CLIMATOLOGY IN ARID ZONE RESEARCH 71 
petent to raise the humidity in the air to any significant degree. 
Here, most of the energy used in evaporation comes from the air. 
The Weather Bureau Class A evaporation pan is 4 feet in 
diameter, and on a summer day in a dry situation it may 
evaporate 2 gallons of water. Solar radiation contributes an 
important share of the energy for evaporation, the amount 
depending on the turbidity of the water and on the albedo of the 
pan, which varies greatly with type, age, and condition of the 
material of which the pan 1s made. Additional energy for evapora- 
tion is available from the air. The amount of water evaporated 
from the pan will do little to modify the moisture content of the 
air; but immediately over the water surface the humidity is 
raised, the moisture gradient reduced, and the evaporation im- 
peded. The extent of this influence depends on the rate at which 
fresh air passes across the evaporating surface from outside. 
An extreme example of the effect of evaporation on the humid- 
ity of the air is taken from a series of observations made during 
the period 1907-10 in connection with an investigation of the 
evaporation from Salton Sea in the Colorado desert of California. 
The sea had been formed accidentally in 1904 while the Colorado 
River was out of control and pouring into the dry bed of Salton 
sink. When the river was again confined to its channel in 1907, 
Salton Sea consisted of a sheet of fresh water 45 miles long and 
10 to 15 miles wide with 440 square miles of surface area. The sea 
was expected to disappear in 10 or 12 years so 1t was recognized 
as presenting an unexcelled opportunity to study evaporation on a 
large scale in an arid climate (1). 
Evaporation pans were exposed at various heights on towers 
over the sea surface and over the land at several locations away 
from the water. Unfortunately, the pans were not all the same 
size and were not uniform as to height or exposure. Nevertheless, 
the results for 1910, the only ones that I have been able to find, 
are very interesting. Table 1 gives the annual total evaporation 
for the calendar year. Four other stations located up to 40 miles 
away from the sea gave results that are similar to those from 
Salton Sea Tower 1. At Indio, the evaporation from a 2-foot 
pan, 10 feet above the ground, was more than 200 inches. 
The actual evaporation from Salton Sea was determined by 
