220 Evaporation from the Surface of the Sea and the Water Budget of the Earth 



2. Direct Measurement of the Evaporation on Board Ship and Methods for Obtaining 

 Corresponding Values for the Sea Surface 



The evaporation from the surface of the ocean can only be measured from ships 

 under way and this involves — probably even more than in direct measurements ashore 

 — a number of sources of error that require special attention. Evaporation differs 

 from other meteorological factors such as barometric pressure, temperature, wind, 

 and cloudiness, in that the apparatus used to measure it is able to exert a very con- 

 siderable influence on the observed values. This greatly increases the difficulty of 

 getting reliable and useful results, because the values obtained are always only relative 

 vahies which give correct absolute values only after the application of suitable correc- 

 tions. 



Measurements on board a moving ship are made by using a vessel filled with sea- 

 water and hung in a cardan suspension. Mohn (1883) used a volumetric method of 

 determining the amount of water evaporated at a given moment. The loss in weight 

 due to evaporation of the cylindrical evaporation vessel was replaced by refiUing it 

 with fresh water to bring it back to a zero mark ; the evaporation height could thus be 

 determined. A more accurate and reliable method is the determination of evaporation 

 by observing the change in salinity which occurs in the evaporating water as a conse- 

 quence of evaporation. Following a suggestion of Penck, a cylindrical glass vessel is 

 used which has a cross-section of 288 cm^ and a volume of 2400 cm^; it is filled with 

 sea water and placed within a white-painted or nickel-plated mantle that protects the 

 vessel against direct radiation. Chlorine titration before and after the evaporation 

 period gives the increase in salinity and allows a very accurate determination of the 

 evaporation. The mean error in a single measurement is seldom more than 3%; it 

 derives from the uncertainties of the salinity determination and in refilling the vessel, 

 while the diminution in volume during the observation can be disregarded. 



Denoting with g^ and gg the weights of the sea-water at the beginning and at the end 

 of the evaporation time, withg^ and^j,, those of evaporated water and of pure water and 

 finally with gs that of the salt at the beginning of the evaporation, then 



gi = gw + gs and gz = (gu, — ge) + gs- 

 The salinity at the beginning and at the end of the measurement is then 



jj = 103 X — ^ — and s^ = 10^ x 



gw i g s \gw ge) ~r g s 



If p is the specific weight (density) of sea-water at the beginning of the measurement 

 and J is the volume of the evaporated water amount from the vessel, it follows that, 

 from the above relations, 



Si S<2, ^1 



g2 = gi- and ge= pJ — ^ — . 



If p is the specific weight (density) of distilled water at the mean temperature t^ 

 at the time of the measurement and o is the area of the evaporating surface, then the 

 evaporation height becomes 



J p S2 — Sj^ 



«<. = -- 



op s^ 



