What is Buys Ballot Law?

C. H. D. Buys Ballot was a Dutch meteorologist, who published empirical evidence in support of the thesis first put forward by William Ferrel in 1856 and Ferrel's Law which we address elsewhere.

The law states that, in the Northern Hemisphere, if a person stands with his back to the wind, then the atmospheric pressure is low to the left, high to the right.

The rule of thumb tends to be to your left and slightly forward.

This is because wind travels counterclockwise around low pressure zones in the Northern Hemisphere. It is reversed in the Southern Hemisphere. Whilst this rule of thumb is generally true, especially at higher latitudes, the centre of the low pressure system is not at right angles, especially at low latitudes.

The law is used by sailors to assist them in steering the vessels away from the centre of depressions or to identify their location with relation to the dangerous quadrant of a TRS or Hurricane. The observed wind speed of a TRS or Hurricane is at its highest (in the Northern Hemisphere) in the leading right quadrant of the system. This is because the wind speed is then the sum of the speed of wind in the system's circulation plus the velocity of the system's forward movement.

In the Southern Hemisphere, low pressure systems and TRS revolve clockwise and therefore, the dangerous quadrant will be the left leading quadrant.

The Buys Ballot Law can also be used in conjunction with the Cross Winds Rule, to help sailors identify whether they can expect better or worsening conditions in the near future. We talk about the Cross Wind Rule elsewhere.