of nearly all the heat of the earth's surface. The atmos- 

 phere absorbs a small part of the heat of the sun's rays 

 as they first pass through it, while the remainder is inter- 

 cepted by the earth. Here it is partly absorbed and 

 partly reflected back into the atmosphere. The propor- 

 tion of the rays absorbed to those reflected varies with 

 different parts of the earth's surface. Much more heat is 

 reflected from water than from soil, and an increasing 

 proportion is reflected as the angle of incidence of the 

 rays increases. When the rays are vertical, the greatest 

 percentage of the incident radiant heat is absorbed ; this 

 gradually decreases until at sunset nearly all the heat is 

 reflected. The reflected radiant heat aids in warming 

 the air, but most of the heat of the atmosphere is received 

 by conduction from the earth. The lower portions of the 

 atmosphere absorb heat much better than the higher 

 portions for the reasons that the air near the earth is 

 more dense and is also filled with impurities, such as dust, 

 heavy gases, and water vapor, which absorb heat readily. 

 Each little particle of dust in the atmosphere becomes a 

 secondary source of heat and heats the air around it. 

 As the lower layer of the atmosphere becomes heated, 



