Coriolis force

Also called: Coriolis effect · Topic: Atmospheric Pressure, Winds and General Circulation · NCERT: Class 11, Ch 4 "Climate"; Class 11, Ch 6 "Natural Hazards and Disasters: Causes, Consequences and Management"; Class 11, Ch 6 "Landforms and their Evolution"; Class 11, Ch 9 "Atmospheric Circulation and Weather Systems"; Class 11, Ch 13 "Movements of Ocean Water"

Meaning

The Coriolis force (also called the Coriolis effect) is an apparent force caused by the Earth's rotation. It bends moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere [4]. It is zero at the equator and greatest at the poles [4].

It matters because it sets the direction of the planetary winds. It explains why the trade winds blow from the north-east and south-east, and how the south-east trades become India's south-west monsoon. It also explains why cyclones spin, and why they cannot form close to the equator.

Explanation

How it works: cause → process → result

• Cause: the Earth spins.
• Air moves over the Earth's surface.
• The ground under the moving air keeps turning.

• So the path of the air looks bent to anyone standing on the ground.

• It is not a real push. Nothing pushes the air sideways. The bending only appears because we watch the air from a spinning Earth.

• Direction (Ferrel's law):
• Moving air turns to the right in the Northern Hemisphere.

• It turns to the left in the Southern Hemisphere [4].

• It changes direction, not speed. The force always acts at right angles to the direction of motion. So it turns the wind but does not speed it up or slow it down [3].

• Who described it: Gaspard-Gustave de Coriolis (1792–1843), a French scientist. His 1835 paper, "On the Equations of Relative Motion of Systems of Bodies", showed that a force acts at right angles on a body moving over a rotating surface [3]. (NCERT gives the year as 1844.)

What decides its strength

• Wind speed: the faster the wind, the more it is turned.
• Latitude: the rule is roughly 2 × (Earth's spin rate) × wind speed × sin(latitude).
• sin 0° = 0 → the force is zero at the equator.

• sin 90° = 1 → the force is greatest at the poles [4].

• NCERT wording: NCERT says the force is "directly proportional to the angle of latitude". The correct rule uses the sine of latitude, not the angle itself.

• Friction weakens it near the ground.
• Friction (drag from hills, forests and buildings) slows surface wind.
• Slower wind means a weaker Coriolis force.
• The pressure gradient force (PGF: the push from high to low pressure) now wins, so surface wind crosses the isobars at an angle, towards low pressure [2]. Isobars are map lines that join places with the same pressure.

How it works with the other forces on wind

• Three forces control surface wind: 1. The pressure gradient force starts the wind and sets its speed. 2. The frictional force slows it near the ground. 3. The Coriolis force turns it.

• Near the ground (below about 1–3 km): PGF = Coriolis + friction. The wind crosses the isobars towards low pressure [2].

• Higher up (above about 2–3 km): there is no friction.
• The PGF and the Coriolis force become equal and pull in opposite directions.
• The wind blows parallel to straight isobars. This is the geostrophic wind [2].

• The upper-air westerlies and jet streams are close to geostrophic.

• If the Earth did not spin and there were no friction, wind would blow straight across the isobars from high to low pressure.

Effects it produces

• Trade winds (the NCERT exercise):
• The pressure gradient runs north to south, from the subtropical high (about 30° N) to the equatorial low.
• Air starts moving south. The Coriolis force turns it to the right, towards the west.
• It reaches the equator as the north-east trades.

• In the Southern Hemisphere, air is turned left and becomes the south-east trades.

• The spin of pressure systems:

• Away from the equator, the Coriolis force sets the spin of cyclones. They spin anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere [4].

• Anticyclones (highs) spin clockwise in the Northern Hemisphere. The spin is reversed in the Southern Hemisphere.

• No cyclones near the equator:

• The Coriolis force there is zero or very weak.
• Air rushing into a low is not turned. It flows straight in and fills the low.

• No spinning system can build up, so tropical cyclones do not form within about 5° of the equator and almost never cross it [4].

• Buys Ballot's law follows from it: in the Northern Hemisphere, stand with your back to the wind and low pressure is on your left. In the Southern Hemisphere, low pressure is on your right.

Where it occurs

• Everywhere except the equator. It is zero at the equator, gets stronger towards the poles and is greatest at the poles [4].
• India: the south-west monsoon.
• In summer, the south-east trades of the southern Indian Ocean cross the equator.
• In the Northern Hemisphere the deflection switches to the right.

• They reach India as the south-west monsoon, which brings most of India's rain.

• India: cyclones in the Bay of Bengal and the Arabian Sea.

• These cyclones form a little away from the equator and spin anticlockwise [4].

• Isobars crowd tightly round the centre of a Bay of Bengal cyclone, so winds there are very strong.

• World: planetary winds.

• North-east trades in the Northern Hemisphere and south-east trades in the Southern Hemisphere.

• Trade winds and westerlies are steadiest over the open oceans, where friction is lowest.

• Upper air: the upper-air westerlies and jet streams (above about 2–3 km) are near-geostrophic winds, shaped by the balance between the PGF and the Coriolis force [2].

• Southern Hemisphere: cyclones spin clockwise [4].

Don't confuse with

• Pressure gradient force: this force starts the wind and sets its speed, from high to low pressure, at right angles to the isobars. The Coriolis force only turns the wind and never starts it.
• Frictional force: friction slows the wind and works only up to about 1–3 km. The Coriolis force changes direction only, not speed [3], and works at all heights.
• Geostrophic wind: this is a wind, not a force. It is the result when the PGF and the Coriolis force balance above about 2–3 km, so the wind blows parallel to the isobars [2].
• Buys Ballot's law vs Ferrel's law: Ferrel's law states the direction of deflection (right in the NH, left in the SH). Buys Ballot's law is a way to find low pressure: in the NH, stand with your back to the wind and low pressure is on your left.

Prelims Hooks

• The Coriolis force depends on wind speed × sine of latitude. It is zero at the equator and greatest at the poles [4]. "Directly proportional to latitude" is a trap.
• It acts at right angles to motion, so it changes the wind's direction, not its speed [3].
• Gaspard-Gustave de Coriolis described it in 1835, not 1844 [3].
• Ferrel's law: wind turns right in the NH and left in the SH [4]. Cyclones spin anticlockwise in the NH and clockwise in the SH [4].
• No tropical cyclones within about 5° of the equator, because the Coriolis force there is too weak [4].
• SE trades → SW monsoon: after the SE trades cross the equator, they are turned right and reach India from the south-west.

Mains Points

• How it shapes planetary winds: the PGF starts the wind, the Coriolis force turns it and friction slows it.
• Together they produce the NE and SE trades, the westerlies and the near-geostrophic jet streams aloft.

• Surface winds cross the isobars towards low pressure because friction weakens the Coriolis force [2]. (GS-I)

• Coriolis force and disasters: cyclones cannot form within about 5° of the equator [4].

• Indian cyclones form a little farther north, in the Bay of Bengal and the Arabian Sea, and spin anticlockwise.

• This helps explain where cyclone risk lies, which feeds into cyclone risk mapping and IMD early warnings for the east and west coasts. (GS-I and GS-III, disaster management)

• Monsoon link: the south-east trades cross the equator and are turned to the right. They become the south-west monsoon, which brings most of India's rain.

• This shows how a force caused by the Earth's rotation shapes India's climate, farming and water supply. (GS-I)

Related concepts

• Wind
• Pressure gradient force
• Frictional force
• Geostrophic wind
• Wind vane
• Anemometer

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