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Atmospheric Circulation and Weather Systems – Class 11 Geography notes, important questions and MCQs

NCERT Class 11 Geography, chapter 9. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Differences in air pressure cause air to move as wind from high-pressure to low-pressure areas. Wind direction is affected by the pressure gradient force, the Coriolis force and friction. The general circulation of the atmosphere creates pressure belts and planetary winds such as the trade winds, westerlies and polar easterlies. Air masses, fronts, extra-tropical cyclones, tropical cyclones and thunderstorms are important weather systems.

Pressure and winds

Air pressure is measured in millibars with a barometer and decreases with height; isobars join places of equal pressure.
The pressure gradient force drives winds; the Coriolis force, caused by the Earth's rotation, deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere; friction slows winds near the surface.
Winds blowing parallel to isobars in the upper atmosphere are called geostrophic winds.

General circulation

Pressure belts include the equatorial low (ITCZ), subtropical highs at about 30°, subpolar lows at about 60° and polar highs.
Planetary winds are the trade winds (from subtropical highs to the equator), the westerlies (towards subpolar lows) and the polar easterlies.
The three cells of circulation are the Hadley, Ferrel and polar cells; El Niño, a warming of the eastern Pacific, alters this circulation and affects the Indian monsoon.

Weather systems

Air masses are large bodies of air with uniform temperature and humidity; fronts form where two air masses meet (cold, warm, stationary and occluded fronts).
Extra-tropical cyclones form along fronts in middle latitudes; tropical cyclones form over warm oceans above 27°C and are called hurricanes, typhoons or cyclones in different regions.
Thunderstorms and tornadoes result from intense convection.

Key terms

Coriolis force: the apparent force due to the Earth's rotation that deflects moving air
ITCZ: the Inter-Tropical Convergence Zone, a low-pressure belt near the equator
Air mass: a large body of air with uniform temperature and humidity
Front: the boundary between two different air masses
Tropical cyclone: an intense low-pressure storm forming over warm tropical oceans

Important questions with answers

What is the Coriolis force?2 marks

The Coriolis force is the apparent force caused by the Earth's rotation, which deflects moving air and water 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.

What are the factors affecting the speed and direction of wind?3 marks

Wind is affected by the pressure gradient force, which determines its speed from high to low pressure; the Coriolis force, which deflects its direction due to the Earth's rotation; and friction with the Earth's surface, which reduces its speed near the ground.

Describe the formation and characteristics of tropical cyclones.5 marks

Tropical cyclones are violent storms that originate over warm tropical oceans and move towards coastal areas. Conditions for their formation include large sea surfaces with temperatures above about 27°C, the presence of the Coriolis force (so they do not form at the equator), small differences in vertical wind speed, a pre-existing weak low-pressure area or disturbance, and upper-level divergence. Warm, moist air rises, condenses and releases latent heat, which further intensifies the low pressure. A mature tropical cyclone has a calm centre called the eye, surrounded by the eye wall, where the strongest winds, towering clouds and heaviest rainfall occur; spiral rain bands extend outward. Wind speeds can exceed 250 km per hour. When cyclones reach land, they cause heavy rainfall, strong winds and storm surges that flood coastal areas, but they weaken over land because they lose their moisture supply. They are called cyclones in the Indian Ocean, hurricanes in the Atlantic, typhoons in the western Pacific and willy-willies in Australia. The coasts of Odisha, Andhra Pradesh and West Bengal are frequently affected.

Quick revision

Pressure gradient, Coriolis force, friction.
Pressure belts; trade winds, westerlies, polar easterlies.
Hadley, Ferrel, polar cells; El Niño.
Air masses, fronts, cyclones, thunderstorms.

Exam tips

Draw the pressure belts and planetary winds on a globe sketch.
Remember: deflection is to the right in the Northern Hemisphere.

Practice MCQs

Air pressure is measured with a
1. thermometer
2. anemometer
3. hygrometer
4. barometer
Show answer
Answer: (d) barometer. It is measured in millibars.
Lines joining places of equal pressure are
1. isobars
2. isohyets
3. isotherms
4. isohalines
Show answer
Answer: (a) isobars. They show pressure distribution.
Winds blow from
1. low pressure to high pressure
2. poles to the Moon
3. east to west always
4. high pressure to low pressure
Show answer
Answer: (d) high pressure to low pressure. The pressure gradient drives them.
In the Northern Hemisphere the Coriolis force deflects winds to the
1. left
2. upward
3. right
4. downward
Show answer
Answer: (c) right. It deflects to the left in the Southern Hemisphere.
Winds blowing from subtropical highs towards the equator are the
1. trade winds
2. polar easterlies
3. westerlies
4. jet streams
Show answer
Answer: (a) trade winds. They converge at the ITCZ.
Tropical cyclones usually form over oceans warmer than about
1. 15°C
2. 20°C
3. 27°C
4. 35°C
Show answer
Answer: (c) 27°C. Warm water supplies energy and moisture.
The circulation cell between the equator and about 30° latitude is the
1. Ferrel cell
2. Hadley cell
3. polar cell
4. Walker cell only
Show answer
Answer: (b) Hadley cell. Air rises at the equator and sinks at 30°.
Why are tropical cyclones absent exactly at the equator
1. water is too cold
2. it is always dry
3. there is no wind
4. the Coriolis force is zero there
Show answer
Answer: (d) the Coriolis force is zero there. Rotation needs the Coriolis effect.

All chapters of Class 11 Geography

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.