Maarg Manthan · Topic 7.3

Forces Affecting Wind Movement

Geography › Pressure, Winds and Atmospheric Moisture · Topic 7.3

Wind is air in horizontal motion, and its speed and direction are decided by three forces: the pressure gradient force, which starts the movement from high to low pressure, the Coriolis force, which deflects it to the right in the northern hemisphere and to the left in the southern, and friction with the surface, which slows it. Above the friction layer, balance of the first two gives the geostrophic wind, blowing parallel to the isobars.

6 min read · UPSC Prelims · Geography · Pressure, Winds and Atmospheric Moisture

At a Glance

Pressure gradient force Perpendicular to isobars; starts the wind
Coriolis force Deflects to the right in the north, left in the south
Friction Slows wind near the surface
Geostrophic wind Parallel to isobars above 1 to 3 km
Coriolis at equator Zero
Exam link UPSC Prelims, Geography: Pressure, Winds and Atmospheric Moisture

Where Does It Fit?

This is the third topic of the category on pressure, winds and atmospheric moisture. The previous topic mapped the pressure belts, and this one explains how differences of pressure produce wind. The next topic covers the general circulation of the atmosphere. Previous topic: Pressure Belts and Pressure Cells. Next topic: General Circulation of the Atmosphere.

What Is Wind and What Starts It?

In short: wind is moving air; pressure differences start it, and the Earth’s rotation and friction shape its path.

Wind is the horizontal movement of air relative to the Earth’s surface, and the vertical movement is called an air current. Winds are named after the direction from which they blow: a westerly blows from the west. Wind speed is measured with an anemometer and direction with a wind vane. Winds arise because the Sun heats the Earth unevenly, producing differences in air pressure, as described in Atmospheric Pressure, Isobars and Vertical Variation. Three main forces control the direction and speed of a wind.

What Is the Pressure Gradient Force?

The pressure gradient force is the force produced by the difference of pressure between two places. It acts at right angles to the isobars, from high to low pressure. The stronger the gradient, which is shown by isobars close together, the stronger the force and the greater the wind speed. If this were the only force, air would flow straight from high to low pressure, and the pressure difference would quickly disappear. But the other forces change the wind path. The movement of air then forms a regular pattern of flow around pressure systems.

What Is the Coriolis Force?

The Coriolis force is an apparent force produced by the rotation of the Earth, named after the French scientist Gaspard-Gustave de Coriolis, who described it in 1835. Because the Earth rotates from west to east, any freely moving object, such as air or water, appears to be deflected from its path relative to the surface. It is explained in Rotation of the Earth and Its Effects.

Feature	Coriolis force
Direction in the northern hemisphere	Deflection to the right of the direction of motion
Direction in the southern hemisphere	Deflection to the left
Latitude effect	Zero at the equator; increases with latitude; maximum at the poles
Speed effect	Stronger for faster winds
Acts	At right angles to the wind; changes direction, not speed

The statement of deflection to the right in the north and left in the south is known as Ferrel’s law. Since the Coriolis force is almost nil near the equator, tropical cyclones do not form within about 5 degrees of the equator, as in Tropical Cyclones – Formation and Conditions. It also explains why winds turn into spirals around pressure centres, and why ocean currents form gyres, as in Ocean Currents – Causes and Types.

What Is Friction and How Does It Matter?

Friction is the drag of the surface on the moving air. It is greatest at the surface and negligible at about 1 to 3 km above, and it slows the wind and reduces the effect of the Coriolis force, so surface winds cross the isobars at an angle towards low pressure. Friction is greater over rough land, with hills, forests and cities, than over smooth seas, so winds are stronger and steadier over the oceans. The layer of the atmosphere affected by friction is the planetary boundary layer. For this reason, surface wind speeds over land are about half those over the sea for the same pressure gradient.

What Is Geostrophic Wind and How Does Circulation Form?

Above the friction layer, the pressure gradient force and the Coriolis force are in balance, and the wind blows parallel to the isobars. This is the geostrophic wind. Upper-level winds, including jet streams, are close to geostrophic. Near the surface, friction reduces the Coriolis force, and the wind crosses the isobars at an angle.

System	Circulation in the northern hemisphere	Circulation in the southern hemisphere	Surface air movement
Low pressure (cyclone)	Anticlockwise	Clockwise	Converges and rises
High pressure (anticyclone)	Clockwise	Anticlockwise	Diverges and sinks

Buys Ballot’s law, formulated by the Dutch meteorologist Buys Ballot in 1857, says that if one stands with one’s back to the wind in the northern hemisphere, the low pressure is on the left, and in the southern hemisphere it is on the right. The wind pattern of the globe is built from these forces, as in General Circulation of the Atmosphere. As an analytical point, the Coriolis force is not a force in the strict physical sense but a consequence of observing motion from a rotating Earth, which is why it can deflect but never speed up or slow down air.

Why Is This Topic Important?

Concepts Coriolis and geostrophic wind asked
Cyclones Explains circulation
Navigation Winds and currents

What Came Next?

The next topic describes the general circulation of the atmosphere. Next topic: General Circulation of the Atmosphere.

Key People and Terms

Pressure gradient force Force from pressure difference
Coriolis force Apparent deflecting force
Geostrophic wind Wind parallel to isobars
Anemometer Wind speed instrument
Ferrel’s law Deflection right in north, left in south
Boundary layer Friction layer near the ground

Exam Corner

Points to Remember

Coriolis deflects right in the north, left in the south.
Coriolis force is zero at the equator.
Pressure gradient force acts across the isobars.
Geostrophic wind blows parallel to isobars.
Northern low pressure rotates anticlockwise.

Do Not Confuse With

Pressure gradient and Coriolis force Starts the wind versus turns it
Geostrophic and surface wind Parallel to isobars versus crossing them
Cyclone and anticyclone Low versus high

Memory Hook

Gradient pushes, Coriolis turns, friction drags; parallel to isobars is geostrophic.

Mains Angle

Questions ask for forces and their effect on winds. These points add depth.

Forces Gradient, Coriolis, friction
Pattern Cyclone and anticyclone
Limit Coriolis fails at the equator

A Question You May Face

An original practice question, not a past paper question.

“Explain the forces that act on winds and show how they produce the circulation around low and high pressure systems.”

How to Answer

Gradient Starts the flow.
Coriolis Deflects.
Friction Slows and crosses isobars.
Circulation Cyclone and anticyclone.

GS Relevance

Prelims: Coriolis force, geostrophic wind, direction of deflection, Buys Ballot's law. GS Paper 1: climatology.

Frequently Asked Questions

What forces affect the movement of wind?

Three main forces affect wind: the pressure gradient force, which starts the movement from high to low pressure; the Coriolis force from the Earth's rotation, which deflects the wind; and friction with the surface, which slows it and makes it cross the isobars.

What is the Coriolis force?

The Coriolis force is an apparent force caused by the rotation of the Earth that deflects moving air and water to the right in the northern hemisphere and to the left in the southern hemisphere. It is zero at the equator and greatest at the poles.

What is the geostrophic wind?

The geostrophic wind is the wind that blows parallel to the isobars above the friction layer, where the pressure gradient force is balanced by the Coriolis force. Upper-level winds, including the jet streams, are close to geostrophic.

Why do winds spiral around a low pressure area?

Air moves toward the low pressure under the gradient force, but the Coriolis force deflects it. In the northern hemisphere it is deflected to the right, so the winds spiral inward in an anticlockwise direction around a low, and clockwise in the southern hemisphere.

What is Buys Ballot's law?

Buys Ballot's law, stated in 1857, says that in the northern hemisphere, if you stand with your back to the wind, the area of low pressure lies to your left. In the southern hemisphere the low pressure is on your right.

PYQ Practice — Statement Analysis

1 The Coriolis force is greatest at the equator.

False

It is zero at the equator.

2 In the northern hemisphere moving air is deflected to the right.

True

In the southern hemisphere to the left.

3 Friction is greater over the sea than over the land.