Buys-Ballot's law, in meteorology, is the name given to a law which 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.

Because of the Earth's spherical shape and its rotation, weather is affected by the planet's centrifugal force and the Coriolis effect. When a low pressure system develops, air from the north and south of the low pressure area must flow into it. In the Northern Hemisphere, air further to the south is traveling faster because it is closer to the equator and therefore further from the Earth's axis of rotation, which means it travels further in a day (rotational period) than the air to the north of it. When that air from the south is drawn north into a low pressure system, it will be moving faster than the ground below it, since that northern ground has slower eastward motion than the ground to the south. So as the southern air moves north it will also move east due to its higher speed. Similarly, the air to the north is moving slower than the air to the south. So when the air from the north is forced south toward the low pressure system, it lags behind the ground in the south, causing it to also move west. Of course, the low pressure system will continue to draw the air, which causes it to swirl around the low pressure area in a counterclockwise direction. In the Southern Hemisphere this is reversed, and the air swirls in a clockwise direction.

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.

Some of this article's text is from the public domain 1911 Encyclopædia Britannica.