EVAPORATION FROM THE OCEANS 
evaporation is small and the same probably applies to 
the South Atlantic down to latitude 55°S. 
The third approach has been used by Jacobs [8, 9] 
in computing the evaporation from the data contained 
in the Atlas of Climatic Charts of the Oceans |27}. 
Tasie Il. Evaporation Factor K [K = EH/w (es — éq)] 
(Computed from the meteorological observations on 
board the Meteor and the evaporation rates 
determined by Wiist) 
Siren Evapora- 
i Tatitud umber w eee! tion 
Region siaey S| *tmb) (Ute) || 28 
day™) 
NE—trade | 10°-25°N 67 | 6.5* | 6.4 3.58 | 0.086 
Calm belt 10°N-0° 40 | 5.2 7.6 3.20 | 0.082 
SH—trade 0°-25°S 107 | 6.5 7.6 3.48 | 0.071 
Transition | 25°-35°S 56 | 7.9 4.9 3.05 | 0.079 
Westerlies 35°-55°S 86 | 9.4 2.0 1.75 | 0.093 
* Reduced to the probable average value. The observed 
value was 8.3 m see, but observations were made in mid- 
winter. 
Data of this character may contain small systematic 
errors in sea-surface and air temperatures, and in 
humidities and wind velocities. It is, therefore, best to 
establish the evaporation factor on a strictly empirical 
basis, requiring that evaporation computed from mete- 
orological data shall agree with that obtained from 
energy considerations. Jacobs followed this procedure. 
By undertaking a comparison between results from 
four limited areas he found 
K = 0.142 
and could show that by using this value the total 
evaporation from the North Atlantic and North Pacific 
Oceans agrees with that computed on the energy basis. 
Tt should be observed that the value 0.142 has no 
general significance, because it applies to the specific 
data used by Jacobs and may not apply if other cli- 
matological compilations are used. Also it should be 
mentioned that the factor K used by Jacobs may not 
be a constant, but may depend on the wind velocity. 
If so, some of the details of Jacobs’ results are possibly 
in error, but the main conclusions can be expected to be 
correct. 
Results. Usmg the equation 
E = 0.142 (e, — e,)Wa (87) 
where ¢ is in mb and w in m sec~, Jacobs has by means 
of the data in the Atlas of Climatic Charts of the Oceans 
[27] computed regional and seasonal values of the evap- 
oration from the North Atlantic and North Pacific 
Oceans. 
The average values between 0° and 60°N are entered 
in Table I. The average evaporation in this latitude 
range agrees with the corresponding average from the 
energy relations, indicating that a correct value of the 
factor K was obtained by means of results from four 
limited areas. Between 0° and 30°N they are higher. 
The observed evaporation rates (Table I, Wiist) display 
the same feature, which indicates that in low latitudes 
part of the radiation surplus is stored as heat in the 
(mm per 24 hr), 
1079 
water, is carried north by ocean currents, and is used 
for evaporation in middle and higher latitudes. The 
energy that is transported by the ocean currents across 
the parallel of 30°N amounts, according to Jacobs’ 
values, to 1.4 X 10° cal min. The observed evapora- 
tion rates give a similar transport to the north, and at 
the same time they show no consistent transport in the 
Southern Hemisphere. 
Tt is well known that the earth receives a radiation 
surplus in lower latitudes and a deficit in higher lati- 
tudes [26]. In order to maintain a steady average 
temperature distribution, heat must therefore be trans- 
ported from the equator towards. the poles across the 
parallels of latitude, and at 30°N this transport amounts 
to 3.6  10'§ cal min. It has been generally assumed 
that this transport takes place mainly by means of the 
air currents, but the results referred to above indicate 
that m the Northern Hemisphere about one-third of 
the transport takes place by means of ocean currents 
and two-thirds by means of air currents, whereas in the 
Southern Hemisphere the transport by ocean currents 
is negligible. This conclusion seems reasonable because 
currents that carry warm water mto higher latitudes 
are far better developed in the Northern than in the 
Southern Hemisphere. 
From the preceding discussion it is evident that the 
computation of evaporation from meteorological ob- 
servations has so far not rendered an independent check 
on results obtamed by other methods because energy 
considerations have been used to establish the evapora- 
tion factor to be applied. Still, the approach has greatly 
increased our knowledge as to the evaporation from the 
oceans because it has furnished a basis for a detailed 
examination of the energy exchange between the ocean 
and the atmosphere. This question is dealt with m an 
adjoming article in this Compendium, which also pre- 
sents the available formation as to the evaporation 
from different ocean areas in different seasons. 
CONCLUSIONS! 
The problem of evaporation from the oceans has been 
attacked by three different methods: (1) observations 
of evaporation from pans on board ship; (2) computa- 
tions on the basis of the available energy; and (3) 
computations of vertical flux of water vapor. 
1. Observations of evaporation from pans on board 
ship have rendered fairly consistent values, but the 
reduction of these values to true rates of evaporation 
from the sea surface is so uncertain that the two other 
methods of approach appear to be superior. 
2. Different computations based on energy considera- 
tions have given similar results as to the average annual 
1. When the present article was in press the author re- 
ceived a copy of the paper: Anderson, H. R., and others, A 
Review of Evaporation Theory and Development of Instrumenta- 
tion. (Lake Mead Water Loss Investigations.) Interim Rep. 
Navy Electronics Laboratory, Rep. No. 159, pp. 1-70, Febru- 
ary 1950. This report includes considerations applicable to a 
limited body of water and deals with effects of stability. Where 
the problems are similar, the conclusions agree with those in 
the present paper. The report contains a long list of references. 
