Estimated Wind speeds from isobars

Calculating Wind Speed and Direction

Calculating Wind from Isobars by Table Lookup

There are at least two formulas I know of to estimate wind speed based on spacing of isobars on a surface analysis. Both are based on the relationship between pressure gradient (e.g. millibars per degree) and the latitude of interest. For those that are interested, a discussion of each follows. Fortunately, it is much easier to use a table where someone else has done the math for you.

Estimated Wind speeds from isobars
Estimated wind speeds based on surface analysis spacing of isobars. Assumes a spacing of 4mb per isobar. Wind speeds with tightly curved isobars may vary by up to 30%.

Calculating Wind Speed

Measurement of the spacing of isobars can be done manually, or with the assistance of a tool. Here, a Surface Analysis of the Northeastern Pacific was loaded and displayed using OpenCPN, a chart plotter program. The shaded circle highlights the area of interest which is located on latitude 40.

Isobar Measurement
Portion of a Surface Analysis. Isobar measurement completed with OpenCPN chart plotter program.

The two black arrows point to each end of a measurement tool. The yellow box indicates a measurement of 102nm. Divide 102 by 60 to get the the number of nautical miles in 1°. In the example, there are 1.7° between the two isobars, falling between two table columns. 40° latitude at 1.5 isobars/degree equals 33 kts winds. 2.0 isobars/degree equals 25 kts.

30 kts is a reasonable estimate of winds in the area.

Estimated Wind Direction

Estimate wind direction by looking at the direction of the isobars. Wind blows out of the high by 15° to 30° and into the low by the same. Interestingly, in both cases, subtract 15° to 30° from the direction of travel of the isobar for an estimate of wind direction.

Estimate wind direction in the example by taking the angle of the measurement (325°) and subtracting 90°. Resulting in the direction the isobar is coming from (235°). The wind travels into the low by an angle of 15° to 30°. Resulting in an estimated wind direction of 205° to 220°.

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