The geostrophic wind is stronger than the gradient wind around a low and is weaker than a gradient wind around a high. This is why the isobar spacing and contour spacing, for a curved flow, differs from that determined by a geostrophic wind scale. If the flow under con-sideration is around a high-pressure cell, the isobars are farther apart than indicated by the geostrophic wind scale. If the flow is around a low-pressure cell, the isobars are closer together than indicated by the geostrophic wind scale. Geostrophic and gradient wind scales are used to determine the magnitude of these winds (based on isobar or contour spacing) and to deter-mine the isobar or contour spacing (based on observed wind speeds).

There are a number of scales available for measuring geostrophic and gradient flow of both surface and upper air charts.

Many weather plotting charts used by the Naval Oceanography Command have geostrophic wind scales printed on them for both isobaric and contour spacing.

The most common scales in general use can be used for both surface and upper air charts. The scales are in 4mb and 60m intervals. An example of a geostrophic wind scale is shown in figure 3-1-13. Note that latitude accounts for the increases in gradients. In tropical regions, the geostrophic wind scales become less reliable because pressure gradients are generally rather weak. Use of these scales is discussed in Unit 7.