ATMOSPHERE 125 



that the humidity is high. If there is very little, we say 

 it is low. When there is as much water vapor in the 

 air at a given time as it can hold, the air is said to be 

 saturated. 



Dew point. If saturated air at any temperature is 

 cooled, a part of the water vapor immediately con- 

 denses into water. When water vapor is condensed in 

 the air it becomes visible. In this manner dew, frost, 

 fog, clouds, rain, and snow are formed. The tempera- 

 ture at which the water vapor in the air begins to 

 condense is called the dew point. 



Dew and frost. Due to radiation, the temperature of 

 the surface of the ground, and especially of the vege- 

 tation, becomes lower than that of the surrounding air. 

 This occurs frequently in the clear still nights of 

 summer and autumn. This causes the invisible water 

 vapor to condense as dew. We thus see that the old 

 saying that "dew falls" is incorrect. 



Frost is formed in much the same way as dew, 

 although in this case the formation takes place below 

 the freezing point of water. The water vapor then 

 passes directly from the gaseous state to the solid 

 state. 



Anything which will check the cooling of the ground 

 and the lower atmosphere tends to prevent the forma- 

 tion of dew and frost. A cloudy sky prevents excessive 

 radiation. Winds constantly change the air and thus 

 hinder cooling. As a result, it is rare to have frost on 

 cloudy or windy nights. 



