RADIATION 



91 



How Moisture and Dust Interfere with Radiation. During the 



passage of radiant energy through the air, some of it is absorbed 

 by the dust particles and moisture which keep the air from being 

 absolutely transparent. The amount of heat obtained by the 

 atmosphere directly is small, and especially so on high mountains 

 where there is little dust and moisture in the air. The important 

 thing is the interference with radiation during the night by moisture 

 and dust. More heat is retained near the earth on hazy and muggy 

 days than during clear days and nights. On high mountains the 

 earth will become heated very quickly during the day, but at night 

 the heat radiates just as quickly, since the atmosphere is far more 

 transparent than that down in the valleys. 



Radiometer. Darken the room. Place a radiometer on the 

 table near a bright flame. The radiometer vanes will be seen to move 

 very rapidly. If the light is a Bunsen burner, 

 allow air enough to enter to make the flame 

 blue. The vanes on the radiometer will go 

 around more slowly. The radiant energy from 

 the bright flame passes directly through the 

 glass, and is absorbed very quickly by the 

 black vanes, but is reflected by the bright 

 vanes. Much of the air has been exhausted 

 from the bulb of the radiometer. The tiny 

 particles or molecules of air remaining strike 

 the blackened surface and rebound with 

 greater velocity than from the other side, thus 

 exerting great pressure. 



Water will boil much more quickly over a 

 blue flame than over a yellow flame. This 

 shows that a blue flame is very much hotter. 

 Less radiant heat is given off by flames which 

 give off little light. 



Season Due to the Slant of the Sun's Rays. If we allow AB 

 to represent a section of the earth during the summer time, and 

 CD a section of the earth during the winter time, it is easily seen 

 that more radiant heat from the sun will be absorbed by the section 

 of the earth in the summer than in the winter. In the summer 



FIG. 70 



