In physics, the Coriolis force is an inertial or fictitious force[1] that acts on objects that are in motion
within a frame of reference that rotates with respect to an inertial frame. In a reference frame
with clockwise rotation, the force acts to the left of the motion of the object. In one with anticlockwise
(or counterclockwise) rotation, the force acts to the right. Deflection of an object due to the Coriolis
force is called the Coriolis effect. Though recognized previously by others, the mathematical
expression for the Coriolis force appeared in an 1835 paper by French scientist Gaspard-Gustave de
Coriolis, in connection with the theory of water wheels.[2] Early in the 20th century, the term Coriolis
force began to be used in connection with meteorology.
Newton's laws of motion describe the motion of an object in an inertial (non-accelerating) frame of
reference. When Newton's laws are transformed to a rotating frame of reference, the Coriolis
and centrifugal accelerations appear. When applied to massive objects, the respective forces are
proportional to the masses of them. The Coriolis force is proportional to the rotation rate and the
centrifugal force is proportional to the square of the rotation rate. The Coriolis force acts in a
direction perpendicular to the rotation axis and to the velocity of the body in the rotating frame and is
proportional to the object's speed in the rotating frame (more precisely, to the component of its
velocity that is perpendicular to the axis of rotation). The centrifugal force acts outwards in the radial
direction and is proportional to the distance of the body from the axis of the rotating frame. These
additional forces are termed inertial forces, fictitious forces or pseudo forces.[3] By accounting for the
rotation by addition of these fictitious forces, Newton's laws of motion can be applied to a rotating
system as though it was an inertial system. They are correction factors which are not required in a
non-rotating system.[4]
In popular (non-technical) usage of the term "Coriolis effect", the rotating reference frame implied is
almost always the Earth. Because the Earth spins, Earth-bound observers need to account for the
Coriolis force to correctly analyze the motion of objects. The Earth completes one rotation for each
day/night cycle, so for motions of everyday objects the Coriolis force is usually quite small compared
with other forces; its effects generally become noticeable only for motions occurring over large
distances and long periods of time, such as large-scale movement of air in the atmosphere or water
in the ocean; or where high precision is important, such as long-range artillery or missile trajectories.
Such motions are constrained by the surface of the Earth, so only the horizontal component of the
Coriolis force is generally important. This force causes moving objects on the surface of the Earth to
be deflected to the right (with respect to the direction of travel) in the Northern Hemisphere and to
the left in the Southern Hemisphere. The horizontal deflection effect is greater near the poles, since
the effective rotation rate about a local vertical axis is largest there, and decreases to zero at
the equator.[5] Rather than flowing directly from areas of high pressure to low pressure, as they would
in a non-rotating system, winds and currents tend to flow to the right of this direction north of
the equator (anticlockwise) and to the left of this direction south of it (clockwise). This effect is
responsible for the rotation and thus formation of cyclones (see Coriolis effects in meteorology).
For an intuitive explanation of the origin of the Coriolis force, consider an object, constrained to
follow the Earth's surface and moving northward in the Northern Hemisphere. Viewed from outer
space, the object does not appear to go due north, but has an eastward motion (it rotates around
toward the right along with the surface of the Earth). The further north it travels, the smaller the
"diameter of its parallel" (the minimum distance from the surface point to the axis of rotation, which is
in a plane orthogonal to the axis), and so the slower the eastward motion of its surface. As the object
moves north, to higher latitudes, it has a tendency to maintain the eastward speed it started with
(rather than slowing down to match the reduced eastward speed of local objects on the Earth's
surface), so it veers east (i.e. to the right of its initial motion).[6][7]
Though not obvious from this example, which considers northward motion, the horizontal deflection
occurs equally for objects moving eastward or westward (or in any other direction).[8] However, the
theory that the effect determines the rotation of draining water in a typical size household bathtub,
sink or toilet has been repeatedly disproven by modern-day scientists; the force is negligibly small
compared to the many other influences on the rotation.[9][10][11]