Climatology
What it is
Climatology studies the long-term state of the atmosphere: how solar energy drives temperature, pressure, winds and storms, and how climates are classified and change. Unit II runs from atmospheric structure and the heat budget through circulation, fronts and cyclones to Köppen, Thornthwaite, ENSO, weather hazards and climate change. UGC-NET tests definitions, thresholds and code decoding.
Core concepts
Composition and Structure of Atmosphere. Dry air is about 78 per cent nitrogen, 21 per cent oxygen and 0.93 per cent argon by volume, with carbon dioxide near 0.04 per cent and rising; water vapour varies up to about 4 per cent. The troposphere (about 8 km at the poles, 18 km at the equator) holds the weather and cools upward at an average 6.5 °C per km. The stratosphere (to about 50 km) warms with height as ozone absorbs ultraviolet light, so it is stable. The mesosphere (to about 80–85 km) is the coldest layer; the thermosphere warms again and contains the ionosphere, which reflects radio waves; the exosphere lies beyond.
Insolation. Insolation is incoming shortwave solar radiation. The solar constant is about 1,361 W/m² (older texts: 1.94 calories per cm² per minute). Earth is nearest the Sun at perihelion (about 3 January) and farthest at aphelion (about 4 July), so seasons come from the 23.5° axial tilt, not distance. Surface insolation peaks over the cloud-free subtropical deserts, not the equator.
Heat Budget of Earth. In the 100-unit NCERT model, 35 units are reflected as albedo (27 by clouds, 2 by snow and ice, 6 scattered), 14 are absorbed by the atmosphere and 51 by the surface. The surface returns its 51 units: 17 straight to space and 34 to the atmosphere (6 directly, 9 by convection and turbulence, 19 as latent heat). The atmosphere radiates 48 units (14 + 34), so 35 + 17 + 48 = 100 leave. Satellite budgets put albedo nearer 30 per cent. A surplus between roughly 40° N and 40° S is carried poleward by winds and currents.
Temperature. Rising unsaturated air cools at the dry adiabatic lapse rate (DALR), about 9.8 °C per km; saturated air cools more slowly at the saturated rate (SALR), about 5–6 °C per km on average, because condensation releases latent heat. Air is absolutely stable when the environmental lapse rate is below the SALR, absolutely unstable above the DALR, and conditionally unstable between. Inversions form on long, clear, calm nights (radiation type), and also by advection, subsidence under anticyclones, fronts and valley cold-air drainage, which makes frost pockets.
Pressure and Winds. Mean sea-level pressure is 1013.25 hPa. The equatorial low and polar highs are thermal; the subtropical highs (near 30°, the horse latitudes) and subpolar lows (near 60°) are dynamic. The Coriolis force deflects winds right in the Northern Hemisphere and left in the Southern (Ferrel's law); it is zero at the equator, greatest at the poles. Aloft, pressure gradient and Coriolis force balance in the geostrophic wind, parallel to isobars. Buys Ballot's law: in the Northern Hemisphere, back to the wind, low pressure is on the left. Planetary winds are the trades, the westerlies (Roaring Forties, Furious Fifties) and polar easterlies. Valley breezes are anabatic, mountain breezes katabatic.
Atmospheric Circulation: Air Masses, Fronts and Upper Air. The three-cell model has the direct Hadley cell (to about 30°), the indirect Ferrel cell (30°–60°) and the direct polar cell. Bergeron (1928) classed air masses by source: continental (c) or maritime (m) with Arctic, Polar, Tropical or Equatorial, giving cP, mT and so on. A cold front is steep, with cumulonimbus and short, heavy showers. A warm front is gentle, with cirrus, cirrostratus, altostratus then nimbostratus and steady rain. An occlusion forms when a cold front overtakes a warm one. Aloft, the westerlies meander in Rossby waves, with subtropical and polar front jets, plus the tropical easterly jet over peninsular India in summer.
Cyclones and Anticyclones (Tropical and Temperate). Cyclones are lows with inflowing winds, anticlockwise in the Northern Hemisphere; anticyclones are highs with subsiding air and clear skies. Tropical cyclones need sea surfaces above about 26–27 °C, a latitude beyond about 5° for the Coriolis force, low wind shear and a prior disturbance. Latent heat drives them, and they decay over land. The IMD calls a system a cyclonic storm at 62 km/h (34 knots) or more (as of 2026-10). Temperate cyclones form on the polar front; they are larger, frontal and eyeless, move west to east, and peak in winter.
Climatic Classification of Köppen and Thornthwaite. Köppen's empirical scheme (1900, revised to 1936) uses monthly temperature and rainfall keyed to vegetation. A: coldest month 18 °C or above. B: dry (BW desert, BS steppe). C: coldest month between 18 °C and −3 °C. D: coldest below −3 °C, warmest above 10 °C. E: warmest below 10 °C (ET tundra, EF ice cap). Second letters give the dry season (f none, m monsoon, w winter, s summer); third letters include a (warmest month above 22 °C), h (hot dry) and g (Ganges type, hottest month before the solstice). In India, Amw is the west coast south of Goa, As the Coromandel coast and BWhw extreme western Rajasthan. Thornthwaite (1931) summed a P-E index, 115 (P/(T − 10))^(10/9) over 12 months (P in inches, T in °F), into provinces A wet (128 and above), B humid (64–127), C subhumid (32–63), D semi-arid (16–31) and E arid (below 16), with a thermal-efficiency index. His 1948 scheme used potential evapotranspiration.
ENSO Events (El Niño, La Niña and Southern Oscillation). In El Niño the trades weaken, the central and eastern equatorial Pacific warms, the Walker cell shifts east, Peruvian upwelling and fisheries fail, and Indonesia and Australia dry out. La Niña reverses this. Gilbert Walker identified the Southern Oscillation, the Tahiti–Darwin pressure see-saw, in the 1920s; the SOI (Tahiti minus Darwin) is negative in El Niño, positive in La Niña. Jacob Bjerknes (1969) linked ocean and atmosphere into ENSO, recurring every 2–7 years. El Niño tends to weaken the Indian monsoon, but not always: in 1997 a positive Indian Ocean Dipole offset a strong El Niño.
Meteorological Hazards and Disasters. Cyclones kill mostly through storm surge. Thunderstorms pass through cumulus, mature and dissipating stages (Byers and Braham, 1949); India's pre-monsoon Kalbaisakhi strike Bengal and Assam. Tornadoes, mostly from supercells, are rated EF0–EF5, and hail grows in layers in strong cumulonimbus updrafts. The IMD declares a plains heat wave at a maximum of at least 40 °C and 4.5 °C or more above normal, or 45 °C or more outright (as of 2026-10); cold waves are the winter counterpart. Drought is meteorological, hydrological, agricultural or socio-economic. A cloudburst (IMD) is 100 mm or more of rain in an hour over about 20–30 km² (Leh, 2010). A glacial lake outburst flood (GLOF) is the sudden failure of a moraine- or ice-dammed lake, as at South Lhonak, Sikkim (2023).
Climate Change. Past climates are read from ice cores (about 800,000 years in Antarctica's EPICA core), sea-floor foraminifera, tree rings and pollen. Natural causes include Milankovitch cycles of eccentricity (about 100,000 years), obliquity (22.1°–24.5°, about 41,000 years) and precession (about 19,000–23,000 years), confirmed by Hays, Imbrie and Shackleton (1976); solar lows (the Maunder Minimum, 1645–1715); and volcanic aerosols (Tambora, 1815). Human impact works through greenhouse gases, carbon dioxide having risen from about 280 ppm pre-industrial (Keeling curve, Mauna Loa, since 1958), plus land-use change, aerosols, CFCs (Montreal Protocol, 1987) and urban heat islands. The IPCC's AR6 (2021) found 2011–2020 about 1.1 °C warmer than 1850–1900.
Worked example
Föhn warming. Air at 25 °C at sea level crosses a 3,000 m range and saturates at 1,000 m. Take DALR = 10 °C per km (9.8 rounded) and SALR = 6 °C per km, with condensed moisture falling as windward rain.
- Dry ascent: 25 − (1 × 10) = 15 °C.
- Saturated ascent: 15 − (2 × 6) = 3 °C at the crest.
- Dry descent: 3 + (3 × 10) = 33 °C at the leeward foot.
- Check: gain 33 − 25 = 8 °C equals (10 − 6) × 2 km = 8 °C.
The gain is windward latent heat: lee air arrives warmer and drier.
Common traps
- Mixing thermal and dynamic belts: equatorial low and polar highs are thermal; subtropical highs and subpolar lows are dynamic.
- Thinking Coriolis force peaks at the equator: it is zero there, so cyclones do not form within about 5°.
- Swapping fronts: cold fronts bring short, heavy showers; warm fronts long, steady rain.
- Reversing the SOI sign: El Niño goes with a negative SOI.
- Treating Köppen and Thornthwaite alike: Köppen uses temperature and rainfall thresholds; Thornthwaite uses precipitation effectiveness and evapotranspiration.
Speed technique
- Decode Köppen left to right: group, then dry season, then summer heat or regime.
- "Back to the wind, low on the left" gives Buys Ballot's law and Northern Hemisphere cyclone rotation.
- El Niño means Tahiti low, Darwin high, SOI negative, monsoon at risk; La Niña reverses each.
Check yourself
- In which layer does temperature rise with height because ozone absorbs ultraviolet light?
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Stratosphere — the ozone heating makes it stable. - Which two pressure belts are dynamic in origin?
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Subtropical highs and subpolar lows — they come from circulation, not surface heating. - In Köppen's As, what does "s" denote, and where in India is As found?
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Dry summer — the Coromandel coast of Tamil Nadu, which gets its main rain in winter. - What sign does the Southern Oscillation Index take in El Niño?
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Negative — pressure falls at Tahiti and rises at Darwin. - What hourly rainfall does the IMD use to define a cloudburst?
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100 mm or more in an hour — over a small area of about 20–30 km².
Try it: Climatology questions
Real questions from the NET Geography bank on exactly this skill. Pick an answer to see the full solution — the intuition, the worked steps, the faster methods and the traps.
In which layer of the atmosphere does temperature rise with height because ozone absorbs ultraviolet radiation?
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Answer: option B
The ozone layer lies within the stratosphere, which extends to about 50 km, and it absorbs incoming ultraviolet radiation, heating the surrounding air.
Temperature therefore increases with height through the stratosphere, making it a stable layer, unlike the troposphere below, which cools upward.
So the layer is the stratosphere, option B.
How does the strength of the Coriolis force vary with latitude?
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Answer: option A
The Coriolis force depends on the sine of latitude, so it vanishes at the equator and reaches its maximum at the poles.
Because the deflection is negligible near the equator, tropical cyclones do not form within about 5° of it.
So the Coriolis force is zero at the equator and greatest at the poles, option A.
Earth is nearest the Sun (perihelion) on about:
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Answer: option A
Earth's orbit is slightly elliptical, bringing it nearest the Sun at perihelion around 3 January and farthest at aphelion around 4 July.
Because the Northern Hemisphere has winter at perihelion, the seasons are caused by the 23.5° tilt of the axis, not by distance from the Sun.
So perihelion falls on about 3 January, option A.
Match the air-mass codes of Bergeron's classification in List – I with their typical character in List – II and choose the correct answer from the options. List – I (Air mass) | List – II (Typical character) --- | --- (a) cP | (i) Cool and moist (b) mT | (ii) Hot and dry (c) cT | (iii) Warm and moist (d) mP | (iv) Cold and dry
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Answer: option B
The small letter gives the source surface, c for continental (dry) and m for maritime (moist), and the capital gives the latitude, P for polar (cold) and T for tropical (warm).
Combining the two letters, cP is cold and dry (a – iv), mT warm and moist (b – iii), cT hot and dry (c – ii) and mP cool and moist (d – i).
So matching List I, from "cP", gives a – iv, b – iii, c – ii, d – i, option B.
Match the climatologists in List – I with their contributions in List – II and choose the correct answer from the options. List – I (Climatologist) | List – II (Contribution) --- | --- (a) Tor Bergeron | (i) Climatic classification keyed to vegetation, using monthly temperature and rainfall (b) Byers and Braham | (ii) Precipitation-effectiveness (P-E) index for climatic provinces (c) C.W. Thornthwaite | (iii) Cumulus, mature and dissipating stages of a thunderstorm (d) Wladimir Köppen | (iv) Classification of air masses by source region
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Answer: option D
Bergeron (1928) classified air masses by source region (a – iv), and Byers and Braham (1949) described the cumulus, mature and dissipating stages of a thunderstorm (b – iii).
Thornthwaite (1931) built his climatic provinces on the precipitation-effectiveness index (c – ii), while Köppen's empirical scheme uses monthly temperature and rainfall thresholds keyed to vegetation (d – i).
So matching List I, from "Tor Bergeron", gives a – iv, b – iii, c – ii, d – i, option D.
Answer above — every one shows its working.