The Ocean 7 



where p^ is the air pressure at the surface, g is the gravitational acceleration and |^o 

 is the deviation of the surface from ideal sea-level. At another place it will be 



P = Pi + gPi(fh + O- 

 The pressure difference between the two places will then be 



^P = -g(po - PiVh - gipo^o - Pi^i)- (I-O 



For a completely homogeneous sea (pq = pi) the relative deviation of the sea-level 

 from the geoid will be 



J^=-A.zlp. (1.2) 



gp 



If the average density for sea-water is taken as 1 -028 then 



J I in dynamical cm = — 0-973/1/7; (Ap in mbar),^ 



}■ (1.3) 



J ^ in cm = —0-993Ap; (Ap in mbar). J 



The numerical factor in the last equation will be 1 -326 when Ap is expressed in mm Hg, 

 because 1 millibar (mbar) corresponds to 0-75 mmHg. 



For a steady difference in air pressure the displacement of the sea-level from the 

 geoid in cm will be 0-993 times the local variation in atmospheric pressure measured 

 in mbar, in the opposite direction. From a knowledge of the steady pressure distribu- 

 tion at sea-level the deviation from ideal sea-level can easily be found. In January the 

 barometric pressure in the high-pressure cell near the Azores is about 1020 mbar, in 

 the Icelandic low-pressure area it is about 990 mbar. It can therefore be expected that 

 the sea-level in the area of the Irming Sea will be about 30 cm higher than at the 

 Azores. Comparisons between changes in barometric pressure and changes in sea- 

 level made at polar stations, where the ice covering allows them to be followed more 

 easily, have shown satisfactory agreement between observed and calculated values of 

 sea-level (Hessen, 1931; Wegener, 1924). 



Other effects due to the inhomogeneity of the water in the ocean and to the currents 

 associated with it, and also to phenomena caused by the blocking of ocean currents 

 at continental coasts (water level rise, Anstau) are harder to deal with. All these aperi- 

 odic stationary deviations of the actual sea-level from the ideal are included in the 

 concept of the physical sea-level. This physical sea-level is, under steady conditions, the 

 true boundary between the ocean and the atmosphere. 



The methods used to fix the position of the physical sea-level relative to the surface 

 of the geoid will be described later (Part II). The effect by itself of different distributions 

 of density in different water masses within the ocean can be found using equation (I.l). 

 Assuming the barometric pressure being the same at both stations (Ap = 0) it follows 

 approximately 



P 



where h^ is the depth at which the pressure difference within the water mass vanishes. 

 For a density difference of 10~^ and a water volume of 100 m vertical extent, the 



