Geostrophic equilibrium

From Glossary of Meteorology

geostrophic equilibrium

A state of motion of an inviscid fluid in which the horizontal Coriolis force exactly balances the horizontal pressure force at all points of the field so described:
where Ω is the vector angular velocity of the earth, vg the geostrophic wind velocity, α the specific volume, p the pressure, and H the horizontal del operator.

With respect to cyclone-scale motions in extratropical latitudes, the free atmosphere frequently approaches a state of geostrophic equilibrium.