Ferrel's law

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Ferrel's law, named after William Ferrel, is a natural law in physical geography and meteorology.

If a body moves in any direction on the earths surface, there is a deflecting force arising from the earths rotation, which deflects it to the right in the northern hemisphere and to the left in the southern hemisphere. This law applies to every body that is set in motion upon the surface of the rotating earth, but usually the duration of the motion of any body due to a single impulse is so brief, and there are so many frictional disturbances, that it is not easy to observe the results of this deflecting force. The movements of the atmosphere, however, are upon a scale large enough to make this observation easy, and the simplest evidence is obtained from a study of the direction of the air movements in the great wind systems of the globe.

In Ferrel's first article about Meteorology and Oceanography, published in 1856, he formulated it as follows:

when, from any cause, a particle of air has been put in motion toward the north or south, the combination of this motion with the rotatory motion of the earth produces a third force, which causes a deflection of the motion to the east when the motion is to the north, and a deflection to the west when it is toward the south.

The fourth and last force arises from the combination of a relative east or west motion of the atmosphere with the rotatory motion of the earth. In consequence of the atmosphere's revolving on a common axis with that of the earth, each particle is impressed with a centrifugal force, which, being resolved into a vertical and a horizontal force, the latter causes it to assume a spheroidal form conforming to the figure of the earth. But, if the rotatory motion of any part of the atmosphere is greater than that of the surface of the earth, or, in other words, if any part of the atmosphere has a relative eastern motion with regard to the earth's surface, this force is increased, and if it has a relative western motion, it is diminished, and this difference gives rise to a disturbing force which prevents the atmosphere being in a state of equilibrium, with a figure conforming to that of the earth's surface, but causes an accumulation of the atmosphere at certain latitudes and a depression at others, and the consequent difference in the pressure of the atmosphere at these latitudes very materially influences its motions. [1]

  1. ^ Ferrel, W. 'An essay on the winds and the currents of the Oceans', Nashville journal of medicine and surgery, 1856.

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

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