Wind speed and spacing of isobars

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WIND SPEED AND SPACING OF ISOBARS

Wind speed is also a reflection of isobaric spacing. After you have drawn your first complete isobar, the next one should more or less be parallel to the first. How close or how far away it is from the first one can be correlated to the wind speed. The spacing of isobars is inversely proportional to the wind speed. In other words, the greater the wind speed, the smaller the spacing and vice versa. Some additional relationships also exist. l For a given wind speed, the spacing be-tween isobars decreases with increasing latitude. Table 7-2-1 shows the spacing of isobars, at 4-mb

Table 7-2-1.—Geostrophic Wind Distance between Isobars over Ocean at 4-mb Intervals for Various Wind Speeds and Latitudes

intervals, for geostrophic wind speed versus latitude. At latitude 40° with wind of 20 knots, the isobars, at intervals of 4 millibars, should be separated by 179 nautical miles. At 60° with 20 knots, the isobars should be 133 nautical miles apart.

l For a given wind speed (at the same latitude), the space between isobars will normally, be greater with anticyclonic curvature (isobars turning in a clockwise direction) than with cyclonic curvature.

Isobars are spaced closer together in areas where the wind is strong and further apart where the wind is light. This principle helps when obser-vations are sparse and is illustrated in figure 7-2-3. It should also be noted that in drawing isobars be-tween two stations, if one station has a consider-ably stronger wind than the other, the isobars will be more closely spaced near the one with the strong wind.

Wind information is of great importance to successful and accurate isobaric analysis. Even over sparse data areas, pressure patterns can be closely approximated with just wind information. This is possible through the use of Buys Ballot’s law and a geostrophic wind scale. The former per-mits location of high- and low-pressure systems based on wind direction, and the geostrophic wind scales permit us to determine correct isobaric spacing.

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