104 THE ATMOSPHERE AND ITS SERVICE TO MAN 



ture. It is important not to think of the dew point as a 

 fixed point of temperature, like that of freezing or boiling. 

 The dew point depends not only upon 

 the temperature of the air but also 

 upon the amount of vapor in the air. 



Condensation of Moisture of the Air. 

 Moisture of the air may condense 

 into little droplets high above the 

 earth's surface, making clouds. If 

 these droplets form near the surface of 

 the earth, the cloud of moisture is 

 called fog. If it collects on objects on 

 or near the ground, it is called dew. 

 When droplets in the clouds become so 

 large that they are too heavy to remain 

 suspended in the air, they fall as rain. 

 Rain and dew can form only when the 

 dew point is higher than the freezing 

 point. When the dew point falls below 

 the freezing point, moisture of the 

 atmosphere condenses as snow, sleet, or 

 frost. Thus a fall of snow on a moun- 

 tain is sometimes accompanied by rain 

 FIGURE 50. AN HY- in the valley. 



GEOMETER 



Cooling by Evaporation. Experi- 

 ment 40. Mark with a rubber band the height of the water col- 

 umn in an air thermometer (Figure 51). Let fall a few drops of 

 ether or alcohol on the bulb, and notice the change in the height 

 of the column. Place a little ether on the back of the hand. What 

 kind of sensation does it give ? (Be careful to use only a few drops 

 of ether, as it is bad to breathe it too freely.) 



