236 THE WEATHER 



temperature. More water will, therefore, change to 

 steam and mix itself with the air of the jar. But if we 

 lower the temperature of air that is saturated, enough of 

 the steam will be condensed to leave the air just saturated 

 at the lower temperature. 



266. Humidity. The weight of the water that is present in a given 

 volume of air is called the humidity of the air. The humidity is stated 

 as so many grains of water for each cubic foot of air, or as so many 

 milligrams of water for each liter of air. A cubic foot of air or space 

 at 50 F. can hold 4 grains of water vapor; if saturated at 80 F., it 

 can hold 11 grains. The cubic foot saturated at 50 F. with 4 grains 

 of water is unsaturated at 80 F. It has, in fact, only yn, or 36 per 

 cent, of the amount needed to saturate it at 80 F. 



The amount of water actually present in a volume of air or space, 

 divided by the amount the air or space would hold if it were saturated, 

 is called the relative humidity of the air or space. In the case given, the 

 relative humidity is 36 per cent (cf. 249 and 267). 



267. Dew and Frost. Dew and frost are moisture 

 deposited directly from the air, upon cool objects. We 

 see dew formed when a pitcher of cold water "sweats" 

 on a hot day. In Nature, dew is commonly formed after 

 sunset, when the air and bodies near the ground radiate 

 their heat, and grow cool. Stones, grass blades, etc., cool 

 more rapidly than the air, and their temperature falls 

 below that at which the air is saturated. They therefore 

 cool the air near them, so that it deposits some of its 

 moisture upon them. The temperature to which air 

 must be cooled so that it will just deposit moisture is 

 called the dew point. If the dew point is below C., frost 

 is formed instead of dew. 



A clear night favors the forming of dew and frost, 



