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Predicting winds from a synotic chart

Given a forecast synoptic chart, it is possible to predict the likely strength and direction of the wind in a given area by examining the isobars in the vicinity

Direction

To find the wind direction, measure the direction of the nearest isobars, bearing in mind that the wind blows so that the lowest pressure is on its left.

In open water, subtract 10°-20° from the direction of the isobars to find the direction of the surface wind.

Inland, subtract 20°-40° from the direction of the isobars to find the direction of the surface wind.

Strength

Measure the distance (in nautical miles) between the two nearest isobars, and use the table below to find the geostrophic wind strength.

Over sea, the surface wind speed is about two thirds of the geostrophic wind. Over land, it is about half the geostrophic wind.

Note that the table refers to isobars at 2 mb intervals. If your chart has intervals at 4 mb intervals, you should halve the measured distance (to reduce it to 2 mb intervals) before referring to the table.

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