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.
Everyone is familiar with the unremitting force of gravity—its powerful pull toward the center of Earth influences all vertical motion that takes place near Earth’s surface. Much less well known, however, because of its inconspicuous nature, is a pervasive influence on the direction of objects moving horizontally around Earth—a phenomenon known as the Coriolis effect.
Due to the Coriolis effect, all things moving over the surface of Earth or in Earth’s atmosphere appear to drift sideways as a result of Earth’s rotation beneath them. As a result of the rotation of Earth, the path of any free-moving object appears to deflect to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere.