The earth rotation deflection force (geostrophic deflection force for short) refers to the force that all horizontally moving objects on the earth deflect due to the earth rotation. The biasing force of this horizontal motion was first studied and determined by the French mathematician Coriolis, so it is also called Coriolis force.

It is known from physics that if a particle moves relative to a reference frame rotating at a constant speed, it will be subjected to an inertial force, which depends on the relative speed, the rotational angular velocity of the reference frame and the mass of the particle. This inertial force is called Coriolis force. Similarly, due to the rotation of the earth, when an object moves relative to the ground, the observer standing on the ground will feel that the direction of the object's movement has changed. Imagine that an object is subjected to a force, which is called geostrophic deflection force, that is, Coriolis force in physics. In addition to revolving around the sun, the earth keeps rotating counterclockwise around its axis. The angular velocity is expressed by ω, and the size is:

Ω = 7.29× l0-5 (radian/second)

The direction of Ω is the ground plane perpendicular to the North Pole along the earth axis (the "day" here refers to sidereal day, and a sidereal day is equal to 23: 56). Because of the rotation of the earth, the ground plane is rotating everywhere. As far as the northern hemisphere is concerned, taking the polar surface as an example, due to the rotation of the earth, the polar surface rotates counterclockwise around its vertical axis, and its angular velocity is the angular velocity. If an object moves relative to the polar regions, the observer standing on the ground will feel that the object is acted by a force parallel to the ground plane, which is called horizontal eccentric force. Take the equatorial plane as an example. On the equator, the ground plane tangent to any point on it rotates around an axis passing through the point and parallel to the earth axis with the rotation of the earth. Only the angular velocity in the horizontal direction. When the earth rotates, the equatorial horizon "falls" on the east side and "rises" on the west side. If an object moves relative to the ground plane, the observer on the earth feels that the object moving eastward is subjected to an upward force, while the object moving westward is subjected to a downward force. Therefore, it is considered that the object moving on the equatorial plane is only subjected to vertical force, which is the vertical bias force.

At latitudes between the equator and the poles, the earth's rotation makes the ground plane rotate everywhere. This rotation can be divided into rotation around the vertical axis (equivalent to the polar plane) and rotation around the horizontal axis (equivalent to the equatorial plane). The geostrophic angular velocity has components in both vertical and horizontal directions. Therefore, if an object is acted in both vertical and horizontal directions at the same time. Therefore, it is considered that objects moving in mid-latitude areas are affected by both horizontal and vertical deflection.

The physical significance of geostrophic deflection force is analyzed from the daily rotation of the ground plane. This force is the force moved by the observer standing on the rotating earth, which acts on the air particles moving relative to the earth due to its rotation.