What is hydrostatic head effect?
This effect is not the oceanography adiabatic temperature gradient, but rather the effect of pressure on the triple-point-of-water (TPW) (pressure effect on the phase point). It is unfortunate that oceanographers and metrologists have a common terminology for 2 different physical effects. The adiabatic lapse rate (oceanography hydrostatic head effect) for pure water (+0.010 C, +6.11 mbar) is -0.3059 microK/cm. The pressure effect on the TPW is -7.3 microK/cm and does not require the presence of all 3 phases (i.e., applies to ice/water interface only). The reason the adiabatic lapse rate does not affect measurements in the TPW cell is because the water at the inner edge of the ice mantle and the water surrounding the thermometer sheath does not convect. This motionless water therefore comes into thermal equilibrium with the vertical temperature gradient at the ice interface (the metrology hydrostatic head effect).
