Barotropic

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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.

More generally, a barotropic fluid is an idealized fluid in which the pressure is a function of the density only. Examples include an isothermal ideal gas or an isentropic ideal gas.

A barotropic flow is a generalization of the barotropic atmosphere described above: It is a flow in which the pressure is a function of the density only, or in other words, a flow in which isobaric surfaces are isopycnic surfaces and vice versa. One may have a barotropic flow with a non-barotropic fluid, but a barotropic fluid must always follow a barotropic flow.

Barotropic fluids are also important idealized fluids in astrophysics, such as in the study of stellar interiors or of the interstellar medium. One common class of barotropic model used in astrophysics is a polytropic fluid. Typically, the barotropic assumption is not very realistic.

Contrast baroclinic. In particular, for a barotropic fluid or a barotropic flow (such as a barotropic atmosphere), the baroclinic vector is always zero.