An equation for the vertical variation of the geostrophic wind in hydrostatic equilibrium which may be written -(∂V/∂p ) = (R /pf )k × ∇_pT, where V is the vector geostrophic wind, p the pressure (used here as the vertical coordinate), R the gas constant for air, f the Coriolis parameter, k a vertically directed unit vector, and ∇_p the isobaric del operator.