Once air has been set in motion by the pressure gradient force, it undergoes an apparent deflection from its path, as seen by an observer on the earth. This apparent deflection is called the "Coriolis force" and is a result of the earth's rotation.

The Coriolis effect is the apparent deflection of moving objects from a straight path when they are viewed from a rotating frame of reference.

The Coriolis effect is caused by the Coriolis force, which appears in the equation of motion in a rotating frame of reference.

This force is a pseudo force .

 In a rotating frame, the Coriolis and centrifugal forces are needed in the equation to correctly describe the motion.

At a given rate of rotation of the observer, the magnitude of the Coriolis acceleration of the object is proportional to the velocity of the object and also to the sine of the angle between the direction of movement of the object and the axis of rotation.

The Coriolis acceleration is perpendicular both to the direction of the velocity of the moving mass and to the rotation axis. So in particular:

· if the velocity is parallel to the rotation axis, the Coriolis acceleration is zero

·  if the velocity is straight inward to the axis, the acceleration is in the direction of local rotation

·   if the velocity is straight outward from the axis, the acceleration is against the direction of local rotation

· if the velocity is in the direction of local rotation, the acceleration is outward from the axis

· if the velocity is against the direction of local rotation, the acceleration is inward to the axis

The Coriolis effect exists only when using a rotating reference frame.

The Coriolis effect exhibited by a moving object can be interpreted as being the sum of the effects of two different causes of equal magnitude.

The first cause is the change of the velocity of an object in time.

 The same velocity (in an inertial frame of reference) will be seen as different velocities at different times in a rotating frame of reference. The apparent acceleration is proportional to the angular velocity of the reference frame  and to the velocity of the object. This gives a term - ( w x v ) . The minus sign arises from the definition of the cross product , and from the sign convention for angular velocity vectors.

The second cause is change of velocity in space.

 Different points in a rotating frame of reference have different velocities (as seen from an inertial frame of reference). In order for an object to move in a straight line it must therefore be accelerated so that its velocity changes from point to point by the same amount as the velocities of the frame of reference. The effect is proportional to the angular velocity (which determines the relative speed of two different points in the rotating frame of reference), and the velocity of the object perpendicular to the axis of rotation (which determines how quickly it moves between those points). This also gives a term - ( w x v ).