GeographyISC Class 11 · Atmospheric Pressure and Winds

Pressure Distribution, Pressure Belts and Planetary Winds (ISC Class 11 Geography)

Updated 17 October 2026 · Fact-checked

Pressure falls with height, about 1 mb per 10 m in the lower atmosphere, and varies with temperature, rotation and moisture. The belts are the equatorial low, subtropical highs, subpolar lows and polar highs. Winds blow from high to low pressure, deflected by the Coriolis force (Ferrel's law), as trades, westerlies and polar easterlies.

Understand Pressure Distribution, Pressure Belts and Planetary Winds

Pressure. About 1,013 mb at sea level, measured by barometer (1 mb = 1 hPa). Falls with height as the air column shortens and thins: roughly 1 mb per 10 m in the lower atmosphere; half the mass lies below about 5.5 km.

Horizontal factors. Temperature (thermal: heated air rises, low pressure; cold air sinks, high pressure); rotation and air movement (dynamic: subtropical highs, subpolar lows); water vapour (moist air is lighter); altitude and land-sea distribution (summer lows and winter highs over land).

Isobars. Lines of equal pressure reduced to sea level. Close spacing: steep gradient, strong wind. They never cross. Closed isobars with lowest pressure at the centre show a low; with highest at the centre a high.

Belts. Equatorial low (about 5 degrees N to 5 degrees S, thermal, doldrums, calm); subtropical highs (about 30 to 35 degrees, dynamic, horse latitudes, dry, deserts); subpolar lows (about 60 to 65 degrees, dynamic, stormy); polar highs (thermal). Belts shift about 5 to 10 degrees with the sun.

Coriolis. Winds deflect right in the northern hemisphere and left in the southern (Ferrel's law); zero at the equator, greatest at poles. Buys Ballot: in the northern hemisphere, with back to the wind, low pressure is on the left.

Planetary winds. Trade winds (subtropical highs to the equatorial low; north-east in the north, south-east in the south); westerlies (subtropical highs to subpolar lows; south-westerlies north, north-westerlies south; roaring forties, furious fifties, shrieking sixties); polar easterlies (polar highs to subpolar lows).

Pressure Distribution, Pressure Belts and Planetary Winds: key facts

Pressure with height: About 1 mb fall per 10 m | Sea level about 1,013 mb
Pressure gradient: $G r a d i e n t = p r ess u r e d i f f er e n ce / d i s t an ce ∣ S t ee p er m e an ss t r o n g er w in d$
Pressure belts: Equatorial low 0
Subtropical highs 30-35
Subpolar lows 60-65
Polar highs 90; Thermal: equatorial, polar; dynamic: subtropical, subpolar.
Planetary winds: Trades 30 to 0
Westerlies 30 to 60
Polar easterlies 90 to 60; Right in NH, left in SH.

Equatorial low	0
Subtropical highs	30-35
Subpolar lows	60-65
Polar highs	90

Trades	30 to 0
Westerlies	30 to 60
Polar easterlies	90 to 60

How to answer Pressure Distribution, Pressure Belts and Planetary Winds questions

Expect 4-mark belt and wind descriptions plus gradient and height sums.

1State the factor (thermal or dynamic) for each belt.
2Give latitude, weather and wind outflow of each belt.
3Apply Ferrel's law to name wind directions.
4For sums, write the formula with units (mb, km, mb per km).

Pressure Distribution, Pressure Belts and Planetary 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.

Wind speed hint

Suppose two stations 250 km apart show 1,008 mb and 998 mb, and another pair 600 km apart shows 1,016 mb and 1,004 mb.

Which gives stronger winds?

Try it first, then show the solutionSolution

First gradient = 10/250 = 0.04 mb per km.
Second = 12/600 = 0.02 mb per km.
The first is steeper, so winds are stronger.

Answer: The first pair (0.04 mb per km).

Memory hook: Closer isobars, stronger wind.

Mountain pressure

Suppose a camp is 2,200 m above sea level where pressure is 1,013 mb.

Estimate camp pressure using 1 mb per 10 m.

Try it first, then show the solutionSolution

Fall = 2,200 / 10 = 220 mb.
Pressure = 1,013 - 220 = 793 mb (approximate, since the fall slows with height).

Answer: About 793 mb.

Memory hook: Ten metres, one millibar.

Tricks to remember Pressure Distribution, Pressure Belts and Planetary Winds

Short memory aids. Say each one aloud once, then use it on a question the same day.

TD
Equatorial and polar are Thermal; subtropical and subpolar are Dynamic.
Use it when: Belt origin.
Right in the North
Winds deflect right in the north, left in the south.
Use it when: Ferrel's law.

Pressure Distribution, Pressure Belts and Planetary Winds: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Belt identification · 1 mark · MCQBoard level

The horse latitudes are associated with:
(a) equatorial low
(b) subtropical highs
(c) subpolar lows
(d) polar highs

Try it first, then show the solutionSolution

30 to 35 degrees.

Answer: (b) Subtropical highs.

Example 2: Ferrel's law · 2 marks · Short answerBoard level

State Ferrel's law and the zone where the Coriolis force is zero.

Try it first, then show the solutionSolution

Moving air is deflected to the right in the northern hemisphere and left in the southern.
The Coriolis force is zero at the equator.

Answer: Right in the north, left in the south; zero at the equator.

Example 3: Gradient sums · 4 marks · Long answerHarder board level

Two stations 400 km apart record 1,014 mb and 998 mb; a second pair 800 km apart records 1,020 mb and 1,004 mb. Find each pressure gradient and decide which area has stronger winds.

Try it first, then show the solutionSolution

First: 1,014 - 998 = 16 mb; 16/400 = 0.04 mb per km.
Second: 1,020 - 1,004 = 16 mb; 16/800 = 0.02 mb per km.
The first gradient is twice the second.
So the first area has stronger winds.

Answer: 0.04 and 0.02 mb per km; first area stronger.

Example 4: Belts · 4 marks · Long answerHarder board level

Describe the subtropical high and the subpolar low belts, stating their origin, latitude and weather.

Try it first, then show the solutionSolution

Subtropical highs: about 30-35 degrees; dynamic; air from the equator cools and sinks; dry, calm, clear; hot deserts.
Subpolar lows: about 60-65 degrees; dynamic; air is spun outward and rises where westerlies meet polar easterlies.
Weather there is stormy.
Belts shift 5-10 degrees with the sun.

Answer: Subtropical highs: dynamic, sinking air, deserts; subpolar lows: dynamic, rising air, storms.

Example 5: Hill station barometer · 5 marks · Case-basedHOTS / competency

Case: A hill resort is 1,800 m above sea level. (a) Estimate its pressure if sea-level pressure is 1,010 mb using 1 mb per 10 m. (b) State why the real pressure may differ. (c) Explain why high altitude brings lower pressure.

Try it first, then show the solutionSolution

(a) Fall = 1,800/10 = 180 mb; pressure = 1,010 - 180 = 830 mb.
(b) The fall per metre is greater near the ground and slows with height, and temperature and weather also vary, so the real pressure differs. The true value at 1,800 m is a little lower (about 815 mb) than the 830 mb estimate, because 1 mb per 10 m is only a rough rule.
(c) The column of air above is shorter and the air thinner.

Answer: (a) About 830 mb (b) the 1 mb per 10 m rule is rough; the real value is a little lower (about 815 mb) (c) shorter, thinner column.

Where students lose marks in Pressure Distribution, Pressure Belts and Planetary Winds

The slips examiners see most often, and what to write instead.

Mistake: Reversing deflection direction
Left and right are swapped.
Wrong working
Winds deflect left in the north
Right working
Right in the north
Do this: Right in the north, left in the south.
Usually costs: 1 mark.
Mistake: Calling the equatorial low dynamic
Origin confused.
Do this: Equatorial and polar belts are thermal.
Usually costs: 1 mark.
Mistake: Using 1 mb per 100 m
Height rule is misquoted.
Do this: About 1 mb per 10 m in the lower atmosphere.
Usually costs: 2 marks.

Exam tips

Mark each belt as thermal or dynamic.
Quote latitudes for belts and winds.
Apply Ferrel's law for each wind direction.
Use mb per km for the gradient.

Practice questions on Pressure Distribution, Pressure Belts and Planetary 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
The trade winds blow from the:
(a) polar highs to subpolar lows
(b) subtropical highs to the equatorial low
(c) equatorial low to subtropical highs
(d) subpolar lows to polar highs
Hint
Towards the equator.
Answer
(b) Subtropical highs to the equatorial low.
Q2 · 3 marks · Short answerHarder board level
A station at sea level records 1,013 mb. Estimate pressure at a hill station 1,200 m above using 1 mb per 10 m.
Hint
Fall = height / 10.
Answer
Fall = 1,200 / 10 = 120 mb; pressure = 1,013 - 120 = 893 mb (approximately).
Q3 · 4 marks · Long answerHarder board level
Why are the subtropical high-pressure belts dynamic and associated with deserts?
Hint
Sinking air.
Answer
They arise from dynamic causes: air that rose at the equator cools and sinks near 30-35 degrees. Descending air is dry, giving clear skies and hot deserts.
Q4 · 4 marks · Long answerHarder board level
Explain why the westerlies are stronger and steadier in the southern hemisphere.
Hint
Few landmasses.
Answer
The southern hemisphere has few landmasses, so there is little friction and little seasonal pressure disturbance; the westerlies blow strongly as the roaring forties, furious fifties and shrieking sixties.
Q5 · 4 marks · Case-basedHOTS / competency
Case: A sailor stands with his back to the wind in the northern hemisphere and finds low pressure to his left. (a) Name the law. (b) State the direction of the wind deflection. (c) Name the Coriolis effect at the equator.
Hint
Buys Ballot.
Answer
(a) Buys Ballot's law. (b) To the right in the northern hemisphere. (c) The effect is zero at the equator.

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.Sea-level pressure?Show answer
About 1,013 mb.
2.Doldrums are the?Show answer
Equatorial low.
3.Trades blow towards?Show answer
The equator.
4.Polar highs origin?Show answer
Thermal.
5.Closed isobars with low centre?Show answer
Cyclone.

Pressure Distribution, Pressure Belts and Planetary Winds: frequently asked questions

Is 1 mb per 10 m exact?

It is a rough rule for the lower atmosphere.

What is a hPa?

Equal to 1 mb.

Why do belts shift?

They follow the sun.