It is a deflecting force experienced due to rotation of earth. Because of Coriolis the air appears to turn towards its right in the northern hemisphere and towards its left in the southern hemisphere. The coriolis always acts in the perpendicular direction of the motion of air. It is zero at the equator and increases towards the poles.
The following are four basic points to remember about the Coriolis effect:
Regardless of the initial direction of motion, any freely moving object appears to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
The apparent deflection is strongest at the poles and decreases progressively toward the equator, where the deflection is zero.
The Coriolis effect is proportional to the speed of the object, and so a fast-moving object is deflected more than a slower one.
The Coriolis effect influences direction of movement only; it does not change the speed of an object.