Coriolis effect

The Coriolis effect is a (fictitious) force which acts upon any moving body (an object or an parcel of air) in an independently rotating system, such as the Earth. In meteorology, the horizontal component of the Coriolis force is of primary importance, as the most well known application of the Coriolis force is the movement or flow of air and ocean currents across the Earth. The effect is named after the French physicist Gaspard de Coriolis (1792-1843), who first analyzed the concept mathematically.

The Earth rotates about its axis from west to east once every 24 hours. This daily rotation of the earth means that in 24 hours a point on its equator moves a distance of some 40 000 kilometres, giving it a tangential velocity of about 1670 kilometres per hour (or roughly 1000 mph). A point at the latitude of, say, Rome, travels a shorter distance in the same time and therefore has a lower tangential velocity - about 1340 kph (840 mph), while the relative tangential speed at the poles is zero. Consequently, an object or current moving above the Earth in a generally northerly or southerly direction (away from the equator) will have an greater eastward velocity than the ground underneath, and so will appear to be deflected in relation to the rotation of the Earth. This deflection acts towards the right (or clockwise) in the, in the Northern Hemisphere and towards the left (or anti-clockwise) in the Southern Hemisphere.

Moving air undergoes an apparent deflection from its path, as seen by an observer on the Earth. This apparent deflection is the result of the Coriolis acceleration (or Coriolis force). The amount of deflection the air makes is directly related to both the latitude and the speed at which the air is moving. Therefore, slowly blowing winds will be deflected only a small amount, while stronger winds will be deflected more. Likewise, winds blowing closer to the poles will be deflected more than winds at the same speed closer to the equator. The Coriolis force is zero right at the equator and becomes a maximum at the poles.

The Coriolis force only acts on large objects like air masses moving considerable distances. Small objects, for example ships at sea, are too small to experience significant deflections in direction due to the Coriolis force. Therefore the Coriolis force is particulary significant with regards to winds, ocean currents and tidal streams. The idea of the Coriolis effect was developed independently by William Ferrel in America.

Related feature:
Gaspard Coriolis

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