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

 if the so-called 'friction velocity'' is introduced according to Karman: 



The values for Zq and u^ follow from measurements of the wind over the surface and 

 depend on the character of this surface. 



In the turbulent layer the water vapour transport E (expressed in g cra"^ sec~^) 

 directed upwards is due to the turbulent exchange process and is given by 



dz 



where q is the specific humidity which decreases upwards, q may be replaced in this 

 formula by the vapour pressure e according to the well-known formula 



0-623 



P 



and one obtains with sufficient accuracy 



0-623 de 



E = A -T . 



p dz 



The process of evaporation must be regarded as stationary (E = constant), so that 

 with the above value for ^, if c is a constant, 



de c 



dz (z + Zo) * 



Denoting the value of e at the lower boundary of the turbulent layer or at the upper 

 limit of the diffusion layer (thickness d, z = d) with e^, then integration gives 



1 - + -0 



''-'^-'^''d^rj-^' 



On the other hand, the quantity E was found to be 



0-623 . , . ^ dz 0-623 



Je 



E= -— pkou^iz -}- Zo)~ = --^7- pkou^c. 



The transport of water vapour through the diffusion layer is given by the equation 



es — ea 



E' 



d 



where S is the diffusion coefficient of water vapour in the boundary layer with 

 reference to vapour pressure. At the boundary of the two layers the water vapour 

 transport is steady so that the necessary condition 



z = d, E = E' 



must be satisfied. Considering, in addition, 



0-623 



b = K p. 



