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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 FerrelThis page is about the meteorologist; for the comedian, see Will Ferrell. William Ferrel ( 1817 1891), an American meteorologist, developed theories which explained the mid-latitude atmospheric circulation cell in detail, and it is after him that the Ferr, 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 1857Events January 9 Earthquake at Fort Tejon, California with an estimated magnitude of 7. 9 February 16 The National Deaf Mute College (later renamed Gallaudet University) is established in Washington, DC becoming the first school for the advanced education. While William Ferrel theorized this first, Buys-Ballot was the first to provide an empirical validationAn empirical validation of a hypothesis is required for it to gain acceptance in the scientific community. Normally this validation is achieved by the scientific method of hypothesis commitment, experimental design, peer review, adversarial review, reprod.

Some of this article's text is from the public domain 1911 Encyclopædia BritannicaThe Eleventh Edition of the Encyclopaedia Britannica ( 1911) represents in many ways the sum of knowledge at the beginning of the 20th century. The edition is still often regarded as the greatest edition of the Encyclopaedia Britannica with many articles.