A barotropic fluid is one whose pressure and density are related by an equation of state that does not contain the temperature as a dependent variable. Mathematically, the equation of state can be expressed as = () or = ().

A state of the atmosphere where isotherms are parallel to isobars. (This is actually known as equivalent barotropic.) No temperature advection occurs. Long waves are barotropic.

In meteorology, a barotropic atmosphere is one in which the density depends only on the pressure, so that isobaric surfaces are also isopycnic surfaces (i.e. surfaces of constant density). The isobaric surfaces will also be isothermal surfaces, hence (from the thermal wind equation) the geostrophic wind is independent of height. Hence the motions of a rotating barotropic fluid are strongly constrained.