102 THE ATMOSPHERE AND ITS SERVICE TO MAN 



tinually stirring with a chemical thermometer. Note the tem- 

 perature at which a mist begins to appear upon the outside of the 

 dish. When the mist has appeared, add no more ice but stir until 

 the mist begins to disappear. Note this temperature. Take the 

 average of these two temperatures. This average is probably 

 the temperature at which the mist really began to form. This 

 temperature is called the dew point. 



When we wish to dry clothes, we place them in a warm 

 room or in the sunshine. Soon we find that the water has 

 left the clothes. It must have gone into the air. It would 

 thus appear that when the temperature of the air is raised, 

 it has the capacity of taking up more moisture than when it 

 is cold. This was seen in Experiment 37. Both Experi- 

 ments 38 and 39 showed that when air is sufficiently cooled, 

 it begins to deposit moisture. Experiment 39 showed the 

 temperature at which the deposition began. This was 

 the dew point for that time and place. 



This property that air has of taking up a large amount 

 of moisture when heated and of giving it out when cooled 

 is the cause of our clouds and rain. 



Humidity. The condition of the air as regards the 

 moisture it holds is called its humidity. The amount of 

 vapor present in the air is spoken of as its absolute humidity. 

 The amount of vapor in the air as compared with the amount 

 the air would contain if it had all it could hold is known as 

 its relative humidity. For example, air at 80 F. is capable 

 of holding almost 11 grains of water vapor per cubic foot. 

 Suppose it actually contains 6 grains of water vapor per 

 cubic foot. It will be loaded then with about TT, or a little 

 more than | of the moisture it would contain if it were 

 saturated (that is, had all the moisture it could hold). This 

 fraction represents the relative humidity of the atmosphere. 



