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How do I use a geostrophic wind scale?

Find the distance between adjacent isobars in the area that you are interested in - making sure that the isobaric interval is the same as that for which the scale was constructed - often 4 mbar. (Dividers can be used, but a strip of paper suitably marked is just as good.) Using the geostrophic scale for the correct latitude, put one end of your marked distance on the left-hand end of the scale, and read off at the right-hand end the geostrophic wind speed for that isobaric spacing at that latitude.

Remember though that many corrections are needed to find an approximation to the 'real' wind. See the Glossary and any good book relating to meteorology.