30 
COLORADO EXPERIMENT STATION. 
the water surface is 80°, the evaporation would be 3 J times as fast as 
when the temperature is 40°, the excess above dew point being 10° 
in each case. 
But at the low temperatures corresponding to high elevations, 
the dew points are nearer the air temperatures than at higher 
temperatures. In addition, there is reason to believe that the 
water temperature is not so much above the air temperatures as 
at higher temperatures. It is evident that the effect of these 
conditions is to make the difference of vapor pressures corresponding 
to the temperature of the water surface, and of the dew point to be 
less at high elevations than at low, and by so much to reduce the 
evaporation. 
§ 42. On the other hand, the lessened air pressure at the higher 
elevations is favorable to increased evaporation, the increase in evap- 
oration being proportional to the decreased pressure, and the influ- 
ence of elevation being to increase the evaporation by the per cent 
given in the third column. This increase is due to the decreased 
barometric pressure alone. 
Increase in Evaporation 
Elevations. 
Pressures. 
Over Evap. at 5,000 Feet. 
5,000 
feet 
25 
inches 
00 per cent. 
6,000 
u 
24 
3J “ 
7,000 
ii 
_ _ 23.2 
u 
_ __ __ __ 7 
8,000 
9,000 
a 
- _ _ _ 22.3 
u 
_ 11 
(( 
21.4 
ii 
... _ _ 14 
10,000 
u 
20.6 
u 
_ 18 
11,000 
(( 
_19.9 
ii 
- - 20 
12,000 
u 
_ _ _19.1 
(( 
_ 24 
13,000 
u 
_ _18.4 
u 
__26 
14,000 
u 
17.7 
a 
29 “ 
§43. 
Confining these 
effects to elevations less than 10,000 feet, 
which is practically the limit of available storage sites, we find that 
the condition of air and water temperatures materially reduce the 
evaporation, the decreased barometric pressure increases, and the 
wind, if greater, would tend to increase it. The effect of lower tem- 
peratures is greater than the increasing effect of the barometric pres- 
sure and probably greater than the effect of the wind, except in ex- 
posed places. And when we take into account the fact that the water 
is frozen for a much longer period of the year, it is safe to conclude 
that the evaporation for the year is much less than at lower eleva- 
tions. 
§ 44. The amount of decrease cannot be stated with certainty. 
An increase in wind increases the evaporation, each mile of wind 
during the twenty-four hours, increasing the evaporation for that 
day by from 1 to 2 per cent. ; 2 per cent, deduced from Fitz-Gerald’s 
formula from Boston observations, f and nearly 2 per cent, for wind 
of 5 miles per hour, decreasing to 1 per cent, for each mile at 25 
