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heat, and checks evaporation from the earth by absorbing the heat and 
partly arresting the direct rays of the sun. It is the medium for the 
formation of dew, rain, and snow, and when examined in connection 
with temperature, gives the only true indication of the dryness of acli 
mate, 
Relative 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. Lt enables us to form an idea of how 
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 theair. 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 19.54 grains, the limit at 100° IF.” 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 acubie 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. Irom the weather maps published 
by the Signal Service at Washington, we find it (the absolute humidity) 
p be 1.9 grains at Denver, 4 at Los Angeles, 3.5 at New York, and 3 at 
is about the same; at Los Angeles it is one-third greater than at Boston, 
thongh the rain fall at the latter place is three times as much as at the 

