Topic 76: Climate – Insolation, Atmospheric

Q: Explain the three-cell model of atmospheric circulation.

The **three-cell model** comprises: (1) **Hadley Cell (0°–30°):** Hot equatorial air rises, moves poleward, descends at 30°, creating subtropical highs and trade winds. (2) **Ferrel Cell (30°–60°):** Mechanically driven; surface westerlies blow poleward. (3) **Polar Cell (60°–90°):** Cold polar air descends, polar easterlies blow equatorward. These cells drive trade winds, westerlies, and polar easterlies respectively.

Study Material

Paper Paper II

Unit 3

1. Insolation - Solar radiation received by Earth's surface; equatorial regions receive ~2× more than polar - Varies with angle of incidence, day length, atmospheric path length, and albedo - Arctic summer sees 24 hours daylight; oblique rays travel further through atmosphere - Annual insolation at equator: ~1,370 W/m² (solar constant)
2. Albedo - Reflectivity of a surface — fraction of incoming solar radiation reflected back - Fresh snow: 80–90%; ice: 70–80%; desert sand: 35–40%; ocean: 6%; tropical forest: 12–15% - Global average: ~30%; high albedo surfaces cool the climate - Low albedo surfaces (dark ocean, forest) absorb more heat
3. Atmospheric Pressure Belts - Equatorial Low (0°): Intense heating, rising air, low pressure, heavy rain - Subtropical High (30° N/S): Subsiding air, dry, clear skies, deserts - Subpolar Low (60° N/S): Meeting of cold polar and warm subtropical air - Polar High (90°): Extremely cold, dense, descending air
4. Three-Cell Atmospheric Circulation - Hadley Cell (0°–30°): Hot air rises at equator, moves poleward, cools and descends at 30° creating subtropical highs - Ferrel Cell (30°–60°): Indirect cell; surface winds blow poleward - Polar Cell (60°–90°): Cold air descends at poles, flows equatorward
5. Coriolis Effect - Due to Earth's rotation, winds deflect right in Northern Hemisphere, left in Southern Hemisphere (Buys-Ballot's Law) - Trade Winds blow from NE (NH) and SE (SH); Westerlies from SW (NH) and NW (SH) - Cyclones rotate anticlockwise in NH and clockwise in SH
6. Global Wind Systems - Trade Winds (0°–30°): NE Trades in NH, SE Trades in SH; most reliable winds on Earth - Westerlies (30°–60°): Blow toward subpolar low; stronger in SH — "Roaring Forties," "Furious Fifties" - Polar Easterlies (60°–90°): Cold, dry winds from polar high
7. Jet Streams - Fast-flowing, narrow upper-air currents at 9–16 km altitude, speeds 120–400 km/h - Subtropical Jet (30° N/S, ~12 km): Influences subtropical weather year-round - Polar Front Jet (60° N/S, ~9 km): Controls mid-latitude weather systems - Tropical Easterly Jet: Drives Indian Monsoon northward
8. Humidity - Absolute humidity: Mass of water vapour per unit volume (g/m³) - Relative Humidity (RH): Ratio of actual vapour to saturation vapour × 100%; measured by hygrometer/psychrometer - Specific humidity: Mass of water vapour per mass of moist air (g/kg) - Dew point: Temperature at which cooling air reaches 100% RH (saturation)
9. Types of Precipitation - Convectional rain: Heated air rises vertically — equatorial/monsoonal regions; afternoon thunderstorms - Orographic/Relief rain: Windward side of mountains — Western Ghats, Cherrapunji 11,430 mm/year - Frontal/Cyclonic rain: Cold and warm air masses meet — temperate regions, moderate, prolonged - Convective hail: Rapid updrafts freeze water droplets
10. World Climate Classification - A (Tropical): Af, Am, Aw — all months above 18°C - B (Dry): BWh (hot desert), BSh (semi-arid) — evaporation exceeds precipitation - C (Temperate): Csa (Mediterranean), Cfb (Oceanic) — coldest month -3°C to 18°C - D (Continental): Cold winters; E (Polar): ET (Tundra), EF (Ice cap)
11. El Niño–Southern Oscillation / ENSO - El Niño: Anomalous warming of central/eastern Pacific (every 2–7 years); weakens trade winds - Effects: Drought in Australia, India, SE Asia; floods in Peru/Ecuador; warmer global temperatures - La Niña: Opposite — stronger trades, cooler Pacific; strengthens Indian Monsoon, floods in Australia - SOI: Pressure difference between Tahiti and Darwin measures Southern Oscillation
12. Greenhouse Effect and Global Warming - Natural greenhouse effect maintained by CO₂, water vapour, CH₄, N₂O; without it Earth avg = −18°C - Enhanced greenhouse effect from fossil fuel burning — CO₂ at 425 ppm (2024) vs pre-industrial 280 ppm - Global avg temperature risen 1.1°C above pre-industrial level (2024)

Key facts

Insolation — Solar radiation received by Earth's surface; equatorial regions receive ~2× more than polar
Albedo — Reflectivity of a surface — fraction of incoming solar radiation reflected back
Atmospheric Pressure Belts — Equatorial Low (0°): Intense heating, rising air, low pressure, heavy rain
Three-Cell Atmospheric Circulation - Hadley Cell (0°–30°): Hot air rises at equator, moves poleward, cools and descends at 30° creating subtropical hi…
Global Wind Systems — Trade Winds (0°–30°): NE Trades in NH, SE Trades in SH; most reliable winds on Earth

Key Points at a Glance

1
**Insolation **
- Solar radiation received by Earth's surface; equatorial regions receive ~2× more than polar
- Varies with angle of incidence, day length, atmospheric path length, and albedo
- Arctic summer sees 24 hours daylight; oblique rays travel further through atmosphere
- Annual insolation at equator: ~1,370 W/m² (solar constant)
2
**Albedo **
- Reflectivity of a surface — fraction of incoming solar radiation reflected back
- Fresh snow: 80–90%; ice: 70–80%; desert sand: 35–40%; ocean: 6%; tropical forest: 12–15%
- Global average: ~30%; high albedo surfaces cool the climate
- Low albedo surfaces (dark ocean, forest) absorb more heat
3
**Atmospheric Pressure Belts **
- Equatorial Low (0°): Intense heating, rising air, low pressure, heavy rain
- Subtropical High (30° N/S): Subsiding air, dry, clear skies, deserts
- Subpolar Low (60° N/S): Meeting of cold polar and warm subtropical air
- Polar High (90°): Extremely cold, dense, descending air
4
**Three-Cell Atmospheric Circulation **
- Hadley Cell (0°–30°): Hot air rises at equator, moves poleward, cools and descends at 30° creating subtropical highs
- Ferrel Cell (30°–60°): Indirect cell; surface winds blow poleward
- Polar Cell (60°–90°): Cold air descends at poles, flows equatorward
5
**Coriolis Effect **
- Due to Earth's rotation, winds deflect right in Northern Hemisphere, left in Southern Hemisphere (Buys-Ballot's Law)
- Trade Winds blow from NE (NH) and SE (SH); Westerlies from SW (NH) and NW (SH)
- Cyclones rotate anticlockwise in NH and clockwise in SH
6
**Global Wind Systems **
- Trade Winds (0°–30°): NE Trades in NH, SE Trades in SH; most reliable winds on Earth
- Westerlies (30°–60°): Blow toward subpolar low; stronger in SH — "Roaring Forties," "Furious Fifties"
- Polar Easterlies (60°–90°): Cold, dry winds from polar high
7
**Jet Streams **
- Fast-flowing, narrow upper-air currents at 9–16 km altitude, speeds 120–400 km/h
- Subtropical Jet (30° N/S, ~12 km): Influences subtropical weather year-round
- Polar Front Jet (60° N/S, ~9 km): Controls mid-latitude weather systems
- Tropical Easterly Jet: Drives Indian Monsoon northward
8
**Humidity **
- Absolute humidity: Mass of water vapour per unit volume (g/m³)
- Relative Humidity (RH): Ratio of actual vapour to saturation vapour × 100%; measured by hygrometer/psychrometer
- Specific humidity: Mass of water vapour per mass of moist air (g/kg)
- Dew point: Temperature at which cooling air reaches 100% RH (saturation)
9
**Types of Precipitation **
- Convectional rain: Heated air rises vertically — equatorial/monsoonal regions; afternoon thunderstorms
- Orographic/Relief rain: Windward side of mountains — Western Ghats, Cherrapunji 11,430 mm/year
- Frontal/Cyclonic rain: Cold and warm air masses meet — temperate regions, moderate, prolonged
- Convective hail: Rapid updrafts freeze water droplets
10
**World Climate Classification **
- A (Tropical): Af, Am, Aw — all months above 18°C
- B (Dry): BWh (hot desert), BSh (semi-arid) — evaporation exceeds precipitation
- C (Temperate): Csa (Mediterranean), Cfb (Oceanic) — coldest month -3°C to 18°C
- D (Continental): Cold winters; E (Polar): ET (Tundra), EF (Ice cap)
11
**El Niño–Southern Oscillation / ENSO **
- El Niño: Anomalous warming of central/eastern Pacific (every 2–7 years); weakens trade winds
- Effects: Drought in Australia, India, SE Asia; floods in Peru/Ecuador; warmer global temperatures
- La Niña: Opposite — stronger trades, cooler Pacific; strengthens Indian Monsoon, floods in Australia
- SOI: Pressure difference between Tahiti and Darwin measures Southern Oscillation
12
**Greenhouse Effect and Global Warming **
- Natural greenhouse effect maintained by CO₂, water vapour, CH₄, N₂O; without it Earth avg = −18°C
- Enhanced greenhouse effect from fossil fuel burning — CO₂ at 425 ppm (2024) vs pre-industrial 280 ppm
- Global avg temperature risen 1.1°C above pre-industrial level (2024)

Introduction and Syllabus Scope

Overview

Topic 76 covers the physical foundations of world climate — the driving forces behind all weather and climate patterns. While it has a relatively low PYQ frequency (appeared only in 2013 and 2016, scoring 7 marks total), it provides the conceptual backbone for Topics 77 (Environmental Issues), 80 (Indian Climate/Monsoon), and 84 (Rajasthan Climate).

Why This Topic Matters for 2026

The 2026 pattern eliminates 2-mark questions, meaning even "low priority" topics now require substantive 50-word answers if asked. More importantly, the climate topic intersects with environmental issues (Topic 77), which is one of the highest-scoring topics (avg 7–10 marks/year). Understanding insolation, atmospheric circulation, and the greenhouse effect is essential for answering environmental issue questions well.

Scope

World scope — global atmospheric processes, pressure belts, wind patterns, humidity, and precipitation types. This topic does NOT include India's monsoon specifically (that is Topic 80) but provides the conceptual tools.

Sign up free to claim an intro topic

The first gated topic you open stays yours; the rest needs a Study Pack or Complete Course.

PREDICTED Predicted RAS Questions

Based on PYQ trends and 2026 syllabus analysis

1 5M Explain the three-cell model of atmospheric circulation. 5 marks · 50 words

Model Answer

The three-cell model comprises: (1) Hadley Cell (0°–30°): Hot equatorial air rises, moves poleward, descends at 30°, creating subtropical highs and trade winds. (2) Ferrel Cell (30°–60°): Mechanically driven; surface westerlies blow poleward. (3) Polar Cell (60°–90°): Cold polar air descends, polar easterlies blow equatorward. These cells drive trade winds, westerlies, and polar easterlies respectively.

~50 words • 5 marks