Climatology
World Climatology
World climatology covers the pressure belts, wind systems, ocean currents, and climate zones that govern global weather patterns. The Koeppen classification (A through E) and three-cell atmospheric circulation model form the exam backbone. UPSC tests ENSO impacts, jet stream roles, and ocean current-climate linkages, while SSC/RRB asks about atmospheric layers, trade winds, and the Coriolis effect.
Key Dates
Koeppen Climate Classification — divides world into 5 major groups: A, B, C, D, E
Inter-Tropical Convergence Zone — equatorial low-pressure belt where trade winds converge
Earth's rotation deflects winds — right in Northern Hemisphere, left in Southern
Periodic warming of eastern Pacific Ocean — causes droughts in India, floods in Peru
Periodic cooling of eastern Pacific — typically leads to stronger Indian monsoon
High-altitude (9-12 km) westerly winds at about 200 km/h; influence weather patterns
2015 — global commitment to limit warming to 1.5-2°C above pre-industrial levels
Atmospheric Structure & Heating
Five atmospheric layers stack by temperature behaviour. The Troposphere (0-12 km; 8 km at poles, 16 km at equator) holds 75% of atmospheric mass and generates all weather; temperature drops at 6.5°C/km (Normal Lapse Rate) up to the Tropopause. The Stratosphere (12-50 km) contains the ozone layer (15-35 km) that absorbs UV radiation, causing temperature to rise with altitude; jet aircraft cruise here in calm, stable air. The Mesosphere (50-80 km) cools sharply to -90°C at the mesopause and burns up incoming meteors. The Thermosphere (80-700 km) heats rapidly as it absorbs solar radiation and houses the ionosphere, which reflects radio waves for long-distance communication. The Exosphere (above 700 km) merges into outer space. Earth receives insolation and reflects 30% back as albedo. Greenhouse gases (CO2, methane, water vapour, N2O) trap outgoing infrared radiation, keeping surface temperature at 15°C instead of -18°C. Over time, Earth maintains radiative equilibrium, emitting as much energy as it absorbs.
Pressure Belts & Wind Systems
Four pressure belts encircle the Earth. The Equatorial Low (Doldrums, 0-5°N/S) generates rising air, persistent clouds, and heavy rainfall at the ITCZ. The Subtropical High (Horse Latitudes, 25-35°N/S) brings descending air that creates dry, calm conditions beneath the world's major deserts: Sahara, Thar, and Atacama. The Sub-polar Low (60°N/S) marks where warm subtropical and cold polar air converge, spawning frontal zones and cyclonic storms. The Polar High (90°N/S) caps each pole with cold, dense, descending air under permanent ice. Three wind systems flow between these belts: Trade Winds (NE in NH, SE in SH) blow from subtropical highs to equatorial lows as the most consistent winds on Earth; Westerlies (35-65°) carry moisture from subtropical highs toward sub-polar lows, watering temperate western coasts; Polar Easterlies flow from polar highs toward sub-polar lows. The Coriolis Effect deflects all moving air right in the Northern Hemisphere and left in the Southern, reaching zero at the equator and maximum at the poles.
Climate Zones & Koeppen Classification
Koeppen classified world climates into 5 major groups based on temperature and precipitation: (A) Tropical — no winter, temp >18°C year-round; types: Af (Tropical Wet — Amazon, Congo, SE Asia), Am (Monsoon — India, Myanmar), Aw (Tropical Savanna — East Africa, Central America); (B) Dry/Arid — evaporation exceeds precipitation; types: BWh (Hot Desert — Sahara, Thar, Atacama), BSh (Semi-Arid/Steppe — Sahel, parts of India); (C) Warm Temperate/Mesothermal — mild winters; types: Cfa (Humid Subtropical — SE USA, eastern China, Buenos Aires), Cfb (Marine West Coast — NW Europe, New Zealand), Cs (Mediterranean — California, S Europe, S Australia); (D) Cold/Microthermal — severe winters; types: Dfa/Dfb (Humid Continental — NE USA, central Europe), Dfc (Subarctic/Taiga — Siberia, Canada); (E) Polar — no true summer; types: ET (Tundra — Arctic coasts), EF (Ice Cap — Greenland, Antarctica). India covers Am, Aw, BSh, BWh, Cwg, and others — showing its climatic diversity.
Ocean Currents & Their Effects
Ocean currents are large-scale movements of water driven by winds, temperature differences, salinity, and Earth's rotation. Warm currents flow from tropics toward poles; cold currents flow from poles toward tropics. Major warm currents: Gulf Stream (North Atlantic — keeps NW Europe warm), Kuroshio/Japan Current (North Pacific), Brazilian Current (South Atlantic), East Australian Current. Major cold currents: Labrador Current (NW Atlantic), California Current (NE Pacific), Canary Current (NE Atlantic), Peru/Humboldt Current (SE Pacific), Benguela Current (SE Atlantic), West Australian Current. Effects on climate: Warm currents make coastal areas warmer and wetter (e.g., Western Europe is warmer than eastern Canada at same latitude due to Gulf Stream); Cold currents make coastal areas cooler and drier (e.g., Atacama Desert — the world's driest — is partly caused by the cold Humboldt Current). Mixing zones of warm and cold currents are rich fishing grounds — Grand Banks (Gulf Stream + Labrador), Dogger Bank (North Sea), Sea of Japan (Kuroshio + Oyashio).
ENSO, IOD & Global Climate Phenomena
El Nino-Southern Oscillation (ENSO) is the most important coupled ocean-atmosphere phenomenon affecting global weather. El Nino (warm phase): Abnormal warming of eastern Pacific (Peru coast) — weakens trade winds, warm water shifts eastward. Effects: drought in India, Australia, Indonesia; heavy rains/floods in Peru, Ecuador, California; coral bleaching in Pacific. La Nina (cool phase): Abnormal cooling of eastern Pacific — strengthens trade winds. Effects: stronger Indian monsoon (more rainfall), wetter Australia/Indonesia, cooler and drier western Americas. ENSO cycle occurs every 2-7 years. Indian Ocean Dipole (IOD): Temperature difference between western and eastern Indian Ocean. Positive IOD (warm west, cool east) brings more rainfall to India; negative IOD causes drought. IOD can modulate ENSO's effect on Indian monsoon. Other phenomena: North Atlantic Oscillation (NAO) — affects European weather; Madden-Julian Oscillation (MJO) — 30-60 day oscillation affecting tropical rainfall; Arctic Oscillation — affects winter weather in Northern Hemisphere.
Relevant Exams
World climatology is a fundamental UPSC topic. Questions on pressure belts, wind systems, ENSO, ocean currents, and Koeppen classification are common. SSC/RRB exams test atmospheric layers, wind names, and current-climate effects. Understanding climate phenomena like El Nino/La Nina is essential for current affairs linked to monsoon predictions.