227 



heat, and cliecks evaporation from the earth by absorbing the heat and 

 partly arresting the direct rays of the sun» It is the niedinm for the 

 formation of dew, rain, and snow, and when examined in conneetion 

 with temperature, gives the only true indication of the dryness of a cli- 

 mate. 



Eelative humidity, as given in the table, indicates the ratio between a 

 given amount of vapor in the air and that which saturated air at the 

 same temperature could contain. It enables us to form an idea of hovf 

 near to saturation and therefore to possible precipitation the air has 

 come. But it does not represent the actual amount of moisture con- 

 tained in the air. This is dependent upon the temperature, for air when 

 warm takes up more moisture than when cold, and the point of satura- 

 tion may therefore represent more or less actual moisture in the air. 



It has been found that '^ the expansion of air under the influence of 

 heat increases its capacity for retaining invisible vapor from about half 

 a grain in a cubic foot — the limit at zero, with the barometrical pressure 

 at 30 inches— to 19.81 grains, the limit at lOQo P." Therefore, in a cool 

 climate like that of Colorado, the air will absorb and hold much less 

 moisture than in a warmer one. Altitude has also a direct effect upon 

 the amount of atmospheric moisture, for as the air becomes lighter and 

 more rarefied, its capacity for moisture decreases, and in Colorado it 

 will contain much less at the point of saturation for the same tempera- 

 ture than at sea-level. There often occur here what are popularly called 

 '^ dry showers," in which the rain dries so quickly that its effect disap- 

 pears almost immediately. The actual amount of moisture in the air 

 or the number of grains in weight of vapor contained in a cubic foot of 

 air is termed its absolute humidity, and is the only exact measure for com- 

 parison of atmospheric moisture in various climates. Knowing the rela- 

 tive humidity and the temperature and altitude of any place, the abso- 

 lute humidity may be calculated. From the weather maps published 

 by the Signal Service at Washington, v/e find it (the absolute humidity) 

 to be 1.9 grains at Denver, 4 at Los Angeles, 3.5 at ISTew York, and 3 at 

 Boston. 



From these figures it is readily seen that humidity has not necessarily 

 direct connection with rain-fall, for at Denver the humidity is only half 

 of what it is at Los Angeles, although the annual rain-fall in both places 

 is about the same j at Lbs Angeles it is one-third greater than at Boston^ 

 though the rain fall at the latter place is three times as much as at the 

 former. We also find the humidity at Los Angeles to be greatest in sum- 

 mer, although the rain-fall is practically nothing. Air may be quite 

 humid, but, if there are no cold currents, the vapor will not be precipi- 

 tated in rain. 



Temperature has a direct effect upon humidity. The expansion of 

 air under the influence of heat increases its capacity for retaining in- 

 visible vapor from about half a grain in a cubic foot — the limit at zero, 

 with the barometrical pressure at 30 inches — to 19.84 grains, the limit at 



