18 



FOFONOFF 



[chap. 1 



If sea-water is in equilibrium with water vapour at a given temperature, 

 pressure and salinity, the partial chemical potential, /xw, of the water in sea- 

 water must equal the chemical potential of the vapour, [x^. If we compare two 



30 35 S %.40 



Fig. 5. The difference A& between temperature in situ and potential temperature as a 

 function of temperature (°C), salinity (%o) and pressure (db). The difference is given in 

 degrees Celsius. 



such equilibrium systems of slightly different salinities, but at the same tem- 

 perature and geopotentialj we must have for the differences of chemical 

 potentials 



J/XW = /1/Xv ^ -|^ ^Pv = Vy Apv, 



(39) 



where p^ is the vapour pressure of sea-water and Vy is the specific volume of the 

 vapour. 



From equation (11), we have 



J|Hw ^ Vw Apv -s— As, 



(40) 



where Vw is the partial specific volume of water. Equation (39) can then be 

 written 



dfji Apv 



S^ ^ {Vy- Vw) — 7- 



ds As 



= -iVy-V.)—. 



(41) 



