CHAPTER VIII 

 WEATHER; WINDS AND STORMS; CLIMATE 



114. Variations in the Composition of the Atmosphere. 



The only way in which the composition of the air varies 

 is in the amount of water vapor it contains. The air con- 

 tains nitrogen, oxygen, and carbon dioxide in practically the 

 same proportion everywhere and all the time. Water vapor 

 is present in varying quantities, according to the location. 

 The air is never without water vapor even in deserts. 



It is therefore more correct to speak of the humidity 

 that is, moisture than of the dryness of the air. The air 

 is said to be saturated when, at a given temperature, there 

 is as much vapor in the air as it can hold at that temperature. 

 The higher the temperature, the more vapor the air is able 

 to hold. If the air contains T % as much vapor as would 

 saturate it, its condition is expressed as relative humidity 

 60 per cent, which means that it contains 60 per cent as 

 much vapor as there could be at that temperature. 



When the relative humidity is high, if the air is cooled, 

 a part of the vapor is condensed and rain falls, or dew is 

 deposited. Blades of grass cool earlier in the evening than 

 stones or earth; therefore the vapor is condensed upon the 

 grass first. Dew is often seen on the outside of a pitcher of 

 ice water. The cold pitcher cools the surrounding air to 

 the temperature at which dew is deposited the dew point. 

 This is not a uniform temperature like the freezing point, 

 but varies with the relative humidity. 



Change in relative humidity and change in temperature 

 are the two principal causes of change in the pressure of the 

 atmosphere. Cold, dry air is of the greatest density and 



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