NCERT Solutions for Class 11 Geography Chapter 9: Atmospheric Circulation and Weather Systems (2026–27)

These Class 11 Geography Chapter 9 solutions cover Atmospheric Circulation and Weather Systems from Fundamentals of Physical Geography, the NCERT textbook for the 2026–27 session. The chapter explains the causes of pressure differences, the forces that control wind (pressure gradient, frictional and Coriolis force), the general circulation of the atmosphere (Hadley, Ferrel and polar cells), local and seasonal winds, air masses and fronts, and the violent tropical and extra-tropical cyclones, thunderstorms and tornadoes. Below you get step-by-step answers to every NCERT exercise question, clear notes on key concepts, extra practice, MCQs, Assertion–Reason questions and FAQs.

Class: 11 Subject: Geography Book: Fundamentals of Physical Geography Chapter: 9 Title: Atmospheric Circulation and Weather Systems Session: 2026–27
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Class 11 Geography Chapter 9 – Overview

Chapter 9, Atmospheric Circulation and Weather Systems, explains how the uneven heating of the Earth produces variations in atmospheric pressure, which in turn set the air in motion as wind, blowing from high to low pressure. It describes how pressure decreases with height (about 1 mb per 10 m), how isobars reveal the horizontal pressure distribution, and the seven global pressure belts (equatorial low, subtropical highs, sub-polar lows, polar highs). Three forces—the pressure gradient force, frictional force and Coriolis force—determine wind speed and direction, producing geostrophic winds aloft and cyclonic/anti-cyclonic circulations. The chapter then covers the general circulation through the Hadley, Ferrel and polar cells, its link to oceans and El Niño–Southern Oscillation (ENSO), local winds (land and sea breezes, mountain and valley winds), air masses and fronts, and the formation, structure and destructive power of extra-tropical and tropical cyclones, thunderstorms and tornadoes.

Key Concepts & Terms

Atmospheric pressure: the weight of a column of air contained in a unit area from mean sea level to the top of the atmosphere; expressed in millibars (mb). At sea level the average pressure is about 1,013.2 mb.

Vertical variation of pressure: in the lower atmosphere pressure falls by about 1 mb for every 10 m rise in elevation; the strong vertical pressure gradient is balanced by gravity, so we do not feel strong upward winds.

Isobars: lines on a map joining places of equal pressure (reduced to sea level). Closely spaced isobars mean a steep pressure gradient and strong winds; widely spaced isobars mean weak winds.

Pressure gradient force: force produced by the difference in pressure; it acts perpendicular to the isobars, from high to low pressure, and largely controls wind speed.

Frictional force: resistance offered by the Earth’s surface that slows the wind; greatest at the surface, extending up to 1–3 km, and minimal over the sea.

Coriolis force: the deflecting force caused by the Earth’s rotation (described by Coriolis in 1844); it deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. It is zero at the equator and maximum at the poles, and acts perpendicular to the pressure gradient force.

Geostrophic wind: when isobars are straight and there is no friction (in the upper atmosphere, 2–3 km above the surface), the pressure gradient force is balanced by the Coriolis force, and the resulting wind blows parallel to the isobars.

Pressure belts: equatorial low, subtropical highs (30° N & S), sub-polar lows (60° N & S) and polar highs; they shift north and south with the apparent movement of the Sun.

General circulation cells: the Hadley cell (tropics), the Ferrel cell (mid-latitudes) and the polar cell; together they transfer heat from the equator towards the poles.

ENSO: the combined phenomenon of El Niño (warm water off the Peru coast) and the Southern Oscillation (pressure changes over the Central Pacific and Australia); a strong ENSO causes large-scale weather variations worldwide.

Air mass: a large body of air with little horizontal variation in temperature and moisture, formed over homogeneous source regions; types include maritime tropical (mT), continental tropical (cT), maritime polar (mP), continental polar (cP) and continental arctic (cA).

Front: the boundary zone where two different air masses meet (frontogenesis); the four types are cold, warm, stationary and occluded fronts.

Tropical cyclone: a violent storm originating over warm tropical oceans (sea-surface temperature above 27°C), with a calm centre (the eye), a surrounding eye wall of strongest winds and torrential rain; called Cyclones in the Indian Ocean, Hurricanes in the Atlantic, Typhoons in the Western Pacific and Willy-willies in Western Australia.

NCERT Exercises — Full Solutions

All questions below are reproduced verbatim from the NCERT textbook’s end-of-chapter Exercises. Answers are original, written in exam-ready style.

1. Multiple choice questions.

(i) If the surface air pressure is 1,000 mb, the air pressure at 1 km above the surface will be: (a) 700 mb    (b) 1,100 mb    (c) 900 mb    (d) 1,300 mb

ANSWER (c) 900 mb. In the lower atmosphere pressure decreases by about 1 mb for every 10 m of rise in elevation. Over 1 km (1,000 m) the decrease is about 1,000 ÷ 10 = 100 mb, so 1,000 mb − 100 mb = 900 mb.

(ii) The Inter Tropical Convergence Zone normally occurs: (a) near the Equator    (b) near the Tropic of Cancer    (c) near the Tropic of Capricorn    (d) near the Arctic Circle

ANSWER (a) near the Equator. The Inter Tropical Convergence Zone (ITCZ) is the low-pressure belt near the equator where the trade winds (easterlies) from both hemispheres converge, the air rises due to high insolation, and the Hadley cell begins.

(iii) The direction of wind around a low pressure in northern hemisphere is: (a) clockwise    (b) perpendicular to isobars    (c) anti-clock wise    (d) parallel to isobars

ANSWER (c) anti-clock wise. Around a low pressure (cyclone) in the Northern Hemisphere the wind blows anticlockwise (cyclonic circulation), because the Coriolis force deflects the inward-blowing winds to their right.

(iv) Which one of the following is the source region for the formation of air masses? (a) the Equatorial forest    (b) the Himalayas    (c) the Siberian Plain    (d) the Deccan Plateau

ANSWER (c) the Siberian Plain. Source regions are large, homogeneous surfaces such as vast plains or oceans over which air can rest long enough to acquire uniform temperature and moisture. The extensive, snow-covered Siberian Plain is such a region; the Himalayas (mountains), equatorial forest and Deccan Plateau are too varied to act as source regions.

2. Answer the following questions in about 30 words.

(i) What is the unit used in measuring pressure? Why is the pressure measured at station level reduced to the sea level in preparation of weather maps?

ANSWER Atmospheric pressure is measured in millibars (mb). Since pressure decreases rapidly with altitude, readings taken at different station heights are reduced to sea level so that the effect of altitude is removed and pressure values can be fairly compared on a weather map.

(ii) While the pressure gradient force is from north to south, i.e. from the subtropical high pressure to the equator in the northern hemisphere, why are the winds north easterlies in the tropics.

ANSWER As the winds blow from the subtropical high towards the equatorial low, the Coriolis force deflects them to their right in the Northern Hemisphere. A wind moving southward is therefore turned towards the south-west, so it appears to blow from the north-east—these are the north-east trade winds.

(iii) What are the geotrophic winds?

ANSWER Geostrophic winds are the winds of the upper atmosphere (about 2–3 km above the surface) where there is no friction and the isobars are straight. Here the pressure gradient force is exactly balanced by the Coriolis force, so the wind blows parallel to the isobars.

(iv) Explain the land and sea breezes.

ANSWER During the day the land heats faster than the sea, so air rises over the land (low pressure) and cooler, higher-pressure air blows from the sea to the land as the sea breeze. At night the land cools faster, pressure becomes higher over the land, and the wind reverses to blow from land to sea as the land breeze.

3. Answer the following questions in about 150 words.

(i) Discuss the factors affecting the speed and direction of wind.

ANSWER The velocity and direction of wind are the net result of three forces acting on air set in motion by pressure differences. 1. Pressure gradient force: air moves from high to low pressure, and this force acts perpendicular to the isobars. Where isobars are closely spaced the gradient is steep and winds are strong; where they are widely spaced winds are weak. A higher pressure gradient also produces greater deflection. 2. Frictional force: the roughness of the Earth’s surface slows the wind. It is greatest at the surface and its influence extends to about 1–3 km; over the smooth sea surface friction is minimal, so winds there are faster. 3. Coriolis force: caused by the Earth’s rotation, it deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. It is zero at the equator and maximum at the poles, increases with wind speed, and acts perpendicular to the pressure gradient force. When these forces balance over straight isobars without friction, the resulting geostrophic wind blows parallel to the isobars; near the surface friction reduces the deflection so winds cross the isobars at an angle.

(ii) Draw a simplified diagram to show the general circulation of the atmosphere over the globe. What are the possible reasons for the formation of subtropical high pressure over 30° N and S latitudes?

ANSWER Simplified diagram (described in words): The general circulation can be shown as three cells in each hemisphere. Near the equator (0°) air rises at the ITCZ (equatorial low) and moves poleward aloft, sinking at about 30° (subtropical high) to form the Hadley cell; the returning surface winds are the trade winds (easterlies). Between 30° and 60° sinking air at the subtropical high and rising air at the sub-polar low (60°) form the Ferrel cell, with westerlies at the surface. Beyond 60°, cold air sinks at the poles (polar high) and rises at the sub-polar low to form the polar cell, with polar easterlies at the surface. Reasons for the subtropical high at 30° N and S: (a) Air rising at the equator moves towards the poles in the upper troposphere (up to about 14 km) and accumulates over 30° N and S; part of this accumulated air sinks to the surface, raising the pressure. (b) On reaching 30° the air also cools, becoming denser and sinking. This descending (subsiding) air produces the belt of subtropical high pressure.

(iii) Why does tropical cyclone originate over the seas? In which part of the tropical cyclone do torrential rains and high velocity winds blow and why?

ANSWER Why tropical cyclones originate over the seas: they form and intensify over warm tropical oceans where the sea-surface temperature is above 27°C. The energy that powers the storm comes from the latent heat released when water vapour condenses in the towering cumulonimbus clouds around the centre, and the warm sea supplies a continuous flow of moisture. Other favourable conditions are the presence of the Coriolis force, small variations in vertical wind speed, a pre-existing weak low-pressure area, and upper-level divergence. On reaching land the moisture supply is cut off, so the storm weakens and dissipates—hence cyclones can only originate over the seas. Where torrential rain and the strongest winds blow: they occur in the eye wall, the ring of cloud immediately surrounding the calm central eye. Here there is a strong spiralling ascent of air to great heights (the tropopause), so the wind reaches its maximum velocity (up to about 250 km/h) and the heavy uplift of moist air causes torrential rainfall. The eye itself is a region of calm with subsiding air, so it has light winds and little rain.

Project Work

(i) Collect weather information over media such as newspaper, TV and radio for understanding the weather systems.

PROJECT This is a hands-on activity to be done by the student. Maintain a weather diary for a week: each day note the temperature (maximum and minimum), the forecast (sunny, cloudy, rain), humidity and any cyclone/depression warnings from the newspaper, TV news or radio (or the IMD app/website). Tabulate the data and observe how pressure, temperature and wind change from day to day, and how the systems described in this chapter (low/high pressure, fronts, cyclones) appear in real forecasts.

(ii) Read the section on weather in any newspaper, preferably, one having a map showing a satellite picture. Mark the area of cloudiness. Attempt to infer the atmospheric circulation from the distribution of clouds. Compare the forecast given in the newspaper with the TV coverage, if you have access to TV. Estimate, how many days in a week was the forecast were accurate.

PROJECT Carry out this comparison yourself. On the newspaper’s satellite map, shade the cloud-covered areas; bands of cloud usually mark convergence/low-pressure zones and fronts, while clear skies mark high-pressure (sinking air) regions—use this to infer the circulation. Each day, write down the newspaper forecast and the TV forecast side by side, then check the next day’s actual weather, and count how many of the seven days’ forecasts proved accurate.

Extra Practice Questions

Short Answer Type Questions

Q1. Define atmospheric pressure and state its unit.

ANSWERAtmospheric pressure is the weight of a column of air of unit cross-section extending from mean sea level to the top of the atmosphere. It is measured in millibars (mb); the average sea-level pressure is about 1,013.2 mb, recorded with a mercury or aneroid barometer.

Q2. What are isobars? What does their spacing indicate?

ANSWERIsobars are lines on a map joining places of equal atmospheric pressure (reduced to sea level). Closely spaced isobars show a steep pressure gradient and strong winds, while widely spaced isobars show a weak gradient and gentle winds.

Q3. Why is the Coriolis force absent at the equator?

ANSWERThe Coriolis force is directly proportional to the latitude, so it is maximum at the poles and zero at the equator. Because it is absent at the equator, winds there blow perpendicular to the isobars and low pressure gets filled instead of intensifying—this is why tropical cyclones do not form right at the equator.

Q4. Differentiate between a cyclone and an anticyclone with respect to wind direction.

ANSWERA cyclone has low pressure at the centre; its winds blow anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. An anticyclone has high pressure at the centre; its winds blow clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere.

Q5. What is a front? Name its four types.

ANSWERA front is the boundary zone formed where two different air masses meet; its formation is called frontogenesis. The four types of fronts are the cold front, warm front, stationary front and occluded front.

Long Answer Type Questions

Q1. Describe the general circulation of the atmosphere and the three cells that constitute it.

ANSWERThe general circulation of the atmosphere is the pattern of planetary winds, which depends on the latitudinal variation of heating, the pressure belts, their seasonal migration, the distribution of continents and oceans, and the Earth’s rotation. It is organised into three cells in each hemisphere. In the Hadley cell, air at the ITCZ rises due to high insolation, moves poleward aloft to about 30°, sinks to form the subtropical high, and returns to the equator as the easterlies (trade winds). In the Ferrel cell (middle latitudes), warm air from the subtropical high rises and cold air sinks, with westerlies at the surface. In the polar cell, cold dense air subsides at the poles and blows towards the middle latitudes as the polar easterlies, rising again at the sub-polar low. These three cells together transfer heat energy from lower to higher latitudes, maintaining the planet’s heat balance, and they also drive the large, slow-moving ocean currents.

Q2. Explain the formation and life cycle of an extra-tropical cyclone, and state how it differs from a tropical cyclone.

ANSWERExtra-tropical (middle-latitude) cyclones form along the polar front, where warm tropical air meets cold polar air. Initially the front is stationary; when pressure drops along it, warm air pushes north and cold air pushes south, setting up an anticlockwise cyclonic circulation. A warm front and a cold front develop, with a wedge of warm air (warm sector) between them. The warm air glides up over the cold air ahead of the warm front, producing layered clouds and steady precipitation; the faster-moving cold front pushes the warm air up sharply, forming cumulus clouds. Eventually the cold front overtakes the warm front, the warm air is completely lifted (occluded front), and the cyclone dissipates. Differences: extra-tropical cyclones have a clear frontal system, can form over land or sea, cover a larger area, and move west to east; tropical cyclones lack fronts, form only over warm seas, are smaller but far more violent, and move east to west.

Q3. Describe the structure of a mature tropical cyclone and the conditions favourable for its formation.

ANSWERA mature tropical cyclone has a strong, spirally circulating wind system around its centre. At the core lies the eye, a calm region of subsiding air, about 150–250 km across in diameter of the inner system. Surrounding it is the eye wall, where air spirals strongly upward to the tropopause; here winds reach their maximum velocity (up to about 250 km/h) and torrential rain falls. From the eye wall, rain bands of cumulus and cumulonimbus clouds radiate outward; the whole storm over the Bay of Bengal, Arabian Sea and Indian Ocean may be 600–1,200 km across and moves about 300–500 km per day, creating storm surges that flood coastal lowlands. Favourable conditions: (i) a large sea surface with temperature above 27°C; (ii) the presence of the Coriolis force; (iii) small variations in vertical wind speed; (iv) a pre-existing weak low-pressure area or low-level cyclonic circulation; and (v) upper-level divergence above the sea-level system. The storm draws its energy from latent heat released by condensation, and dies out when it makes landfall and loses its moisture supply.

MCQs & Assertion–Reason

1. Atmospheric pressure is measured in:

(a) Celsius    (b) millibars    (c) kilometres    (d) joules

2. In the lower atmosphere, pressure decreases by about 1 mb for every:

(a) 1 m rise    (b) 10 m rise    (c) 100 m rise    (d) 1,000 m rise

3. The Coriolis force is maximum at the:

(a) equator    (b) Tropic of Cancer    (c) poles    (d) 30° latitude

4. A wind that blows parallel to straight isobars in the upper atmosphere is called a:

(a) trade wind    (b) geostrophic wind    (c) sea breeze    (d) katabatic wind

5. The cell of general circulation found in the tropics is the:

(a) Ferrel cell    (b) polar cell    (c) Hadley cell    (d) convection cell

6. The appearance of warm water off the coast of Peru is known as:

(a) La Niña    (b) El Niño    (c) Southern Oscillation    (d) the Hadley effect

7. Which of the following is NOT a type of air mass?

(a) Maritime tropical (mT)    (b) Continental polar (cP)    (c) Continental arctic (cA)    (d) Maritime arctic (mA)

8. A tropical cyclone requires a minimum sea-surface temperature of about:

(a) 15°C    (b) 20°C    (c) 27°C    (d) 40°C

9. The calm central region of a tropical cyclone is called the:

(a) eye wall    (b) eye    (c) front    (d) trough

10. A violent, funnel-shaped storm with very low central pressure that occurs mainly in middle latitudes is a:

(a) hurricane    (b) typhoon    (c) tornado    (d) willy-willy

Answer key: 1-(b), 2-(b), 3-(c), 4-(b), 5-(c), 6-(b), 7-(d), 8-(c), 9-(b), 10-(c).

For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.

A-R 1. Assertion: Winds blow from high pressure to low pressure.

Reason: The difference in atmospheric pressure produces a pressure gradient force that sets the air in motion.

A-R 2. Assertion: Tropical cyclones do not form right at the equator.

Reason: The Coriolis force is zero at the equator, so low pressure gets filled instead of intensifying.

A-R 3. Assertion: We do not experience strong upward winds despite the large vertical pressure gradient.

Reason: The vertical pressure gradient force is generally balanced by a nearly equal but opposite gravitational force.

A-R 4. Assertion: A tropical cyclone weakens and dissipates after it makes landfall.

Reason: On reaching land the supply of moisture from the warm sea is cut off, removing the source of its energy.

A-R 5. Assertion: Friction has the same effect on wind over land and over the sea.

Reason: Frictional force is greatest at the surface and extends up to an elevation of 1–3 km.

Answer key: 1-(A), 2-(A), 3-(A), 4-(A), 5-(D).

Exam Tips & Common Mistakes

How to score full marks in this chapter

Memorise the three forces controlling wind—pressure gradient, frictional and Coriolis—and exactly what each does. For the 1 mb per 10 m rule, practise the standard MCQ calculation (1,000 mb at the surface → 900 mb at 1 km). Learn the seven global pressure belts and the three circulation cells (Hadley, Ferrel, polar) with a neat labelled diagram, as the 5-mark question often asks for it. Remember the Coriolis rule—right in the Northern Hemisphere, left in the Southern—and use Table 9.2 to recall cyclone/anticyclone wind directions. For cyclone questions, clearly separate the eye (calm) from the eye wall (strongest winds and rain), and list the five conditions for tropical cyclone formation.

Common mistakes to avoid

  • Writing that the Coriolis force deflects winds to the left in the Northern Hemisphere—it is to the right in the North, left in the South.
  • Confusing the eye (calm, subsiding air) with the eye wall (maximum winds and torrential rain).
  • Mixing up cyclone (low pressure, converging air) and anticyclone (high pressure, diverging air).
  • Saying tropical cyclones move west to east—they move east to west; extra-tropical cyclones move west to east.
  • Forgetting that pressure must be reduced to sea level before drawing isobars on a weather map.
  • Confusing geostrophic wind (parallel to isobars, no friction) with surface winds that cross the isobars at an angle.

Frequently Asked Questions

What is Chapter 9 of Class 11 Geography about?

Chapter 9, Atmospheric Circulation and Weather Systems, explains how pressure differences set the air in motion, the forces controlling wind (pressure gradient, frictional and Coriolis), the global pressure belts and general circulation cells, local and seasonal winds, air masses and fronts, and the formation of tropical and extra-tropical cyclones, thunderstorms and tornadoes.

Why is the Coriolis force zero at the equator?

The Coriolis force is directly proportional to latitude, so it is maximum at the poles and zero at the equator. Because of this, winds near the equator blow perpendicular to the isobars and low pressure gets filled rather than intensifying, which is why tropical cyclones do not form at the equator itself.

What is the difference between a tropical cyclone and an extra-tropical cyclone?

A tropical cyclone forms only over warm seas (above 27°C), has no fronts, is smaller but extremely violent, and moves from east to west. An extra-tropical cyclone forms along the polar front in middle latitudes, has a clear warm-front and cold-front system, covers a larger area, can form over land or sea, and moves from west to east.

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