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The Coriolis effect is an inertial force first described by Gaspard-Gustave Coriolis, a French scientist, in 1835. When an object is moving in a rotating coordinate system, the path of the object appears to deviate due to the Coriolis effect. If you are in the moving coordinate system, this deviation makes it look like a force is acting upon the object (due to Newton's laws of motion), but actually there is no real force acting on the object, the effect is due to rotation (associated with an acceleration) of the coordinate system itself. A similar effect from a moving frame of reference is the centrifugal force.

In a rotating frame of reference (such as the earth), the apparent force can be described by the formula:

,

where bold indicates vector quantities, m is mass, v is the velocity and Ω is the angular velocity of the coordinate system.

This equation means that the force will be proportional to the velocity of the object and the rotation of the coordinate system. The force will be in a direction perpendicular to the velocity. If an object is travelling on earth in the northern hemisphere, the Coriolis force will deflect the object to the right. In the southern hemisphere the reverse is true, while at the equator the horizontal component of the force is zero for horizontal motions.

The Coriolis force plays a strong role in weather patterns, where it affects prevailing winds and the rotation of storms, as well as in the direction of ocean currents. Above the atmospheric boundary layer, friction plays a relatively minor role, as air parcels move mostly parallel to each other. Here, an approximate balance between pressure gradient force and Coriolis force exists, causing the geostrophic wind, which is the wind effected by these two forces only, to blow along isobars (along lines of constant geopotential height, to be precise). Thus a northern hemispheric low pressure system rotates in a counterclockwise direction, while northern hemispheric high pressure systems or cyclones on the southern hemisphere rotate in a clockwise manner.

The Coriolis effect must also be considered in astrophysics, and stellar dynamics, where it affects phenomena such as the rotational direction of sunspots. The flight paths of airplaness, artillery shells, and missiles must account for the Coriolis effect or risk being off course by significant amounts.

Although the Coriolis force is relatively small and does not have an observable influence on small systems such as the whirlpool of a draining bathtub, toilet or sink  , the Coriolis effect can have a visible effect over large amounts of time and has been observed to cause uneven wear on railroad tracks and cause rivers to dig their beds deeper on one side.

A practical application of the Coriolis force is the mass flow meter, an instrument that measures the mass flow rate of a fluid through a tube. The instrument was introduced in 1977 by Micro Motion Inc. Simple flow meters measure volume flow rate, which is proportional to mass flow rate only when the density of the fluid is constant. If the fluid has varying density, or contains bubbles, then the volume flow rate multiplied by the density is not an accurate measure of the mass flow rate. The Coriolis mass flow meter works by applying a vibrating force to a curved tube through which the fluid passes. The Coriolis effect creates a force on the tube perpendicular to both the direction of vibration and the direction of flow. This force is measured to give the mass flow rate. Coriolis flow meters can also be used with non-Newtonian fluids, which tend to give inaccurate results with volume flow meters. The same instrument can be used to measure the density of the fluid, since this affects the resonant frequency of the vibrating tube. A further advantage of this instrument is that the fluid is contained in a smooth tube, with no moving parts that would need to be cleaned and maintained, and that would impede the flow. EDN Access 2003-06-30  