● Air is set in motion due to the differences in atmospheric pressure.

● The air in motion is called wind.

● The wind blows from high pressure to low pressure.

● The wind at the surface experiences friction. In addition, rotation of the earth also affects the
wind movement.

● The force exerted by the rotation of the earth is known as the Coriolis force.

● Thus, the horizontal winds near the earth surface respond to the combined effect of three forces – the pressure gradient force, the frictional force and the Coriolis force. In addition, the gravitational force acts
downward.

Pressure Gradient Force

● The differences in atmospheric pressure produces a force.

● The rate of change of pressure with respect to distance is the pressure gradient.

● The pressure gradient is strong where the isobars are close to each other and is weak where the isobars are apart.

Frictional Force

● It affects the speed of the wind.

● It is greatest at the surface and its influence generally extends upto an elevation of 1 – 3 km.

● Over the sea surface the friction is minimal.

Coriolis Force

● The rotation of the earth about its axis affects the direction of the wind.

● This force is called the Coriolis force after the French physicist who described it in 1844.

● It deflects the wind to the right direction in the northern hemisphere and to the left in the southern hemisphere.

● The deflection is more when the wind velocity is high.

● The Coriolis force is directly proportional to the angle of latitude.

● It is maximum at the poles and is absent at the equator.

● The Coriolis force acts perpendicular to the pressure gradient force.

● The pressure gradient force is perpendicular to an isobar.

● The higher the pressure gradient force, the more is the velocity of the wind and the larger is the deflection in the direction of wind.

● As a result of these two forces operating perpendicular to each other, in the low-pressure areas the wind blows around it ( that is around the low pressure areas).

● At the equator, the Coriolis force is zero and the wind blows perpendicular to the isobars.

● The low pressure gets filled instead of getting intensified.

● That is the reason why tropical cyclones are not formed near the equator.