Atmospheric Pressure and Winds ICSE Class 9: Belts, Winds, Cyclones and Jet Streams

What Atmospheric Pressure and Winds is about

Weightage: CISCE publishes no chapter weightage and no question-paper layout for ICSE Class 9 Geography. Class IX is examined by your school: one 80-mark paper of 2 hours plus 20 marks of internal assessment (a record file and one project). Atmospheric Pressure and Winds is topic 4(c), Atmosphere, of the 2028 syllabus.Source: CISCE ICSE 2028 Geography syllabus, Class IX

Key concepts

Atmospheric pressure and the factors that affect it

Atmospheric pressure is the force exerted by the weight of a column of air on a unit area of the earth's surface. It is measured with a barometer (mercury or aneroid) in millibars (mb) or hectopascals (hPa). Average pressure at sea level is about 1,013 mb. Lines joining places of equal pressure on a map are isobars.

Factors affecting pressure:
1. Altitude: pressure is highest at sea level and falls with height, because there is less air above. In the lower atmosphere it falls by roughly 1 mb for every 10 m of ascent.
2. Temperature: warm air expands, becomes lighter and rises, so pressure falls (low pressure over hot regions); cold air contracts, becomes denser and sinks, so pressure rises (high pressure over cold regions).
3. Water vapour: moist air is lighter than dry air, so humid air has slightly lower pressure.
4. Rotation of the earth: rotation deflects moving air, so air piles up in some latitudes (sub-tropical highs) and is thrown away from others (sub-polar lows). These belts are therefore called dynamically formed.

Worked example

(2 marks) Why does atmospheric pressure decrease with altitude? Estimate the pressure at a place 500 m above sea level if sea-level pressure is 1,013 mb.

Try it first, then show the solutionSolution

1. With height there is less air above, so its weight is less - 1 mark.
2. Fall ≈ 1 mb per 10 m: 500 ÷ 10 = 50 mb; pressure ≈ 1,013 − 50 = 963 mb - 1 mark.

Answer: Pressure falls with height because there is a smaller column of air above, so it weighs less. At 500 m the pressure is roughly 1,013 − 50 = 963 mb.

Major pressure belts of the world

Seven pressure belts encircle the earth:
1. Equatorial low-pressure belt (about 5°N to 5°S): intense heating all year makes air rise; winds are light and variable, so the belt is called the doldrums. Thermally induced.
2-3. Sub-tropical high-pressure belts (about 30°N and 30°S): air that rose at the equator cools and sinks here, and the earth's rotation piles air up. Calm, dry conditions; called the horse latitudes. Dynamically induced. Most hot deserts lie in these latitudes.
4-5. Sub-polar low-pressure belts (about 60°N and 60°S): rotation throws air away from these latitudes, and warm westerlies meet cold polar air and rise. Dynamically induced; stormy.
6-7. Polar high-pressure areas (around the poles): very cold, dense air. Thermally induced.

The belts shift north in the northern summer and south in the southern summer, following the overhead sun.

Diagram checklist: a circle for the earth; latitudes 0°, 30°N/S, 60°N/S, 90°N/S; label each belt H or L with its name; add the planetary winds between them with arrows deflected right in the north and left in the south.

Worked example

(3 marks) Distinguish between thermally and dynamically induced pressure belts, with one example of each.

Try it first, then show the solutionSolution

1. Thermal: caused by temperature differences; e.g. equatorial low (heating) or polar high (cold) - 1 mark.
2. Dynamic: caused by rotation and the movement of air (sinking or being thrown aside) - 1 mark.
3. e.g. sub-tropical highs (sinking air) or sub-polar lows (air thrown away) - 1 mark.

Answer: Thermally induced belts result from heating or cooling: the equatorial low forms because heated air rises; the polar highs because cold air is dense. Dynamically induced belts result from the earth's rotation and air movement: the sub-tropical highs form where air piles up and sinks, and the sub-polar lows where air is thrown outward and rises.

Factors affecting wind: pressure gradient and Coriolis effect

Wind is the horizontal movement of air from high pressure to low pressure.

Pressure gradient: the rate of change of pressure over a horizontal distance.

• Wind always blows from high to low pressure.
• When isobars are close together the gradient is steep and winds are strong; when they are far apart the gradient is gentle and winds are light. So the pressure gradient controls the velocity (speed) of the wind.

Coriolis effect (force): the deflection of moving air caused by the earth's rotation.

• Winds are deflected to the right in the northern hemisphere and to the left in the southern hemisphere (Ferrel's law).
• The effect is zero at the equator and greatest at the poles.
• So winds blow across the isobars at an angle, not straight from high to low. The Coriolis effect controls the direction of winds.

(Friction with the ground also slows winds near the surface.)

Worked example

(2 marks) State Ferrel's law. Why are there no cyclones right on the equator?

Try it first, then show the solutionSolution

1. Ferrel's law: winds are deflected to the right in the northern and to the left in the southern hemisphere - 1 mark.
2. The Coriolis effect is zero at the equator, so winds cannot spin into a cyclone there - 1 mark.

Answer: Ferrel's law states that moving air is deflected to the right in the northern hemisphere and to the left in the southern hemisphere because of the earth's rotation. At the equator the Coriolis effect is zero, so air cannot be set spinning to form a cyclone.

Permanent (planetary) winds

Permanent winds blow throughout the year in a definite direction between the pressure belts.

1. Trade winds: from the sub-tropical highs towards the equatorial low. Deflected, they blow from the north-east in the northern hemisphere (north-east trades) and from the south-east in the southern hemisphere (south-east trades). They are steady and were used by sailing ships for trade. They bring rain to the eastern coasts of continents in low latitudes and are dry on the western sides (hot deserts).

2. Westerlies: from the sub-tropical highs towards the sub-polar lows. They blow from the south-west in the northern hemisphere and from the north-west in the southern hemisphere. In the southern hemisphere, with few land masses to slow them, they are very strong and stormy; sailors called these latitudes the Roaring Forties, Furious Fifties and Shrieking Sixties. They bring rain to the western coasts of continents in middle latitudes (e.g. western Europe).

3. Polar easterlies: from the polar highs towards the sub-polar lows. They blow from the north-east in the northern hemisphere and from the south-east in the southern hemisphere. They are cold and dry. Where they meet the warmer westerlies, temperate cyclones form.

Worked example

(3 marks) Why do the westerlies blow more strongly in the southern hemisphere than in the northern hemisphere? What are these latitudes called?

Try it first, then show the solutionSolution

1. The southern hemisphere between 40° and 60° is almost entirely ocean - 1 mark.
2. With little land to cause friction or block them, the westerlies blow with great force - 1 mark.
3. Called the Roaring Forties, Furious Fifties and Shrieking Sixties - 1 mark.

Answer: Between about 40°S and 60°S the southern hemisphere is almost all ocean, so there is little land friction or mountain barrier to slow the westerlies, which blow with great force. Sailors named these latitudes the Roaring Forties, Furious Fifties and Shrieking Sixties.

Periodic winds: land and sea breezes, monsoons

Periodic winds change direction with the time of day or the season, because land and water heat and cool at different rates.

Sea breeze (day): the land heats faster than the sea, so low pressure forms over the land and relatively high pressure over the sea; cool air blows from sea to land in the afternoon.
Land breeze (night): the land cools faster than the sea, so high pressure forms over the land; air blows from land to sea at night and early morning. Fishermen sail out with the land breeze before dawn and return with the sea breeze.

Monsoons: seasonal winds that reverse direction between summer and winter; they are land and sea breezes on a giant scale.

• Summer (south-west monsoon, June to September in India): the land of northern India and Asia becomes very hot, creating low pressure; moist winds blow from the high-pressure Indian Ocean towards the land, bringing heavy rain.
• Winter (north-east monsoon, about December to February): the land is cold with high pressure; dry winds blow from land to sea. Picking up moisture over the Bay of Bengal, they bring winter rain to the Tamil Nadu coast.

Monsoons also occur in South-East Asia and northern Australia.

Worked example

(3 marks) Explain the formation of land and sea breezes.

Try it first, then show the solutionSolution

1. By day land heats faster than sea; low pressure over land, higher over sea - 1 mark.
2. Cool sea breeze blows from sea to land - 1 mark.
3. At night land cools faster, pressure becomes higher over land, and the land breeze blows to the sea - 1 mark.

Answer: By day the land heats faster than the sea, so the air over the land rises and pressure there is lower; cooler air flows in from the sea as a sea breeze. At night the land cools faster, pressure over it becomes higher than over the warmer sea, and air flows from land to sea as a land breeze.

Local winds: Loo, Chinook, Foehn and Mistral

Local winds blow over small areas for short periods, due to local differences in temperature and relief.

1. Loo: a hot, dry, dusty wind blowing from the west and north-west over the northern plains of India (Punjab to Bihar) on May and June afternoons. Temperatures can exceed 45 °C, and it can cause heatstroke.

2. Chinook: a warm, dry wind blowing down the eastern (leeward) slopes of the Rocky Mountains in the USA and Canada, usually in late winter and spring. As the air descends it is compressed and warmed. It melts snow quickly, so it is called the 'snow eater', and clears pastures for grazing.

3. Foehn: a warm, dry wind blowing down the northern (leeward) slopes of the Alps, especially into the valleys of Switzerland, mainly in spring. It is warmed by compression as it descends. It melts snow, helps animals graze earlier and helps ripen grapes, but can also cause avalanches and forest fires.

4. Mistral: a cold, dry, strong northerly wind that blows down the Rhône valley into southern France and the Gulf of Lion, mostly in winter and spring, from the cold highlands to the warmer Mediterranean. Funnelled through the valley, it is very strong; farmers plant rows of trees as windbreaks and houses face south.

Chinook and Foehn are similar winds: both are warm and dry because descending air is compressed and heated.

Worked example

(3 marks) Distinguish between the Chinook and the Mistral.

Try it first, then show the solutionSolution

1. Chinook: warm and dry; Mistral: cold and dry - 1 mark.
2. Chinook: down the eastern slopes of the Rockies (USA, Canada); Mistral: down the Rhône valley to southern France - 1 mark.
3. Chinook melts snow and helps grazing; Mistral harms crops, so windbreaks are planted - 1 mark.

Answer: The Chinook is a warm, dry wind descending the eastern slopes of the Rockies; it melts snow and helps cattle graze. The Mistral is a cold, dry, strong northerly wind funnelled down the Rhône valley into southern France; it damages crops, so farmers plant windbreaks.

Variable winds: cyclones and anticyclones; jet streams

Cyclone: a low-pressure system with closed isobars and the lowest pressure at the centre; winds blow inward and spiral upward, anticlockwise in the northern hemisphere and clockwise in the southern. Rising air brings clouds, strong winds and heavy rain.

• Tropical cyclones form over warm seas (above about 27 °C) between 5° and 30° latitude; very violent, with a calm 'eye' at the centre. Called cyclones in the Indian Ocean, hurricanes in the Atlantic and typhoons in the western Pacific. They cause storm surges, floods and damage, as along the coasts of Odisha and Andhra Pradesh.
• Temperate cyclones form in middle latitudes where warm westerlies meet cold polar air; they are larger and less violent and bring cloudy, rainy weather to western Europe and, as western disturbances, winter rain to north-west India.

Anticyclone: a high-pressure system with the highest pressure at the centre; air sinks and blows outward, clockwise in the northern hemisphere and anticlockwise in the southern. It brings calm, dry, clear weather (cold nights and fog in winter).

Jet streams: narrow belts of very fast westerly winds high in the troposphere, near the tropopause (generally above about 12 km). Their speed is often over 100 km/h and can exceed 180 km/h in winter. The sub-tropical westerly jet blows south of the Himalayas in winter.
Importance:

• Aircraft flying eastward use them to save time and fuel; westward flights avoid them.
• They steer cyclones and anticyclones and so affect weather; the sub-tropical jet brings western disturbances to north India in winter, and its shift north of the Himalayas in summer is linked with the onset of the monsoon.

Worked example

(3 marks) Distinguish between a cyclone and an anticyclone.

Try it first, then show the solutionSolution

1. Cyclone: low pressure at the centre; anticyclone: high pressure at the centre - 1 mark.
2. Cyclone: winds inward, anticlockwise (N hemisphere); anticyclone: winds outward, clockwise (N hemisphere) - 1 mark.
3. Cyclone: rising air, clouds and rain; anticyclone: sinking air, clear and dry weather - 1 mark.

Answer: A cyclone has low pressure at its centre, with winds blowing inward (anticlockwise in the northern hemisphere) and rising air that brings clouds and rain. An anticyclone has high pressure at its centre, with winds blowing outward (clockwise in the northern hemisphere) and sinking air that gives calm, clear, dry weather.

Atmospheric Pressure and Winds in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Pressure Belts, Planetary Winds and Local Winds

Atmospheric Pressure and Winds: key facts

Pressure with height

Pressure falls ≈ 1 mb per 10 m (lower atmosphere); sea level ≈ 1,013 mb

Barometer measures pressure.

Pressure belts

0° L (doldrums); 30° H (horse latitudes); 60° L; 90° H

Equatorial and polar: thermal; 30° and 60°: dynamic.

Planetary winds

1. 1.Trades: 30°
2. 2.0° (NE, SE); westerlies: 30°
3. 3.60° (SW, NW); polar easterlies: 90°
4. 4.60° (NE, SE)

Named by direction of origin.

Ferrel's law

Right in the northern hemisphere, left in the southern; zero at the equator

Coriolis effect controls direction; pressure gradient controls speed.

Cyclone vs anticyclone (N hemisphere)

Cyclone: low centre, inward, anticlockwise, rain; anticyclone: high centre, outward, clockwise, fair

Reversed in the southern hemisphere.

Local winds

Loo (hot, N India); Chinook (warm, Rockies); Foehn (warm, Alps); Mistral (cold, Rhône valley)

Chinook = Foehn type.

How the school exam asks Atmospheric Pressure and Winds

The question patterns to expect, and the method that works for each.

Tricks to remember Atmospheric Pressure and Winds

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

• L-H-L-H

  From equator to pole: Low (0°), High (30°), Low (60°), High (90°).

  Use it when: Pressure belts.

• Trades To the equator, Westerlies Wander poleward

  Trades flow towards 0°; westerlies flow towards 60°; polar easterlies flow from 90° to 60°.

  Use it when: Planetary winds.

• Right in the north

  Northern hemisphere: deflect Right; Southern: deflect Left (N-R, S-L).

  Use it when: Coriolis effect.

• Hot Loo, Warm C-F, Cold Mistral

  Loo hot; Chinook and Foehn warm; Mistral cold.

  Use it when: Local winds.

Atmospheric Pressure and Winds: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 7-mark question split into 2 + 3 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Atmospheric Pressure and Winds

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

• ✗Mistake: Naming winds by where they go

  Direction confused.

  Wrong working

  North-east trades blow towards the north-east.

  Right working

  North-east trades blow from the north-east towards the equator.

  ✓Do this: Winds are named after the direction they come from.

  Usually costs: 1 mark.

• ✗Mistake: Wrong deflection in the diagram

  Ferrel's law reversed.

  ✓Do this: Right in the northern hemisphere, left in the southern; check each arrow.

  Usually costs: 1 to 2 marks on the diagram.

• ✗Mistake: Calling the Chinook or Foehn cold

  They come from snowy mountains.

  ✓Do this: Both are warm and dry because descending air is compressed and heated; only the Mistral is cold.

  Usually costs: 1 mark.

• ✗Mistake: Cyclone rotation given without the hemisphere

  Only one direction learnt.

  ✓Do this: Cyclone: anticlockwise (N), clockwise (S); anticyclone: the reverse.

  Usually costs: 1 mark.

• ✗Mistake: Saying jet streams are near the ground

  Mixed with surface winds.

  ✓Do this: They are high in the upper troposphere, near the tropopause.

  Usually costs: 1 mark.

Practice questions on Atmospheric Pressure and Winds

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   The instrument used to measure atmospheric pressure is the:
   (a) thermometer
   (b) barometer
   (c) anemometer
   (d) rain gauge

   Hint

   Pressure.

   Answer

   (b) barometer.

2. Q2 · 1 mark · MCQExam level

   The Foehn is a local wind of:
   (a) the Rockies
   (b) the Alps
   (c) the Himalayas
   (d) the Andes

   Hint

   Switzerland.

   Answer

   (b) the Alps.

3. Q3 · 2 marks · Very short answerExam level

   What are jet streams? Give one importance.

   Hint

   High, fast, westerly.

   Answer

   Jet streams are narrow belts of very fast westerly winds high in the troposphere, near the tropopause. Importance: eastbound aircraft use them to save time and fuel (they also steer weather systems and influence the Indian monsoon).

4. Q4 · 2 marks · Very short answerHarder exam level

   A hill station is 1,800 m above sea level. If sea-level pressure is 1,012 mb, estimate its pressure using the approximate rule for the lower atmosphere.

   Hint

   1 mb per 10 m.

   Answer

   Fall ≈ 1,800 ÷ 10 = 180 mb; pressure ≈ 1,012 − 180 = 832 mb (approximate, as the rate decreases slightly with height).

5. Q5 · 2 marks · Very short answerHarder exam level

   Why are the sub-polar low-pressure belts called dynamically induced?

   Hint

   Not caused by heat.

   Answer

   They lie in cold latitudes, so they are not caused by heating. They form because the earth's rotation throws air outward from these latitudes, and because warm westerlies meet cold polar air and are forced to rise.

6. Q6 · 2 marks · Very short answerHarder exam level

   Give two effects of the Mistral on life in southern France.

   Hint

   Crops, houses.

   Answer

   (1) Its cold, strong blasts damage crops, so farmers plant rows of trees and hedges as windbreaks. (2) Houses are built facing south with few north-facing windows to shelter from it.

7. Q7 · 3 marks · Short answerHOTS / competency

   Why do the trade winds bring rain to the eastern coasts of continents but leave the western sides dry?

   Hint

   Onshore and offshore.

   Answer

   Trade winds blow from the east. Crossing the oceans, they pick up moisture and blow onshore on eastern coasts, rising over the land and giving rain. By the time they reach the western sides of continents they have lost their moisture and blow offshore, so these areas are dry; the major hot deserts lie there.

8. Q8 · 3 marks · Short answerHarder exam level

   State three features of a tropical cyclone.

   Hint

   Eye, winds, rain.

   Answer

   (1) Very low pressure at the centre with a calm, cloudless eye. (2) Violent winds spiralling inward (anticlockwise in the northern hemisphere). (3) Heavy rain and storm surges along coasts; it forms over warm seas and weakens over land.

9. Q9 · 2 marks · Short answerHOTS / competency

   Western disturbances bring winter rain to Punjab. Which winds and which upper-air feature help them reach India?

   Hint

   Temperate cyclones and jet.

   Answer

   Western disturbances are temperate cyclones carried eastward from the Mediterranean region by the westerlies; in winter the sub-tropical westerly jet stream, which lies south of the Himalayas, steers them across north-west India.

10. Q10 · 2 marks · Very short answerHarder exam level

    Distinguish between a land breeze and a monsoon wind on two points.

    Hint

    Time scale and area.

    Answer

    (1) A land breeze changes daily (blows at night), while monsoons change seasonally. (2) A land breeze affects only a narrow coastal strip; monsoons affect whole sub-continents.

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. 1.Average sea-level pressure?Show answer

   About 1,013 mb.

2. 2.Lines joining places of equal pressure?Show answer

   Isobars.

3. 3.Winds between 30° and 60° latitude?Show answer

   Westerlies.

4. 4.Warm wind of the eastern Rockies?Show answer

   Chinook ('snow eater').

5. 5.Cold wind of the Rhône valley?Show answer

   Mistral.

6. 6.Rotation of a cyclone in the northern hemisphere?Show answer

   Anticlockwise.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Pressure: weight of air; ≈ 1,013 mb at sea level; falls ≈ 1 mb per 10 m; barometer.
• ☐Belts: equatorial low (doldrums), sub-tropical highs (horse latitudes), sub-polar lows, polar highs.
• ☐Pressure gradient sets speed; Coriolis sets direction (right N, left S).
• ☐Trades (NE, SE), westerlies (SW, NW; Roaring Forties), polar easterlies.
• ☐Breezes daily, monsoons seasonal; Loo hot, Chinook and Foehn warm, Mistral cold.
• ☐Cyclone low, inward, rain; anticyclone high, outward, fair; jet streams fast westerlies near the tropopause.

Atmospheric Pressure and Winds: frequently asked questions