Estimating Wind Speed From Isobars

Calculating Wind Speed from Isobars

Some charts have a “geostrophic chart” that will allow you to estimate wind speed for isobars with only a pair of dividers.  If the chart does not have one, use this technique for Estimating Wind Speed From Isobars.  This technique works on most charts where the isobars are spaced 4 mb apart. Geostrophic Wind Scale

A change in pressure with horizontal distance is called a pressure gradient. The change is maximum along a line parallel to the isobars. A force results which is called pressure gradient force and is always directed from high to low pressure. Speed of the wind is approximately proportional to this pressure gradient, and can be calculated with a geostrophic chart.

A Note about calculating wind direction from Isobars

Not far above the surface of the Earth (at a point where friction has little or no effect on air flow), wind tends to blow parallel to the isobars (lines connecting points of equal pressure).  This is called geostrophic wind, and occurs when the isobars are straight. However, when the isobars are curved, as is the case around highs and lows, the wind does not generally flow parallel to the isobars; it tends to blow at angle to the isobars.  This is called gradient wind.

Near the surface of the Earth, friction affects the flow of air.  Friction tends to divert the wind from the isobars toward the center of low pressure and away from the center of high pressure – approximately 15 degrees.  At sea, where the effects of friction is less than on land, the wind follows the isobars more closely.  Therefore, be sure to allow for friction when determining wind direction from isobars.

 ACTION EXPLANATION Use a pair of dividers to measure the distance between two adjacent isobars. If the chart provides a distance scale, use it to determine the distance in nautical miles.  If not, use the latitude marks to determine the distance (one degree of latitude equals 60 nautical miles). Use the Geostrophic Wind Scale, take the dividers and place one end at the latitude position on the vertical axis.  This will determine the approximate wind speed.  You may need to interpolate between two lines.  In this case, the wind speed is probably between 30 and 35 knots. You can make quick estimates of wind speeds elsewhere on the chart by noting that when the distance between the isobars is halved, the wind speed is double at the same latitude.  The relative distance between the isobars along the same latitude will yield the same ratio of wind speed.