Maarg Manthan · Topic 5.8

Factors Affecting Wind: Pressure Gradient Force, Frictional Force, Coriolis Force

Geography › Climatology · Topic 5.8

Wind is air in horizontal motion, and it starts because of differences in air pressure. Three forces control its speed and direction. The pressure gradient force pushes air from high to low pressure at right angles to the isobars; the closer the isobars, the stronger the wind. The Coriolis force, caused by the Earth's rotation, deflects the wind to the right in the northern hemisphere and to the left in the southern, and is zero at the equator. Friction slows the wind near the ground, and the three forces together decide the path of the wind across the isobars.

5 min read · UPSC Prelims · Geography · Climatology

At a Glance

Starts with Pressure difference
Pressure gradient force High to low, across isobars
Coriolis force Right in the north; left in the south
Friction Slows winds near the surface
Coriolis at equator Zero
Exam link UPSC Prelims, Geography: Climatology

Where Does It Fit?

This topic follows pressure and explains why air moves and how the Earth’s rotation turns it. It supplies the rules used to explain cyclones, anticyclones and the planetary wind belts that follow. Previous topic: Atmospheric Pressure: Vertical and Horizontal Distribution. Next topic: Cyclonic and Anticyclonic Circulation.

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: Vertical and Horizontal Distribution. 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 Motions of the Earth: Rotation and Revolution and Their 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 Conditions, Paths, Naming Conventions, Early Warning Systems. It also explains why winds turn into spirals around pressure centres, and why ocean currents form gyres, as in Ocean Currents: Horizontal and Vertical Motion; Cold and Warm Currents.

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.

How Do the Forces Combine?

The three forces act together. Starting from rest, air moves down the pressure gradient. As it speeds up, the Coriolis force turns it, until in the free atmosphere the two balance and the wind blows parallel to the isobars, which is the geostrophic wind. Near the surface friction reduces the speed, and the wind therefore crosses the isobars at a small angle towards low pressure. This is why winds seen on a weather map blow almost along the isobars aloft, but spiral across them near the ground. The effect is the basis of the circulation around highs and lows described in Cyclonic and Anticyclonic Circulation.

Why Is This Topic Important?

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

What Came Next?

The next topic applies these forces to cyclonic and anticyclonic circulation. Next topic: Cyclonic and Anticyclonic Circulation.

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: forces on wind, Coriolis force, Ferrel's law. GS Paper 1: atmospheric circulation.

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 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.

What is the pressure gradient force?

The pressure gradient force is the force that arises from the difference in air pressure between two places and pushes air from high to low pressure, at right angles to the isobars. The closer the isobars are, the steeper the gradient, and the stronger the force and the wind.

How does friction affect the wind?

Friction between the moving air and the surface slows the wind down. It is greatest at the surface and is negligible above about 1 to 3 km. Because it reduces the Coriolis effect along with the speed, surface winds cross the isobars at an angle towards low pressure.

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.