MOISTURE IN THE ATMOSPHERE 



13 



FIG. 8. Hair Hygrometer. 



Humidity is just as variable as temperature because of the 

 evaporation of water in the air; the dews, the rains, the winds 

 depend entirely upon fluctuations of the heat in the air. The per- 

 centage of humidity may be anywhere up to 100%, a figure which 

 indicates saturation of the air with vapor. 



How Humidity is Measured. There are two ways of expressing 

 humidity. The actual amount of water 

 vapor in the atmosphere is known as the 

 absolute humidity. Absolute humidity is 

 expressed in grams of water vapor per 

 cubic foot of air. 



The air in a schoolroom 20 feet wide 

 by 30 feet long by 15 feet high would hold 

 at 70 Fahrenheit 10.4 pounds of water 

 vapor, if the air were saturated. 



The relative humidity is measured in 

 per cent. Air saturated would have 100%, 

 and perfectly dry air would have a relative 

 humidity of 0%. 



Relation of Humidity to Bodily Heat. Humidity influences 

 the heat of our bodies in two ways. First, moist air conducts heat 

 from the skin more readily than dry air. In warm weather the 

 difference in temperature between the body and the air is so slight 

 that neither moist nor dry air conducts any large amount of heat 

 from the surface, but when the difference of temperature is greater, 

 the effect of humidity in the conduction of heat is very marked. 

 Moist air at 65 F. is chilly to one sitting still, while dry air at this 

 temperature is very comfortable. When perspiration becomes 

 necessary to maintain the normal temperature of the body, the 

 moisture of the air interferes with the escape of heat from the 

 body, for evaporation of the perspiration does not take place rapidly 

 in moist air. Therefore, a high percentage of humidity makes us 

 feel warmer on a warm day and cooler on a cool day because in 

 the former case it interferes with the evaporation of the perspira- 

 tion from the body, while in the latter instance the heat is conveyed 

 more rapidly from the body, causing a sense of coolness. 



The following table of tests, made to determine the exact bod- 



