Marine Glossary

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Buys Ballots law

A rule useful in locating the center of cyclones and anticyclones. It states that, "With the true surface wind on your back in the Northern Hemisphere, atmospheric pressure decreases toward the left and increases toward the right." See WXT G168.

To point in the direction of the Low, put your back to the surface wind, extend your left arm straight out to your side and then move it forward some 15 to 30° and you will be pointing to the low pressure center. This formulation of the law accounts for the fact that the surface wind is backed relative to the winds aloft because of surface friction acting on the air flow.

In the Southern Hemisphere, the flow of the wind and the statement of the law are reversed. Low pressure in the SH will be to your right — raise your right arm straight up to your side and move it forward to point to the center of the Low.

Look at general circulation around a Low in WXT G180 to see how the law comes about.

An alternative statement of the law is figure the true wind direction numerically and then the Low will be located about 10 points to the right of that direction in the NH, or to the left in the SH. Each compass point is 11.25°. This is equivalent to moving the arm forward in the first version of the law by 2 points, or 22.5°.

This could be fine turned to account for friction, either from land or from rough seas. With much friction, the Low would be more like 12 points to the right and with no friction it would be closer to 8 points (90°). Change right to left in the SH.

For example, assuming the average value of 10 points (112.5°, or round this to, say, 113, or even 115 as many texts do), then if the wind is from 180, the Low is located in the direction 180 + 115 = 295. We would get the same result by putting our back to the wind, left hand out (pointing to 270) and then forward "some," in this case 25°.

This latter form of the law may be the easiest to quantify and apply. It is also perhaps less likely for error, especially in cases where this judgment is most crucial, ie in locating the direction to a tropical cyclone. In the case of a developed hurricane, the wind flow is more parallel to the isobars in the center of the storm than it is near its edges. Hence, the cyclone is about 10 points to the right of the wind when first entering the cyclonic winds of the storm, and then more like 9 points to the right when the barometer has dropped some 10 mb, and then maybe more like 8 points to the right when the pressure is down from the initial value by some 20 mb.

Again, in the SH, the same guesses would apply, but looking to the left of the wind direction to find the storm center.