Wind and the forces that steer it

Detail

What wind is

• Wind is the horizontal movement of air from an area of high pressure to an area of low pressure.
• Air that moves up or down is called an air current, not a wind.

• Two things describe a wind: its speed and its direction.

• Naming rule: a wind takes its name from the direction it comes from, not the direction it goes to.

• A westerly blows from the west towards the east.
• The north-east trades blow from the north-east towards the south-west.
• Exam trap: "south-west monsoon" means wind coming from the south-west and blowing into India.

Measuring wind: instruments and scales

Instrument / scale	What it measures	How it works	Where used
Wind vane	Direction	Its arrow points to the direction the wind comes from	Weather stations, rooftops
Wind sock	Direction (and a rough idea of strength)	A cloth cone. It fills with air and points the way the wind is blowing to, so its open mouth faces where the wind comes from	Airfields, and industries that release gases
Cup anemometer	Speed	Cups catch the wind and spin. More spins mean faster wind. Speed is read in km/h or knots	Weather stations
Beaufort scale	Strength, judged by its effects	Grades wind by what it does to smoke, trees, waves and buildings	Ships, coasts, simple observation

• Knot: 1 nautical mile per hour. 1 knot = 1.852 km/h [4].
• The Beaufort scale was developed in 1805 by Sir Francis Beaufort of the British (UK) Royal Navy [4].
• It runs from Force 0 to Force 12 [4]:

Force	Name	Speed (knots)
0	Calm	< 1
3	Gentle breeze	7–10
6	Strong breeze	22–27
8	Gale	34–40
10	Storm	48–55
11	Violent storm	56–63
12	Hurricane	64 and above (about 118 km/h or more)

(All rows from [4].) The table in Class 9 NCERT is a simplified version of this scale.

Forces that control the wind

• Three forces decide how fast surface wind blows and which way it goes: 1. Pressure gradient force: starts the wind and decides its speed. 2. Frictional force: slows the wind near the ground. 3. Coriolis force: turns the wind.

• Gravity also acts. It pulls air down and holds the atmosphere to the Earth.

1. Pressure gradient force (PGF)

• Pressure gradient: how fast pressure changes over a distance.
• Pressure gradient force: the push this difference gives the air. It moves air from high to low pressure.
• Direction: at right angles to the isobars (isobars: lines on a map that join places with the same pressure), from high to low.
• Strength:
• Isobars close together → steep gradient → strong wind.

• Isobars far apart → gentle gradient → weak wind.

• The force is directly proportional to the pressure difference over a given distance [3].

• Example: around a tropical cyclone over the Bay of Bengal, isobars crowd tightly round the centre, so winds there are very strong.
• If the Earth did not spin and there were no friction, wind would blow straight across the isobars from high to low.

2. Frictional force

• Frictional force: drag from the Earth's surface (hills, forests, buildings) that slows the wind.
• Where it acts:
• It is greatest at the surface.
• It reaches up about 1–3 km. This layer is the friction layer (boundary layer).

• It is least over the sea, because the water surface is smooth.

• Result: winds over oceans are faster and steadier than winds over land. This is why trade winds and westerlies are steadiest over the open oceans.

• Why surface winds cross the isobars:
• Friction slows the wind → the Coriolis force, which depends on speed, gets weaker.
• The PGF now wins over the Coriolis force → the wind is not turned fully parallel to the isobars.
• Surface wind therefore crosses the isobars at an angle, towards low pressure [2].
• The rougher the surface, the bigger the angle. The angle is bigger over land than over sea.

3. Coriolis force

• Coriolis force: an apparent force caused by the Earth's rotation. It is not a real push. It appears because the ground under moving air keeps turning.
• Direction (Ferrel's law): it bends moving air
• to the right in the Northern Hemisphere, and

• to the left in the Southern Hemisphere [6].

• It always acts at right angles to the direction of motion. So it changes the wind's direction, not its speed [5].

• What it depends on:
• Wind speed: faster wind → more deflection.
• Sine of latitude: the formula is roughly 2 × (Earth's spin rate) × speed × sin(latitude).
• sin 0° = 0 → zero at the equator.
• sin 90° = 1 → greatest at the poles [6].

• (NCERT: "directly proportional to the angle of latitude". The correct rule is the sine of latitude, not the angle itself.)

• 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 on a rotating surface a force acts on a moving body at right angles to its motion [5]. (NCERT: 1844.)

Why no cyclones near the equator

• Near the equator the Coriolis force is zero or very weak.
• Air rushing into a low is not turned.
• It flows straight into the low and fills it.

• No spinning system can build up.

• So tropical cyclones do not form within about 5° of the equator. They almost never cross the equator [6].

• Away from the equator, the Coriolis force sets the spin of cyclones:
• anticlockwise in the Northern Hemisphere (for example, Bay of Bengal and Arabian Sea cyclones);
• clockwise in the Southern Hemisphere [6].

Trade winds: the NCERT exercise

• Question: the pressure gradient runs north to south, from the subtropical high (about 30° N) to the equatorial low. So why do the trades blow from the north-east, not from the north?
• Answer:
• Air starts moving south, towards the equator.
• The Coriolis force turns it to the right, that is, towards the west.
• It reaches the equator as the north-east trades.

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

• Indian example: in summer, the south-east trades of the southern Indian Ocean cross the equator. The deflection then changes to the right, so they arrive over India as the south-west monsoon.

Buys Ballot's law

• Rule (Northern Hemisphere): stand with your back to the wind and low pressure is on your left. High pressure is on your right.
• In the Southern Hemisphere it is the other way round: low pressure is on your right.
• This lets you find the nearest low, for example a storm centre, using only the wind.

Geostrophic wind

• Geostrophic wind: wind higher up, about 2–3 km and above, where there is no friction.
• The PGF (pulling towards low pressure) and the Coriolis force (pulling the opposite way) become equal.

• The air stops crossing the isobars and blows parallel to straight isobars [2].

• Near the ground the balance is PGF = Coriolis + friction. The wind crosses the isobars and blows towards low pressure [2].

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

Feature	Surface wind	Geostrophic (upper) wind
Height	Below about 1–3 km	Above about 2–3 km
Forces	PGF, Coriolis, friction	PGF and Coriolis only, balanced
Direction	Crosses isobars towards low	Parallel to isobars
Speed	Slower (friction)	Faster

Wind round pressure systems (Northern Hemisphere)

System	Spin	Air movement
Cyclone (low)	Anticlockwise	Air flows in (converges) and rises
Anticyclone (high)	Clockwise	Air sinks and flows out (diverges)

• In the Southern Hemisphere both spins are reversed.

Prelims Hooks

• A wind is named after the direction it comes from: a westerly blows from the west.
• Wind vane → direction. Cup anemometer → speed (km/h or knots). Wind sock → direction at airfields.
• Beaufort scale: 1805, Sir Francis Beaufort, Force 0 (calm) to Force 12 (hurricane, ≥ 64 knots) [4].
• The Coriolis force depends on sine of latitude × wind speed. It is zero at the equator and greatest at the poles. "Directly proportional to latitude" is a trap.
• Coriolis described the effect in 1835, not 1844 [5].
• Ferrel's law: wind turns right in the Northern Hemisphere and left in the Southern Hemisphere.
• No tropical cyclones within about 5° of the equator, because the Coriolis force there is too weak.
• Buys Ballot's law (NH): back to the wind → low pressure on the left.
• Geostrophic wind blows parallel to isobars above 2–3 km. Surface winds cross isobars towards low pressure because of friction.
• Friction is least over the sea, so ocean winds are faster and steadier.

Mains Points

• How the three forces work together: the PGF starts the wind, the Coriolis force turns it and friction slows it. Together they explain the planetary winds: the NE and SE trades, the westerlies, and the geostrophic jet streams aloft. (GS-I)
• Coriolis force and disasters: cyclones cannot form within about 5° of the equator. Indian cyclones form in the Bay of Bengal and the Arabian Sea a little farther from the equator, and spin anticlockwise. This matters for cyclone risk mapping and IMD early warnings for the east and west coasts. (GS-I and GS-III, disaster management)
• Monsoon link: the SE 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 the planetary wind forces shape the Indian climate. (GS-I)
• Practical uses: wind direction and speed data from anemometers and the Beaufort scale help with aviation safety (wind socks), shipping warnings, siting wind-energy farms and tracking how factory gases spread. (GS-III)