MOISTURE IN THE AIR 107 



temperature may drop many degrees without the dew-point 

 being reached. 



WEIGHT IN GRAINS OF WATER VAPOR PER CUBIC FOOT AT 

 SATURATION 



Temp. F. Grains Temp. F. Grains 



0.481 55 4.849 



5 0.610 60 5.745 , 



10 0.776 65 6.782 



15 0.986 70 7.980 



20 1.235 75 9.356 



25 1.551 80 10.934 



30 1.935 85 12.736 



35 2.366 90 14.790 



40 2.849 95 17.124 



45 3.414 100 19.812 



50 4.076 



94. Humidity. The amount of moisture actually present 

 in a given quantity of air at any time is called its absolute 

 humidity. The absolute humidity is usually expressed in 

 grains of water vapor per cubic foot. The ratio of the absolute 

 humidity in a given space at a given temperature to the amount 

 of moisture the space could hold if saturated gives its relative 

 humidity. When the temperature of the air is lowered it does 

 not affect the absolute humidity provided the dew-point is not 

 reached, for the mere lowering of the temperature cannot 

 affect the amount of moisture actually present, but since the 

 relative humidity is the ratio of the amount present to the 

 amount the space could hold if saturated, lowering the tempera- 

 ture increases the relative humidity and raising the tempera- 

 ture decreases it. To be called moist, the air should have more 

 than 50 per cent, of the vapor it is capable of holding. The 

 amount of moisture in the air over cultivated land areas is 

 between 50 and 60 per cent., and over water, of course, it is 

 much greater. The relative humidity of our dwellings and 



