Buys-Ballot's law

(meteorology) A law describing the relationship of the horizontal wind direction in the atmosphere to the pressure distribution: if one stands with one's back to the wind, the pressure to the left is lower than to the right in the Northern Hemisphere; in the Southern Hemisphere the relation is reversed. Also known as baric wind law.

A law, formulated by the eponymous Dutch meteorologist in 1857, expressing the relationship of horizontal wind direction and pressure patterns; it states that if an observer faces the direction to which the wind is blowing, the lower pressure will be to the left in the Northern Hemisphere, and to the right in the Southern Hemisphere. Wind direction can thus be predicted, given the location of the lower pressure. It is a qualitative statement of the geostrophic wind equation.

Determining the center of an oncoming low-pressure center
This is the law that helps you determine where the center of an approaching depression lies, so that you can take action at sea to avoid it.Christoph Hendrik Diederik Buys Ballot (1817–1890) was a Dutch scientist who founded the Royal Netherlands Meteorological Institute. He realized that if you face directly into the wind in the Northern Hemisphere, the center of the low atmospheric pressure is between 90 and 110 degrees on your right hand. Higher pressure lies to your left, of course.South of the equator, low pressure is on your left hand when you face the wind, and barometric pressure increases on your right hand.If the approaching storm system is still some distance away while you’re at sea, you might be able to place yourself to the north or south of it before it arrives, thereby missing the worst winds and waves.See also Air Masses.

Notice the wind-direction arrows in this Northern Hemisphere low-pressure system. Buys Ballot’s Law says that if you face directly into the wind, the center of the low is between 90 and 110 degrees on your right.

C.H.D. Buys Ballot

Facing the wind, high-pressure area (H) is on your left-hand side on the north hemisphere

In meteorology, Buys-Ballot's law may be expressed as follows: In the Northern Hemisphere, stand with your back to the wind; the low pressure area will be on your left. In other words, wind travels counterclockwise around low pressure zones in the Northern Hemisphere. It is approximately true in the higher latitudes of the Northern Hemisphere, and is reversed in the Southern Hemisphere, but the angle between barometric gradient and wind is not a right angle in low latitudes. See Coriolis effect#Flow around a low-pressure area.

This rule, which was first deduced by the American meteorologists J.H. Coffin and William Ferrel, is a direct consequence of Ferrel's law. The law takes its name from C.H.D. Buys-Ballot, a Dutch meteorologist, who published it in the Comptes Rendus, November 1857. While William Ferrel theorized this first, Buys-Ballot was the first to provide an empirical validation.

External links

• M. Buys-Ballot, "Note sur le rapport de l'intensite et de la direction du vent avec les ecarts simultanes du barometre", Comptes Rendus, Vol. 45 (1857), pp. 765–768.

This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.

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