Forces Affecting the Velocity and Direction of Wind

The air is set in motion because of differences in atmospheric pressure, and that moving air is what we call wind. Wind blows from regions of high pressure to regions of low pressure. But the wind does not travel in a straight line from one to the other, because two additional influences act on it: friction at the surface and the rotation of the Earth. The force produced by the Earth's rotation is called the Coriolis force. So the horizontal winds near the Earth's surface respond to the combined effect of three forces — the pressure gradient force, the frictional force, and the Coriolis force. The gravitational force also acts, but it pulls downward.

The pressure gradient force is the force that arises from the difference in pressure between two points. It is perpendicular to an isobar, meaning it pushes the air straight across the lines of equal pressure, from high to low. The greater the pressure gradient force, the higher the velocity of the wind, and the larger the deflection in the wind's direction.

The Coriolis force is the deflection caused by the Earth's rotation. It deflects the wind to the right in the northern hemisphere and to the left in the southern hemisphere. The deflection is greater when the wind velocity is high. The Coriolis force is directly proportional to the angle of latitude — it is maximum at the poles and completely absent at the equator. Importantly, the Coriolis force acts perpendicular to the pressure gradient force.

Because these two forces operate perpendicular to each other, the wind in low-pressure areas blows around the low rather than straight into it. At the equator, where the Coriolis force is zero, the wind blows perpendicular to the isobars, straight toward the low pressure. As a result, the low pressure gets filled instead of getting intensified. This is the reason tropical cyclones are not formed near the equator — without the Coriolis force, the circulation needed to intensify a storm cannot develop.

The velocity and direction of the wind are the net result of these wind-generating forces. In the upper atmosphere, about 2 to 3 kilometres above the surface, the wind is free from the frictional effect of the surface. There, the wind is controlled mainly by the pressure gradient force and the Coriolis force. When the isobars are straight and there is no friction, the pressure gradient force is exactly balanced by the Coriolis force, and the resultant wind blows parallel to the isobar. This wind is known as the geostrophic wind.

The wind circulation around a low-pressure system is called cyclonic circulation, and around a high-pressure system it is called anticyclonic circulation. The direction of the winds around such systems changes according to their location in the two hemispheres.