The earth’s rotation
creates an apparent force (“Coriolis force”) that deflects moving air to
the right of its initial direction in the Northern Hemisphere and to the
left of its initial direction in the Southern Hemisphere.

The magnitude of the
deflection, or “Coriolis effect,” varies significantly with latitude.
The Coriolis effect is zero at the equator and increases to a maximum at
the poles. The effect is proportional to wind speed; that is, deflection
increases as wind strengthens. The resultant balance between the
pressure force and the Coriolis force is such that, in the absence of
surface friction, air moves parallel to isobars (lines of equal
pressure). This is the geostrophic wind.

The Coriolis force
explains why winds circulate around high and low pressure systems as
opposed to blowing in the direction of the pressure gradient.

The following figure
shows how wind is deflected in each hemisphere:


The effect of the Earth’s rotation on the atmosphere and on all objects on the
Earth’s surface. In the northern hemisphere it causes moving objects and
currents to be deflected to the right; in the southern hemisphere it causes
deflection to the left. 


As air begins flowing from high to low pressure, the Earth rotates under it,
making the wind follow a curved path. In the Northern Hemisphere, the wind turns
to the right of its direction of motion. In the Southern Hemisphere, it turns to
the left. The Coriolis force is zero at the equator.

Gaspard de Coriolis

The effect is named after its discoverer, French mathematician Gaspard de
Coriolis (1792–1843)