AIR AND WATER 



29 



saturation point contains 17.12 grams of water per cubic 

 meter. Therefore we need to determine what per cent 9.93 

 is of 17.12. It is 57.4 per cent, so 57.4 per cent is the rela- 

 tive humidity. By use of this method it is possible to determine 

 with approximate accuracy the humidity of any room or of the 

 open air at any time. 



28. Variations in relative humidity. If the temperature 

 changes, the relative humidity changes also. For instance, 



FIG. 20. Average relative humidity in the United States 

 The figures show relative humidity in percentages. After diagram by H. J. Cox 



a space which is saturated at one temperature may have 

 its relative humidity reduced very greatly by a temperature 

 increase of a comparatively few degrees. In the same way 

 the relative humidity changes during the day as the tempera- 

 ture 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. 20) 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 



