Coriolis Force

The rotation of Earth, a spinning spherical planet, has been observed to cause a change in the trajectory of any moving body or mass (e.g., a current of air or water). This change in direction is caused by the Coriolis force, a force that is in turn created by the rotating frame of reference. The Earth's rotating frame is anticlockwise when viewing the Northern Hemisphere from space but the opposite for the Southern Hemisphere; thus, any mass given velocity across the Earth's surface will experience an apparent force that will veer it to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Understanding the Coriolis force, named after the French engineer Gustave Gaspard de Coriolis, is fundamental to explaining major features of global atmosphere and ocean circulation, including cyclonic flow. However, the topic is often confusing, since it requires an appreciation of relative frames of reference and of scale of motion. Some essential properties of the Coriolis force are explained below.

The Coriolis force does not provide an actual push or pull on an object, nor does it originate from identifiable physical sources such as matter (as for electromagnetic or nuclear forces). Hence, it is referred to as a fictitious force. Nevertheless, within a rotating frame of reference (i.e., noninertial), the Coriolis force has real effects on motion, causing acceleration.

The Coriolis force is proportional to the mass and velocity of the moving object and acts only to change its direction, not to reduce its speed. Therefore, the apparent deflection experienced by moving objects is often referred to as the Coriolis effect.

The magnitude of this effect, or deflection, is a function of the angular velocity of rotation and the relative location or proximity to the axis of rotation. For objects in the Earth system, this means that the Coriolis force scales according to both the rotation rate (or angular velocity, Ω) of Earth and latitude (φ). If the surface of the Earth is considered as a disc centered at any point location of interest, then it would have a spin about the local vertical equal to Ωsinφ. Thus, the extra-apparent Coriolis acceleration given to all object (i.e., air mass) motion around the Earth is equal to

(2Ω sin ϕ)u,

where u is the object's speed. This means that the slower-moving winds will be less deflected than the faster ones. Similarly, a quantity called the Coriolis parameter (f) is defined as

f = 2Ω sin ϕ.

The Coriolis parameter remains constant for a given latitude. Moreover, the Coriolis parameter varies between zero at the equator and maximum at poles. This means that there is no need to account for the Coriolis effect acting at the equator, whereas the deflection of motion is greatest at the highest latitudes.

The Coriolis force must be accounted for in describing the dynamics of the atmosphere and oceans that have planetary scales of motion. The effect is named after the French physicist Gaspard de Coriolis, who first analyzed the phenomenon mathematically.