The study of fluid motion. “Fluid” here refers ambiguously to liquids and gases. Although “classical” hydrodynamics was primarily concerned with incompressible fluids, the term aerodynamics has been reserved for such a specialized aspect of compressible fluid flow that most of meteorological dynamics is best included under the general heading of “hydrodynamics. ” W. And J. Bjerknes refer to the hydrodynamics of compressible fluids as physical hydrodynamics.