Free surface
From Glossary of Meteorology
free surface
The upper surface of a layer of liquid at which the pressure on the liquid is equal to the external atmospheric pressure, assumed constant.
The existence of a free surface is expressed, in hydrodynamics, by the relation Dp/Dt = 0 on the free surface, where p is the total fluid pressure; this relation is often referred to as the free- surface condition and is a special case of a dynamic boundary condition.
