THE WATER OF THE AIR 27 



25. Variations in relative humidity. If the temperature 

 changes, the relative humidity changes also. For instance, you 

 have found that space which is saturated at 48 F. may have 

 a relative humidity of less than 50 per cent if it is warmed 

 to 70 F. In the same way the relative humidity changes 

 during the day as the temperature rises and falls. It also 

 changes because of difference in the actual amount of water 

 present absolute humidity. In most parts of the United 

 States (fig. 18) where it is not desert the relative humidity 

 in summer averages not less than 65 per cent ; in the desert 

 region of the southwest it may average below 30 per cent; 

 and there is a record of relative humidity as low as 3 per 

 cent at Yuma, Arizona, though this very low humidity did 

 not last more than a few hours. 



26. Humidity in houses. In the winter the air of our 

 houses is heated so that it is much warmer than the air out 

 of doors. Usually no water, or very little, is added, and there- 

 fore the air seems very dry; that is, the relative humidity is 

 very low. It is usually below 50 per cent and sometimes as 

 low as 20 per cent. It is seen, therefore, that during the 

 winter those persons who spend most of their time in the 

 house are really living in a desert climate so far as atmos- 

 pheric moisture is concerned. It is believed that we should 

 be much healthier if we could keep the air of our rooms 

 more moist. 



27. Total amount of water in the air. Determine the relative 

 humidity of the room. Measure the room and calculate its 

 cubic contents. By means of the table given in section 22 

 find out the number of grains of water in a cubic foot and 

 calculate the amount of water in the entire room. 



It has been estimated that the amount of water in the at- 

 mosphere of the earth would, if condensed into liquid water, 

 make a layer of liquid water at least an inch deep over the 

 whole surface of the globe. 



