THE WATER VAPOR OF THE AIR 215 



ture of the quart of water 1F. (compare with Art. 150). 1 

 Whenever water evaporates, this large amount of heat is 

 taken from the surrounding air or objects and becomes 

 insensible heat, which we can not feel (see Arts. 149-150). 

 That is why sprinkling the floors or pavements in hot weather 

 makes them and the air cooler. The evaporation of the water 

 uses up some of their heat (review Art. 12, Ex. 10). All the 

 heat thus used in forming water vapor is liberated again when 

 the vapor is condensed into clouds, rain, snow, dew, fog, or 

 frost. This liberated heat helps in the development of certain 

 clouds and storms to be considered later. 



245. Effect of Moisture upon Personal Comfort. With a 

 given temperature, the measure of comfort that we experience 

 varies with the amount of water vapor in the air. A damp 

 day is usually more uncomfortable in either summer or winter 

 than a dry day at the same temperature. (1) In winter, the 

 moisture, by increasing the conductivity of the air, enables it 

 to take away heat faster from the body, and when the body 

 thus loses heat too rapidly we have the uncomfortable 

 sensation of chill. (2) In summer, the body is ordinarily kept 

 cool by the evaporation of its perspiration. Damp air 

 permits less evaporation and therefore less cooling. When the 

 body thus loses too little heat we experience the sensation of 

 uncomfortable warmth. This is why high humidity in summer 

 causes more suffering and increases the danger of sunstroke. 

 For a similar reason indoor air in winter that has enough 

 moisture feels warmer and more comfortable than very dry 

 air that is actually several degrees warmer. It follows that 

 offices, school buildings and homes having a proper humidity 

 are comfortable at a lower temperature. This saves fuel 

 and is also more healthful for lungs and throat. In many 

 modern school and office buildings humidifiers automatically 



1 The quantity of heat needed to evaporate varies a little with the tempera- 

 ture of the water. If the water is at 212F. the heat needed to change it to 

 vapor at 212 is only 966 times that required to raise the temperature of the 

 water 1F. 



