PG = D+ CFCentrifugal force acts with the pressure gradientforce when the circulation is anticyclonic and againstthe pressure gradient force when the circulation iscyclonic. Therefore, wind velocity is greater in ananticyclone than in a cyclone of the same isobaricspacing.VariationsIt has been determined that, given the same density,pressure gradient, and latitude, the wind is weakeraround a low-pressure cell than a high-pressure cell.This is also true for gradient and geostrophic winds.The wind we observe on a synoptic chart is usuallystronger around low cells than high cells because thepressure gradient is usually stronger around thelow-pressure cell.Geostrophic and Gradient Wind ScalesThe geostrophic wind is stronger than the gradientwind around a low and is weaker than a gradient windaround a high. This is why the isobar spacing andcontour spacing, for a curved flow, differs from thatdetermined by a geostrophic wind scale. If the flowunder consideration is around a high-pressure cell, theisobars are farther apart than indicated by thegeostrophic wind scale. If the flow is around alow-pressure cell, the isobars are closer together thanindicated by the geostrophic wind scale.Geostrophic and gradient wind scales are used todetermine the magnitude of these winds (based onisobar or contour spacing) and to determine the isobaror contour spacing (based on observed wind speeds).There are a number of scales available for measuringgeostrophic and gradient flow of both surface and upperair charts.Weather plotting charts used by the NavalOceanography Command has geostrophic wind scalesprinted on them for both isobaric and contour spacing.The most common scales in general use can be used forboth surface and upper air charts. The scales are in 4mband 60m intervals. An example of a geostrophic windscale is shown in figure 3-13. Note that latitude3-13LATITUDEMAP SCALE = ________True at lat. ________ISOBARIC INTERVAL = 4MBDENSITY AT SEA LEVELAG5f0313VELOCITY (KNOTS20252530354045505560657075808510010015050507.57.56655101030302020151590708075406035OOOOOOOOOOOOOO2510Figure 3-13.—Geostrophic wind scale.
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