. 



vii The Atmosphere 107 



that of air, is almost entirely confined to the lowest region of 

 the atmosphere. 



/ 158. Vapour Pressure and Humidity. The fraction 

 of atmospheric pressure exerted by the water-vapour it con- 

 tains is often termed vapour tension, but preferably vapour 

 pressure. The amount of water-vapour in the atmosphere at 

 any place as measured by the hygrometer (see 441), and ex- 

 pressed in the pressure it exerts in inches of mercury or by 

 the number of grains weight in a cubic foot of atmosphere, 

 is called the absolute humidity. In the case of saturated 

 vapour this depends only on the temperature. The vapour 

 in the atmosphere has seldom an opportunity to become 

 saturated, for the air is never at rest. Suppose, for example, 

 ,. that air containing water-vapour saturated at 32, and there- 

 fore exerting a vapour pressure of 0-18 inches, is carried 

 inland to a waterless place and heated up to 50. Or 

 suppose simply that its temperature is raised so rapidly 

 that the somewhat slow process of evaporation has not had 

 time to produce its full effect. The absolute humidity or 

 vapour pressure is consequently only o- 1 8 inches, but 

 evaporation could continue if time and opportunity were 

 given until the amount of vapour would be doubled. Hence 

 this portion of the atmosphere has only one half, or 50 per 

 cent, of the water-vapour it could contain at its temperature. 

 If the same portion of air were cooled without other change 

 to 32 it would contain all the vapour possible at that 

 temperature, or 100 per cent, and have no tendency to 

 evaporate more. If it were heated to 70 it would contain 

 only one quarter, or 2 5 per cent, of what might be present 

 at that temperature, and evaporation would go on rapidly 

 from free surfaces of water. The term relative humidity 

 is applied to the percentage of the whole possible amount of 

 water-vapour which is present at any particular temperature. 

 When the relative humidity is low the atmosphere is "drying" 

 or has a tendency to raise more vapour from water or damp 

 soil when on the other hand the relative humidity is high, 

 there is little tendency to evaporation, and a slight fall 

 of temperature leads to saturation and condensation. 



159- Thermal Changes in Evaporation and Con- 



