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# Factors Affecting the Velocity and Direction of Wind

## Wind movement UPSC

• Horizontal movement of air is called wind.
• Vertical movement of air is called current.
• The wind blows from high pressure to low pressure.
• Wind movement is influenced by a variety of factors like friction, rotation of earth etc.
• The wind movement in near the earth surface is 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 pressure gradient 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.
• So, in a closely spaced gradient, steep pressure change is found due to which there is a strong wind speed. ## Coriolis force

• The rotation of the earth about its axis affects the direction of the wind.
• This rotation produces a force called a Coriolis force, which is named after a French physicist.
• This force deflects the wind to the right direction in the northern hemisphere and to the left in the southern hemisphere, thus following Farrell’s law.
• Also, 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.

## Frictional force

• Friction is caused due to the irregularities of the earth’s surface, which offers resistance to the wind movement.
• Frictional force 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. ## Why tropical cyclones are not formed near the equator?

• As we know that the Coriolis force acts perpendicular to the pressure gradient force.
• The pressure gradient force is perpendicular to an isobar.
• So, higher the pressure gradient force, the more is the velocity of the wind and the larger is the deflection in the direction of wind.
• Due to the two forces operating perpendicular to each other, in the low-pressure areas, the wind blows around it.
• At the equator, the Coriolis force is zero (discussed above) 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.

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