Climatology: Atmospheric Circulation, Pressure Belts & Climate Change [Complete Study Notes]

Climatology is the scientific study of Earth's climate system. It looks at long-term atmospheric patterns, the distribution of heat across the planet, the movement of moisture, and the feedback loops that shape weather conditions over many years. Unlike meteorology, which deals with short-term, day-to-day weather and forecasts, climatology studies planet-scale trends, climate classification, atmospheric circulation and human-made climate change.

For candidates preparing for WBCS, UPSC, SSC, Railway (RRB NTPC/JE), Banking and State PSC exams, climatology is a high-scoring Geography topic. West Bengal aspirants should also know the state's own climatic features – the tropical monsoon regime, Bay of Bengal cyclones, Nor'westers (Kalbaishakhi) and the role of the Himalayas. This guide explains every concept in simple language with a diagram, easy tables, memory tricks, standard textbook references and 20 practice MCQs with answers.

➤ Composition of the atmosphere (dry air): Nitrogen 78.08% · Oxygen 20.95% · Argon 0.93% · Carbon dioxide about 0.04%
➤ Layers of the atmosphere: Troposphere → Stratosphere → Mesosphere → Thermosphere → Exosphere
➤ Coriolis force: Deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere
➤ Global pressure belts: Equatorial Low (Doldrums) · Subtropical Highs (Horse Latitudes) · Subpolar Lows · Polar Highs
➤ Köppen climate groups: A (Tropical) · B (Dry) · C (Warm Temperate) · D (Cold/Continental) · E (Polar)
➤ Major greenhouse gases: Water vapour (H₂O), Carbon dioxide (CO₂), Methane (CH₄), Nitrous oxide (N₂O), Ozone (O₃)

1. Elements and Factors of Climate

The climate of any region is the result of several atmospheric elements working together, controlled by geographical factors such as latitude and altitude.

A. Core Elements of Climate

B. Controlling Factors of Climate

2. Structure of the Atmosphere

The atmosphere is divided into five layers on the basis of how temperature changes with height.

3. Atmospheric Circulation and Pressure Belts

Global atmospheric circulation carries heat from the hot equatorial regions towards the cold poles and keeps the planet's heat balance. Because of uneven heating and Earth's rotation, a pattern of alternating high- and low-pressure belts develops, and winds blow between them (Figure 1).

A. Global Pressure Belts

Figure 1: Global pressure belts, planetary winds and the three circulation cells – winds blow from high (H) to low (L) pressure and are deflected by the Coriolis force

B. Planetary Wind Systems

C. The Three-Cell Model of Circulation

The Hadley Cell operates between 0° and 30° latitude, the Ferrel Cell between 30° and 60°, and the Polar Cell between 60° and 90° in each hemisphere. Jet streams – narrow bands of very fast westerly winds in the upper troposphere, such as the Subtropical Jet and the Polar Front Jet – strongly influence surface weather. The subtropical westerly jet and the tropical easterly jet play an important role in the onset and withdrawal of the Indian monsoon.

D. El Niño, La Niña and the Indian Ocean Dipole

4. Global Climate Patterns and Köppen Classification

Climatologists classify Earth's climates on the basis of temperature, rainfall and natural vegetation. The Köppen Climate Classification, developed by the German climatologist Wladimir Köppen, uses letter codes:

Note: In Köppen's system, the letter "w" means a dry winter and "s" a dry summer. India's warm temperate regions have dry winters (Cw), unlike the Mediterranean (Cs) climate, which has dry summers and wet winters.

5. Climate Change, Impacts and Mitigation

Climate change refers to long-term shifts in global temperature and weather patterns. Since the Industrial Revolution, it has been driven mainly by human activities – burning fossil fuels, deforestation and industrial agriculture – that release greenhouse gases.

A. The Greenhouse Effect

Short-wave solar radiation passes through the atmosphere and warms Earth's surface. The surface then gives off long-wave infrared radiation, part of which is trapped by greenhouse gases (CO₂, CH₄, N₂O and water vapour). This natural greenhouse effect keeps Earth warm enough for life. Extra emissions from human activity strengthen this trapping and cause global warming.

B. Ecological and Social Impacts

C. Global Mitigation Frameworks

6. West Bengal Connection – Climate of Bengal

For more interdisciplinary study, read our guides on Major Grassland and Scrubland Ecosystems of India, Light Reflection and Refraction, Carbon and Organic Chemistry and British Expansion in India. Climate also shapes the economy – see Economic Planning: From Five-Year Plans to NITI Aayog.

20 Practice MCQs on Climatology

Q1. Which atmospheric layer contains about 75% of the atmosphere's mass and almost all weather phenomena?
(a) Stratosphere
(b) Troposphere
(c) Mesosphere
(d) Thermosphere

Answer: (b) – The troposphere is the lowest layer and contains most of the air's mass and water vapour, so clouds, rain and storms form here.

Q2. What causes winds and ocean currents to deflect to the right in the Northern Hemisphere?
(a) Coriolis force due to Earth's rotation
(b) Gravitational pull of the Moon
(c) Thermal expansion at the equator
(d) Earth's revolution around the Sun

Answer: (a) – The Coriolis force deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere; it is zero at the equator.

Q3. The calm equatorial belt of low pressure, light variable winds and heavy convectional rain is called:
(a) Horse Latitudes
(b) Roaring Forties
(c) Doldrums
(d) Polar Front

Answer: (c) – The Doldrums lie near the equator where the north-east and south-east trade winds converge, forming the Inter-Tropical Convergence Zone (ITCZ).

Q4. In the Köppen climate classification, what does the letter "B" stand for?
(a) Tropical humid climate
(b) Polar climate
(c) Cold continental climate
(d) Dry (arid and semi-arid) climate

Answer: (d) – Group B covers dry climates where evaporation exceeds precipitation, such as deserts (BW) and steppes (BS).

Q5. Which gas has the highest percentage by volume in dry air near Earth's surface?
(a) Oxygen
(b) Carbon dioxide
(c) Argon
(d) Nitrogen

Answer: (d) – Nitrogen makes up about 78.08% of dry air, followed by oxygen at about 20.95%.

Q6. What is the normal lapse rate of temperature in the troposphere?
(a) Temperature falls by about 6.5 °C per 1,000 m
(b) Temperature rises by 1 °C per 100 m
(c) Temperature stays constant
(d) Temperature falls by 1 °C per 1,000 m

Answer: (a) – On average, temperature drops about 6.5 °C for every 1 km rise in altitude within the troposphere.

Q7. Fast-flowing, narrow bands of westerly winds in the upper troposphere that steer mid-latitude weather are called:
(a) Trade winds
(b) Monsoons
(c) Jet streams
(d) Doldrums

Answer: (c) – Jet streams, such as the Subtropical Jet and the Polar Front Jet, also influence the onset and withdrawal of the Indian monsoon.

Q8. The "Horse Latitudes" coincide with which pressure belt?
(a) Equatorial Low
(b) Subtropical High
(c) Subpolar Low
(d) Polar High

Answer: (b) – The Horse Latitudes lie around 30°–35° N and S in the subtropical high-pressure belts, known for calm, dry, sinking air.

Q9. Which agreement adopted in 2015 aims to keep global warming well below 2 °C above pre-industrial levels?
(a) Kyoto Protocol
(b) Montreal Protocol
(c) Rio Declaration
(d) Paris Agreement

Answer: (d) – Adopted at COP21 under the UNFCCC, the Paris Agreement also aims to pursue a 1.5 °C limit; each country sets its own targets through Nationally Determined Contributions (NDCs).

Q10. What is the main cause of ozone layer depletion in the stratosphere?
(a) Carbon dioxide from vehicles
(b) Methane from paddy fields
(c) Chlorofluorocarbons (CFCs) and other halogen compounds
(d) Sulphur dioxide from industries

Answer: (c) – CFCs release chlorine atoms in the stratosphere, and each chlorine atom can destroy thousands of ozone molecules.

Q11. Which layer of the atmosphere contains the ozone layer?
(a) Stratosphere
(b) Troposphere
(c) Thermosphere
(d) Exosphere

Answer: (a) – The ozone layer absorbs ultraviolet radiation, which is why temperature increases with height in the stratosphere.

Q12. Winds that blow from the north-east in the Northern Hemisphere and south-east in the Southern Hemisphere towards the equator are:
(a) Westerlies
(b) Polar Easterlies
(c) Monsoon winds
(d) Trade winds

Answer: (d) – Trade winds blow steadily from the subtropical highs to the equatorial low.

Q13. Ocean acidification is mainly caused by the absorption of which gas?
(a) Methane
(b) Carbon dioxide
(c) Nitrous oxide
(d) Sulphur dioxide

Answer: (b) – Seawater absorbs extra CO₂ and forms carbonic acid, lowering pH and harming corals and shellfish.

Q14. In the three-cell model, the circulation between 0° and 30° latitude is called the:
(a) Ferrel Cell
(b) Polar Cell
(c) Hadley Cell
(d) Walker Cell

Answer: (c) – The Hadley Cell is the tropical loop; the Ferrel Cell lies between 30° and 60°, and the Polar Cell between 60° and 90°. The Walker circulation is an east–west loop over the Pacific.

Q15. Abnormal warming of surface water in the eastern equatorial Pacific Ocean is known as:
(a) El Niño
(b) La Niña
(c) Indian Ocean Dipole
(d) Madden–Julian Oscillation

Answer: (a) – El Niño is the warm phase of ENSO and usually weakens the Indian monsoon; La Niña is the cool phase.

Q16. The global ocean conveyor belt (thermohaline circulation) is driven mainly by:
(a) Tides
(b) Submarine volcanoes
(c) Coriolis force alone
(d) Differences in water temperature and salinity

Answer: (d) – Cold, salty water is denser and sinks, setting up deep ocean currents that move heat around the globe.

Q17. Which of the following is a short-lived climate pollutant with a much higher warming potential than CO₂?
(a) Nitrogen
(b) Methane
(c) Argon
(d) Oxygen

Answer: (b) – Methane stays in the atmosphere for about a decade but traps far more heat than CO₂ over a 20-year period.

Q18. The rain-shadow effect on the leeward side of mountains is associated with which type of rainfall?
(a) Convectional rainfall
(b) Cyclonic rainfall
(c) Frontal rainfall
(d) Orographic rainfall

Answer: (d) – Moist air rises over the mountain and rains on the windward side; descending dry air leaves the leeward side dry, as in the rain-shadow region east of the Western Ghats.

Q19. Which international protocol of 1987 phases out ozone-depleting substances like CFCs?
(a) Montreal Protocol
(b) Kyoto Protocol
(c) Basel Convention
(d) Ramsar Convention

Answer: (a) – The Montreal Protocol is considered the most successful environmental treaty; its Kigali Amendment (2016) also targets HFCs.

Q20. The violent pre-monsoon thunderstorms of West Bengal in April–May are locally called:
(a) Loo
(b) Mango showers
(c) Kalbaishakhi
(d) Western disturbances

Answer: (c) – Kalbaishakhi (Nor'westers) come from the north-west; the "Loo" is a hot wind of north India, and "Mango showers" occur in Kerala and Karnataka.

Summary and Exam Focus Points

To score well in climatology questions in WBCS, UPSC, SSC and Railway exams, master four areas. First, learn the layers of the atmosphere, their heights, temperature changes and composition. Second, understand the pressure belts, the Coriolis force, planetary winds and the three-cell model (Hadley, Ferrel and Polar). Third, study the Köppen classification and phenomena such as El Niño, La Niña and jet streams. Finally, revise the greenhouse effect, the impacts of climate change and key agreements – the Montreal Protocol, Kyoto Protocol and Paris Agreement – along with West Bengal's Kalbaishakhi, monsoon and cyclones.

Reference: Savindra Singh, Climatology and Physical Geography; D.R. Khullar, India: A Comprehensive Geography; NCERT Class XI, Fundamentals of Physical Geography and India: Physical Environment.