THE EFFECT OF CLIMATE AND WEATHER 315 



the temperature at which any given condition produces a 

 specified coohng effect. Thus when the air moves 700 feet a 

 minute, saturated air with a temperature of 70° feels as 

 cool as still air with a temperature a trifle below 60°. If the 

 air is not saturated, the dry and wet bulbs of course stand at 

 different levels, and the effect of atmospheric movement is 

 increased, as is indicated by the example which accompanies 

 the chart. 



At temperatures above that of the body, as appears from 

 the crossing of the wind velocity lines, any movement of the 

 air increases the feeling of discomfort. The reason for this is 

 that after still, hot air has touched the body and thereby 

 been cooled, it acts as a sort of blanket to keep away still 

 hotter air. But when the air is in motion new air keeps 

 touching the body, thus tending to heat the body more 

 and more. 



In both Figures 5 and 6 we may well think of a comfort 

 zone lying on either side of the comfort line. This zone is 

 practically identical with the areas of heaviest shading in 

 our climographs. It likewise represents the atmospheric 

 conditions under which factory accidents are least numerous, 

 and various other human conditions are most favorable. 

 Thus from whatever side we approach the matter we find 

 the optimum at a temperature of 6^ or 66° and a relative 

 humidity of approximately 80 per cent in still air, or else 

 under other atmospheric conditions which give the air 

 essentially the same cooling power. Nevertheless high wind, 

 like great dryness, is not so desirable a means of securing 

 the right cooling power as is the correct temperature. The 

 best combination of all is probably air at about 6j°¥. and 80 

 per cent relative humidity, with a barely perceptible move- 

 ment. A departure from these conditions in any direction 

 diminishes people's comfort, reduces their capacity to work, 

 presumably increases their susceptibility to disease, and 

 unquestionably raises their death rate. 



4. Variability. Even yet we have not reached a final 

 definition of the optimum climate. Variability must also be 

 considered. Experiments show that plants kept uniformly 

 at their optimum temperature grow faster than if kept 

 uniformly at any other temperature, but not so well 



