# Geostrophic wind scale

From AMS Glossary

## geostrophic wind scale

A graphical device used for the determination of the speed of the geostrophic wind from the isobar or contour-line spacing on a synoptic chart.

It is a nomogram representing solutions of the geostrophic wind equation: where geopotential height is the vertical coordinate; or where atmospheric pressure is the vertical coordinate. In the above equations,

*V*_{g}is the speed of the geostrophic wind, ρ the density of the air,*f*the Coriolis parameter,*p*the pressure at a fixed geopotential height,*z*the height of a constant-pressure surface, and*n*horizontal distance measured normal to the flow. The*n*axis is directed to the right of the flow in the Northern Hemisphere and to the left of the flow in the Southern Hemisphere. In the nomogram, standard values of ρ or*g*are usually adopted. The gradient of pressure or height is approximated by the finite difference ratio, Δ*p*/Δ*n*or Δ*z*/Δ*n*, in which a standard difference in pressure or height is adopted; Δ*n*then represents the normal distance between isobars or contour lines drawn. The nomogram often utilizes Δ*n*as the abscissa and the latitude as ordinate, so that the speed of the geostrophic wind may be read from a family of lines of the graph.