Pressure Belts, Forces on Wind, General Circulation, ENSO and Local Winds (Class 11 Geography, Fundamentals of Physical Geography, Chapter 9)
Updated 17 October 2026 · Fact-checked
Air pressure is the weight of a column of air, 1,013.2 mb on average at sea level, falling about 1 mb per 10 m. Wind blows because of pressure differences and is shaped by the pressure gradient force, friction and the Coriolis force. The general circulation has Hadley, Ferrel and polar cells; El Nino and local winds such as land and sea breezes add variations.
Air rises at the lows (equator, 60°N) and sinks at the highs (30°N, pole); the Ferrel cell turns opposite to the Hadley and polar cells.
Illustration drawn for QuestionBuddy.
Understand Pressure Belts, Forces on Wind, Circulation Cells, ENSO and Local Winds
Why this topic matters. Coriolis effects, cell diagrams and wind types are repeated Board items.
1. Pressure. Weight of a column of air over a unit area, in millibars; average sea-level pressure 1,013.2 mb; measured by mercury or aneroid barometers. It falls with height, about 1 mb per 10 m in the lower atmosphere. Pressure differences are the primary cause of wind. Vertical pressure gradient force is balanced by gravity, so there is no strong upward wind.
2. Isobars. Lines joining places of equal pressure, reduced to sea level. A low has the lowest pressure at the centre; a high the highest.
3. Belts. Equatorial low; subtropical highs at about 30 N and S; sub-polar lows at about 60 N and S; polar highs. They shift north in northern summer and south in winter with the sun's apparent movement.
4. Forces. (i) Pressure gradient force: strong where isobars are close; acts perpendicular to isobars. (ii) Friction: affects speed, greatest at the surface, up to 1-3 km, minimal over the sea. (iii) Coriolis force: due to rotation; deflects wind to the right in the northern hemisphere and left in the southern; proportional to latitude, maximum at the poles, absent at the equator; acts perpendicular to the pressure gradient. At the equator a low fills up, so tropical cyclones do not form there. Geostrophic wind: in the upper air 2-3 km up, where pressure gradient and Coriolis forces balance, wind blows parallel to isobars.
5. Cyclone and anticyclone. Cyclone around a low: anticlockwise in the northern hemisphere. Anticyclone around a high: clockwise. Over a low air converges and rises; over a high it subsides and diverges.
6. General circulation. Hadley cell in the tropics: air rises at the ITCZ up to 14 km, moves poleward, sinks near 30 N and S; surface easterlies return. Ferrel cell in mid-latitudes: westerlies. Polar cell: cold air subsides near the poles, giving polar easterlies.
7. El Nino. Warm water of the central Pacific drifts to the South American coast replacing the cool Peruvian current. Southern Oscillation is the associated pressure change over the central Pacific and Australia. ENSO = both. In strong years: heavy rain on the arid west coast of South America, drought in Australia and sometimes India, floods in China.
8. Local winds. Sea breeze by day (sea to land), land breeze by night (land to sea). Valley breeze by day, mountain wind by night. Katabatic wind: cool air draining from plateaus and ice fields into valleys. Warm dry leeward wind after crossing a range. Seasonal winds: monsoon.
Pressure Belts, Forces on Wind, Circulation Cells, ENSO and Local Winds: key facts
- Pressure facts
Average sea level 1,013.2 mb Falls about 1 mb per 10 m Unit millibar Instruments mercury and aneroid barometer - Numbers.
- Belts
- Equatorial low
- Subtropical highs about 30 N and S
- Sub-polar lows about 60 N and S
- Polar highs
- Order.
- Three forces
Pressure gradient perpendicular to isobars Friction surface, 1-3 km Coriolis right in north, left in south, zero at equator - Wind control.
- Cells
Hadley tropics, easterlies Ferrel mid-latitudes, westerlies Polar polar easterlies - Surface winds.
How to answer Pressure Belts, Forces on Wind, Circulation Cells, ENSO and Local Winds questions
Circulation answers combine force, cell and deflection.
- 1Start from pressure differences.
- 2Add Coriolis with direction and latitude rule.
- 3Describe the three cells with surface wind.
- 4Mention ENSO effects with places.
- 5Draw local winds in day-night pairs.
Pressure Belts, Forces on Wind, Circulation Cells, ENSO and Local Winds in real life
The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.
Coast breezes
Suppose people at a seaside town feel a cool wind from the sea in the afternoon and a gentle wind from land at night.
Name the two winds and the reason.
Try it first, then show the solutionSolution
- Land heats faster by day, low pressure over land.
- At night land cools faster, so pressure gradient reverses.
Answer: Sea breeze by day and land breeze by night, due to differential heating of land and sea.
Memory hook: Day sea-to-land, night land-to-sea.
Spinning swirl
Suppose water draining from a basin starts to swirl in a fixed direction because of a sideways push.
Which atmospheric force does this resemble?
Try it first, then show the solutionSolution
- Coriolis force deflects moving air because of the earth's rotation.
- Right in the northern hemisphere.
Answer: Coriolis force, deflecting winds to the right in the northern hemisphere.
Memory hook: Rotation turns winds.
Tricks to remember Pressure Belts, Forces on Wind, Circulation Cells, ENSO and Local Winds
Short memory aids. Say each one aloud once, then use it on a question the same day.
1013.2
Average sea-level pressure in mb
Use it when: Pressure.
Right in the North, left in the South
Coriolis
Use it when: Direction.
H-F-P
Hadley, Ferrel, Polar
Use it when: Cells.
Pressure Belts, Forces on Wind, Circulation Cells, ENSO and Local Winds: solved examples
Ordered from board level to HOTS. Try each one before you open the solution.
Example 1: Geostrophic wind · 1 mark · MCQBoard level
Geostrophic wind blows:
(A) perpendicular to isobars
(B) parallel to isobars in the upper air
(C) from land to sea
(D) only at the equator
Try it first, then show the solutionSolution
- Pressure gradient and Coriolis forces balance, so wind is parallel to isobars (1 mark).
Answer: (B)
Example 2: Wind direction and the Coriolis force · 2 marks · Very short answerBoard level
A low-pressure centre lies to the north of a station in the northern hemisphere. State the direction in which the pressure gradient force acts on the air, and the direction in which the Coriolis force deflects it. (2 marks)
Try it first, then show the solutionSolution
- Pressure gradient force acts from high to low, so towards the north, perpendicular to the isobars (1 mark).
- Coriolis force deflects moving air to the right in the northern hemisphere (1 mark).
Answer: The pressure gradient force pushes the air northwards towards the low, across the isobars; the Coriolis force then deflects it to the right of its path.
Example 3: Hadley cell · 3 marks · Short answerHarder board level
Describe the working of the Hadley cell. (3 marks)
Try it first, then show the solutionSolution
- Air rises at the ITCZ by convection (1 mark).
- Moves poleward at the top of the troposphere, accumulates near 30 N and S, sinks forming the subtropical high (1 mark).
- Easterlies blow back to the ITCZ at the surface (1 mark).
Answer: High insolation at the ITCZ makes air rise by convection to the top of the troposphere, up to 14 km. It moves poleward, accumulates near 30 N and S, and part of it sinks as it cools, forming the subtropical high. Surface easterlies flow back to the ITCZ.
Example 4: Assertion and reason: El Nino · 1 mark · Assertion-reasonHarder board level
Assertion (A): El Nino can cause drought in Australia.
Reason (R): It is associated with pressure changes over the central Pacific and Australia, called the Southern Oscillation.
(A) Both true, R explains A
(B) Both true, R does not explain A
(C) A true, R false
(D) A false, R true
Try it first, then show the solutionSolution
- The Southern Oscillation pressure change is linked with ENSO years of drought in Australia (1 mark).
Answer: (A)
Example 5: HOTS: wind direction · 5 marks · Long answerHOTS / competency
Isobars run west-east in the northern hemisphere with high pressure on the south and low on the north. State the direction of the pressure gradient force and the direction in which the Coriolis force deflects the wind. What is the resulting wind direction in the upper air? (5 marks)
Try it first, then show the solutionSolution
- Pressure gradient force acts perpendicular to isobars, from high to low, i.e. northwards (1 mark).
- Coriolis force deflects wind to its right in the northern hemisphere (1 mark).
- Wind moving north is deflected towards the east (1 mark).
- In the upper air, free from friction, the two forces balance (1 mark).
- So the wind blows parallel to the isobars, from west to east (a westerly) (1 mark).
Answer: The pressure gradient force acts perpendicular to the isobars from high to low pressure, so northwards. In the northern hemisphere Coriolis force deflects moving air to the right, so a northward-moving wind turns towards the east. In the upper air, free of friction, the forces balance and the geostrophic wind blows parallel to the isobars, from west to east, as a westerly.
Where students lose marks in Pressure Belts, Forces on Wind, Circulation Cells, ENSO and Local Winds
The slips examiners see most often, and what to write instead.
Mistake: Deflection direction wrong
Both hemispheres confused.
Wrong working
Wind turns left in the northern hemisphere.
Right working
Right in the north, left in the south.
Do this: Use the phrase.
Usually costs: 1 mark.
Mistake: Wrong cell winds
Names similar.
Wrong working
Hadley cell gives westerlies.
Right working
Hadley: easterlies; Ferrel: westerlies; Polar: easterlies.
Do this: Pair cell and wind.
Usually costs: 1 mark.
Mistake: Calling pressure units wrong
Unit confusion.
Wrong working
Pressure in kg.
Right working
Millibar.
Do this: State mb.
Usually costs: 1 mark.
Exam tips
- Draw the three cells with arrows.
- State Coriolis rules with latitude.
- Pair day and night winds in answers.
Practice questions on Pressure Belts, Forces on Wind, Circulation Cells, ENSO and Local Winds
Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.
Q1 · 1 mark · MCQBoard level
Which belt is found near 60 N and S?
(A) Equatorial low
(B) Subtropical high
(C) Sub-polar low
(D) Polar highHint
Between Ferrel and polar cells.
Answer
(C)
Q2 · 2 marks · Very short answerBoard level
If pressure decreases at about 1 mb per 10 m, estimate the pressure at 500 m above a place with 1,000 mb. (2 marks)
Hint
500/10 = 50.
Answer
1,000 - 50 = about 950 mb (approximate, as the rate varies).
Q3 · 3 marks · Short answerHarder board level
Write three differences between a cyclone and an anticyclone circulation. (3 marks)
Hint
Centre and direction.
Answer
A cyclone has a low at the centre, anticlockwise circulation in the northern hemisphere and converging, rising air; an anticyclone has a high at the centre, clockwise circulation in the northern hemisphere and subsiding, diverging air.
Q4 · 3 marks · Short answerHarder board level
Explain the effects of a strong ENSO year in different regions. (3 marks)
Hint
Rain and drought.
Answer
Heavy rain falls on the arid west coast of South America, Australia and sometimes India face drought, and China has floods; ENSO is monitored for long-range forecasting.
Q5 · 4 marks · Short answerHOTS / competency
Why does the Coriolis force have no effect on wind at the equator but strong effect at higher latitudes? (4 marks)
Hint
Proportional to latitude.
Answer
The Coriolis force is directly proportional to the angle of latitude, so it is absent at the equator and maximum at the poles. At the equator winds blow perpendicular to isobars and lows fill up, while at higher latitudes winds are deflected and circulate around lows and highs.
Test your recall
Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.
1.What is the ITCZ?Show answer
The zone where air rises at the equator in the Hadley cell.
2.Who described the Coriolis force?Show answer
A French physicist, in 1844.
3.What is a katabatic wind?Show answer
Cool air from high plateaus and ice fields draining into valleys.
4.Where does the Hadley cell end?Show answer
Near 30 degrees N and S.
Pressure Belts, Forces on Wind, Circulation Cells, ENSO and Local Winds: frequently asked questions
What is the primary cause of wind?
Variation in pressure from place to place.
How high is the Hadley cell?
Air rises up to 14 km.
What is a mountain wind?
A night wind when dense air descends into the valley.