1080 
evaporation between parallels of latitude. Still, in order 
to place the results on a firmer basis, it is very desirable 
to obtain further direct measurements at sea of in- 
coming radiation from sun and sky and of nocturnal 
radiation. Measurements of the reflectivity of the sea 
surface under different conditions are also needed, as 
well as further examination of the Bowen ratio (the 
ratio between heat losses from the sea surface by 
conduction and by evaporation). Even with improved 
knowledge of these conditions, the energy approach 
cannot render information as to evaporation rates under 
given meteorological conditions or seasonal and regional 
values of the evaporation. In order to compute such 
values the third approach has to be adopted. 
3. For the computation of the vertical flux of water 
vapor from the sea surface several assumptions have 
been introduced as to the character of the eddy transfer 
processes near the sea surface. These assumptions lead 
to different formulas from which the evaporation rate 
can be obtained from observations of humidity and 
wind velocity at a standard level above the sea surface 
and observations of the sea surface temperature from 
which the vapor pressure at the sea surface can be 
derived. 
These theoretical equations all presuppose indifferent 
stratification or slight instability. This restriction is 
not very serious, because stable conditions are not 
common over the open ocean and when they occur, 
evaporation is small. 
The different approaches have been reviewed and it 
has been shown that they lead to widely diverging 
results. Furthermore, it has been shown that the avail- 
able empirical evidence is far too scanty and too in- 
consistent to indicate what assump‘ion may be ac- 
ceptable. 
In order to advance our knowledge, it is necessary 
to carry out a large number of detailed measurements 
of conditions directly above the sea surface. These 
should comprise measurements of wind profiles in order 
to examine the validity of the concept that at low wind 
velocities the sea surface is hydrodynamically sm oth, 
measurements of humidity in order to examine the 
validity of the logarithmic law for the decrease of 
water vapor with height, and measurements of tem- 
perature in order to establish the stability under which 
the observations are made. It is possible that such 
extended observations may show that one of the sug- 
gested theoretical equations is applicable, but it is also 
possible that new concepts have to be introduced and 
more variables considered in order to arrive at a satis- 
factory basis for the computation of evaporation during 
short time intervals. 
' The problem is somewhat different if seasonal and 
regional values are to be computed from climatological 
data. In this case it may be assumed that the evapora- 
tion can be computed by means of an equation that is 
based on theoretical considerations, and the unknown 
factor in this equation may be established by means 
of evaporation values that are derived from energy 
considerations. Such an equation cannot be expected 
to give accurate results before the correctness of the 
MARINE METEOROLOGY 
character of the formula has been more firmly estab- 
lished, and before it has been shown that the unknown 
factor is a constant. 
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