How do you use a geostrophic-wind scale?
July 27, 2007 2:26PM
A geostrophic wind scale is a graphical device printed in synoptic weather charts available on weather fax or on the internet, like the ones compiled by Bracknell and other weather services. It enables estimation of the geostrophic wind velocity by measuring the distance between the isobars (lines of constant pressure) in the weather chart and plotting this against the geographic latitude in the wind scale. step 1: determine the geographic latitude of the position you want to estimate the geostrophic wind speed for. step 2: measure the distance between the pressure lines (isobars) shown on either side of that position. step 3: choose the correct latitude line in the geostrophic wind scale or interpolate in the scale, using the result of step 1. step 4: plot the distance measured in step 2 on that line. step 5: read the estimated wind speed from the scale, using the curved lines. NOTE that the geostrophic wind is only a theoretical wind flowing parallel to the isobars in the chart. The true wind always is reduced by friction against the earth or sea surface and will be deflected towards the centre of the low pressure system which is circled by the isobars you used.