54 METEOROLOGICAL RESULTS OF LAST CRUISE OF CARNEGIE 
ner is 3.30 mm. This is greater than the mean deter- 
mined by Wust, who gives a value of 2.64 mm as the 
mean 24-hour depth of evaporation for all oceans. 
The maximum corrected 24-hour value (5.37 mm) 
was measured on August 20-21, 1928, in mean latitude 
20°6 north, mean longitude 38°9 west; the minimum 24- 
hour value (1.05 mm) was recorded on July 19-20, 1928, 
in mean latitude 8°0 north, mean longitude 36/1 west. 
Careful measurements of wind velocity, vapor pres- 
sure, sea-surface temperature, and air temperature 
were made coincidentally with the evaporation meas- 
urements. From these data it is possible to compute 
theoretical evaporation rates according to recent for- 
mulas developed by Sverdrup [39]. For wind measure- 
ments and vapor-pressure determinations conducted at 
a height of 3.6 meters above the sea surface, the fol- 
lowing equation is used 
h= 0.149 (ew- a) u 
where ew is the vapor pressure at the immediate sea 
surface expressed in millimeters, eg is the vapor pres- 
sure at a level 3.6 meters above the surface of the sea 
and also expressed in millimeters, and u is the mean 
wind speed expressed in meters per second. As stated, 
the mean corrected 24-hour evaporation value for the 
Carnegie data is 3.30 mm. Ascomputedfrom Sverdrup’s 
equation, this value is 2.46 mm. On eliminating the five 
measurements conducted during periods when the mean 
24-hour wind speed was less than 2.5 meters per sec- 
ond, the corrected Carnegie value is 3.22 mm, whereas 
the mean as computed after Sverdrup is 2.84 mm, indi- 
cating a better agreement for periods with the higher 
wind velocities. This agreement is all the more re- 
markable when the uncertainty of evaporation measure- 
ments on board ship is considered. 
The number of periods of evaporation measurements 
is too small to determine the variation of evaporation 
with the various meteorological elements with any de- 
gree of certainty. It was found, however, that cloudiness 
tended to decrease the evaporation rates slightly, obvi- 
ously by lowering the sea-surface temperatures. The 
mean for days with an average cloudiness greater than 
0.5 is 2.66 mm per 24 hours (corrected value), whereas 
the mean for days with average cloudiness less than 0.5 
is 2.79 mm per 24 hours. This variation is so slight, 
however, that it can hardly be taken as conclusive. 
The most consistent variation appears to exist with 
air-temperature changes.? The Carnegie data show an 
increasing rate of evaporation with increasing air tem- 
perature up to 25°, but the rate decreases for tempera- 
tures over 25°, apparently because of the fact that the 
higher temperatures occurred only in the tropics, where 
vapor pressures tended to be high also. 
Since much of the original Carnegie meteorological 
data was lost when the vessel burned, it is impossible to 
present further corrected data on evaporation rates as 
related to the various meteorological elements. 
CONCLUSION 
The efforts made to secure accurate evaporation 
data on board the Carnegie illustrate some of the many 
difficulties which are encountered in such observational 
work at sea. The results also emphasize the necessity 
for detailed observations of humidity and wind gradients 
over the sea surface. With fragmentary data available, 
it is difficult to correct the measurements of evapora- 
tion from the small container to represent amounts evap- 
orated from the sea surface [40]. 
@ Sverdrup’s equation indicates that air temperature 
should be a secondary consideration. Wind speed, vapor 
pressure and sea-surface temperature are the control- 
ling factors. 
