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, Vg 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, Δp/Δn or Δz/Δn, in which a standard difference in pressure or height is adopted; Δn then represents the normal distance between isobars or contour lines drawn. The nomogram often utilizes Δn 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.