Climatology

In short

Climatology is the study of the atmosphere's average and long-term behaviour — the composition and structure of the air, the energy balance that drives it, and the patterns of pressure, wind, moisture, precipitation and climate types that result. It differs from meteorology, which deals with day-to-day weather, by its emphasis on statistics over 30-year "normals" and on the mechanisms that make climates differ from place to place. For a BA Geography student it carries large marks in physical geography papers, and it is a standing section of UPSC and State PSC general studies, UGC-NET Geography, and the entrance tests of IMD, NRSC and state disaster-management bodies. Applied climatology underlies crop advisories, flood and cyclone warning, renewable-energy siting, insurance pricing and climate-risk assessment in India.

Explained

Composition and vertical structure of the atmosphere

• Dry air by volume: nitrogen ~78.08%, oxygen ~20.95%, argon ~0.93%, carbon dioxide ~0.042% (420 ppm and rising ~2.5 ppm per year), plus neon, helium, methane, ozone. Water vapour is variable, 0 to about 4% by volume.
• Importance of trace gases: CO₂, water vapour, CH₄, N₂O and O₃ absorb outgoing longwave radiation (the greenhouse effect); ozone in the stratosphere absorbs ultraviolet; aerosols and dust act as condensation nuclei and scatter light.
• Homosphere (surface to ~80 km, gases well mixed by turbulence) and heterosphere (above, gases stratified by molecular weight).
• Troposphere: 0 to about 8 km at the poles and 18 km at the equator; holds ~75% of atmospheric mass and almost all water vapour; temperature falls with height; all weather occurs here. Its upper limit is the tropopause.
• Stratosphere: to about 50 km; temperature rises with height because of ozone absorption; stable, so jet aircraft fly in its lower part. Ozone layer concentrated 15–35 km.
• Mesosphere: to about 80 km; temperature falls again to the coldest point in the atmosphere (mesopause, about −90 °C); meteors burn here.
• Thermosphere / ionosphere: temperature rises steeply; ionised layers (D, E, F) reflect radio waves; aurorae occur here. Beyond is the exosphere.
• Normal (environmental) lapse rate: about 6.5 °C per km. Dry adiabatic lapse rate (DALR): 9.8 °C/km. Saturated adiabatic lapse rate (SALR): 4–6 °C/km (less, because latent heat is released on condensation).
• Temperature inversion: temperature rising with height — radiation (clear calm winter nights), valley or air-drainage inversion (cold dense air sinking into valleys — the "frost hollow"), subsidence inversion in anticyclones, and frontal inversion. Inversions trap pollutants: the basis of Delhi's winter smog episodes.

Insolation, the heat budget and temperature distribution

• Insolation = incoming solar radiation received at a surface. The solar constant at the top of the atmosphere is about 1,361 W/m²; averaged over the rotating globe it becomes about 340 W/m².
• Controls on insolation: angle of incidence (sun's altitude), duration of daylight, transparency of the atmosphere, distance from the sun (aphelion ~4 July, perihelion ~3 January), and nature of the surface (albedo — fresh snow 80–90%, desert sand 25–40%, forest 10–15%, water 5–10% at high sun, Earth's mean ~30%).
• Depletion in the atmosphere: scattering (Rayleigh scattering of short wavelengths makes the sky blue), reflection by clouds, absorption by ozone, water vapour and dust.
• Heat budget (balance of 100 units of incoming shortwave): about 30 reflected back (clouds ~20, atmosphere/dust ~6, surface ~4); about 19 absorbed by the atmosphere; about 51 absorbed by the surface. The surface returns it as longwave radiation, conduction and latent heat of evaporation, and the atmosphere and surface together radiate about 70 units of longwave back to space, which with the 30 reflected returns all 100 — the planet is in near-balance. (Shares differ a little between textbooks and the latest satellite estimates; the structure, not the exact unit, is what is examined.)
• Latitudinal imbalance: surplus of radiation roughly between 40°N and 40°S, deficit polewards. Winds (about 80%) and ocean currents (about 20%) transport the surplus polewards — the engine of general circulation.
• Horizontal distribution: isotherms run roughly east–west, bend over land (continentality), and are far more regular in the Southern Hemisphere (the "water hemisphere"). Annual range of temperature is largest in continental interiors (Verkhoyansk, Siberia) and smallest at the equator.
• Vertical and other controls: altitude, distance from the sea, prevailing winds, ocean currents (warm Gulf Stream keeps NW Europe ice-free; cold Peru current cools coastal Peru), slope aspect, cloud cover, vegetation.
• Inversion of temperature in India's context: Kashmir valley and Himalayan valleys show strong winter inversions; fruit orchards are sited on slopes rather than valley floors to escape frost.

Pressure, winds and the general circulation

• Atmospheric pressure at sea level averages 1,013.25 hPa (mb) = 1 atmosphere = 76 cm of mercury. It falls with height roughly 1 hPa per 10 m near the surface. Measured by mercury and aneroid barometers, barographs; maps use isobars reduced to sea level.
• Four pressure belts in each hemisphere: equatorial low (doldrums, thermally induced), subtropical high (30°, dynamically induced — "horse latitudes"), subpolar low (60°, dynamic), polar high (thermal). Belts shift ~5° with the apparent movement of the sun, following the ITCZ.
• Forces on air: pressure-gradient force (drives motion), Coriolis force (deflects right in the Northern Hemisphere, left in the Southern; zero at the equator, maximum at the poles), friction (surface layer), centripetal force in curved flow.
• Geostrophic wind: balance of pressure gradient and Coriolis, blowing parallel to straight isobars in the free atmosphere. Gradient wind adds curvature. Near the surface friction turns wind across isobars towards low pressure.
• Buys Ballot's law: stand with your back to the wind in the Northern Hemisphere and low pressure is to your left.
• Planetary winds: trade winds (NE and SE, converging at the ITCZ), westerlies (strongest in the Southern Hemisphere — "roaring forties, furious fifties, shrieking sixties"), polar easterlies.
• Tri-cellular model: Hadley cell (thermal, equator to 30°), Ferrel cell (indirect, 30–60°), Polar cell. Rising limbs are wet (equatorial rainforest, mid-latitude frontal rain); descending limbs are dry (Sahara, Thar, Atacama).
• Jet streams: narrow, fast (100–250+ km/h) westerly cores in the upper troposphere — subtropical westerly jet (~25–30°), polar front jet, and the tropical easterly jet over India in summer, which helps the monsoon. The easterly jet's position and the Somali jet (cross-equatorial low-level jet) matter for Indian rainfall.
• Periodic and local winds: land and sea breezes, mountain (katabatic) and valley (anabatic) breezes; loo (hot dry NW India summer), chinook, foehn, mistral, bora, sirocco, harmattan, norwester (kalbaisakhi) of Bengal and mango showers of Kerala and Karnataka.

Moisture, condensation and precipitation

• Evaporation depends on temperature, wind speed, dryness of air and available water. Evapotranspiration adds plant transpiration; potential evapotranspiration (PET) is the demand of the atmosphere, central to Thornthwaite's climate classification and to irrigation scheduling.
• Humidity: *absolute humidity* (g of water vapour per m³ of air), *specific humidity* (g per kg of air — conserved as air rises), *relative humidity* (actual vapour pressure ÷ saturation vapour pressure, as %). RH rises as temperature falls even with no change in moisture content.
• Dew point: temperature at which saturation is reached. Measured with wet-and-dry-bulb (sling psychrometer) readings; hair hygrograph for continuous record.
• Forms of condensation: dew, white frost, fog (radiation, advection, upslope), mist, haze, and clouds. Smog = smoke + fog, a major Indo-Gangetic plain winter hazard.
• Cloud classification (WMO, by height): high — cirrus, cirrostratus, cirrocumulus (above ~6 km, ice); middle — altostratus, altocumulus (2–6 km); low — stratus, stratocumulus, nimbostratus (below 2 km); vertically developed — cumulus and cumulonimbus (thunderstorm cloud, to the tropopause).
• Atmospheric stability: air is *unstable* when the environmental lapse rate exceeds the adiabatic lapse rate (parcel stays warmer and keeps rising — convection, cumuliform cloud, showers); *stable* when ELR is less (stratiform cloud, drizzle, fog); *conditionally unstable* when ELR lies between SALR and DALR.
• Types of precipitation by mechanism: (1) convectional — intense afternoon thunderstorms, equatorial belt and Indian pre-monsoon; (2) orographic — forced ascent on windward slopes, with a rain-shadow leeward (Western Ghats windward Agumbe/Mahabaleshwar very wet, Pune and the Deccan dry); (3) cyclonic / frontal — along fronts of temperate cyclones; (4) convergence in the ITCZ.
• Forms: rain, drizzle, snow, sleet, hail (from strong cumulonimbus updraughts), glaze. Measured by Symons' rain gauge and automatic/tipping-bucket and self-recording gauges; radar and satellite estimate area rainfall. Variability of rainfall is inversely related to its amount — the reason low-rainfall Rajasthan is drought-prone.
• World rainfall regimes: equatorial (double maxima at equinoxes), monsoon (summer maximum), Mediterranean (winter maximum), temperate maritime (all year), continental interior and desert (scanty).

Air masses, fronts and storms

• Air mass: a large body of air with uniform temperature and humidity acquired over a source region. Classified by latitude (Arctic A, Polar P, Tropical T, Equatorial E) and surface (continental c, maritime m): cP, mP, cT, mT, mE. Modified as it moves (k = colder than surface, w = warmer).
• Front: sloping boundary between two air masses. Warm front (gentle slope, nimbostratus and steady rain ahead), cold front (steep, cumulonimbus, short heavy showers, temperature drop), occluded front (cold overtakes warm, lifting it off the surface), stationary front.
• Temperate (extratropical) cyclone: forms on the polar front, 'wave' theory of Bjerknes; large (up to 1,500 km), moves west to east with the westerlies, has fronts, widespread moderate precipitation, and a life cycle — incipient, mature, occlusion, dissipation. Western disturbances are of this family and bring winter rain to NW India and snow to the Himalaya; they are vital for the rabi wheat crop.
• Tropical cyclone: forms over seas warmer than about 26–27 °C, needs latent heat, low vertical wind shear, pre-existing disturbance and Coriolis force (so none within ~5° of the equator). Structure — eye (calm, subsiding), eye wall (strongest winds and rain), spiral rain bands. No fronts. Energy source is latent heat of condensation; it decays rapidly over land.
• IMD classification of cyclonic disturbances over the north Indian Ocean by 3-minute sustained wind: depression 31–49 km/h; deep depression 50–61; cyclonic storm 62–87; severe cyclonic storm 88–117; very severe cyclonic storm 118–166; extremely severe cyclonic storm 167–221; super cyclonic storm 222 km/h and above. IMD is the WMO Regional Specialised Meteorological Centre for the region and names the storms from an agreed 13-country list.
• Thunderstorm and tornado: thunderstorm has cumulus, mature and dissipating stages; a tornado is a violent narrow vortex, rated on the Enhanced Fujita scale — rare but recorded in West Bengal, Odisha and Assam.
• Anticyclone: high pressure, subsiding, clockwise outflow in the Northern Hemisphere, clear dry calm weather; blocking anticyclones cause heat waves and cold waves.

Climate classification, the Indian monsoon and climate change

• Classification approaches: *empirical* (based on observed elements — Köppen), *genetic* (based on causes — Flohn, air-mass based), *applied/rational* (water balance — Thornthwaite).
• Köppen (1918, revised): letters for groups — A tropical, B dry, C warm temperate, D cold/continental, E polar, H highland; second letter for precipitation (f, m, s, w, S steppe, W desert); third for temperature (a, b, c, d, h, k). Indian types include Amw (monsoon with short dry season, Kerala and the west coast), As (Coromandel coast, rain in winter), Aw (savanna, much of peninsular India), BShw (semi-arid steppe, Rajasthan and the leeward Deccan), BWhw (arid desert, west Rajasthan), Cwg (Gangetic plain, dry winter), Dfc (Arunachal), E/ET (Himalaya above the tree line).
• Köppen's merits and limits: quantitative, simple, maps well; but ignores winds, air masses and the cause of climate, uses vegetation-fitted thresholds, and draws sharp boundaries where change is gradual.
• Thornthwaite (1948): uses PET and a moisture index Im = 100(S − D)/PET (S = water surplus, D = deficit); yields types A perhumid to E arid, with thermal efficiency indices. Scientifically rational but data-hungry.
• Indian monsoon — classical theory: differential heating of the Asian landmass and the Indian Ocean creates a thermal low over NW India in summer; the ITCZ shifts north; the SE trades cross the equator, are deflected right and arrive as the SW monsoon. Reversal in winter gives the dry NE monsoon.
• Modern views: monsoon as a manifestation of the migrating ITCZ; the role of the Tibetan plateau as an elevated heat source; the subtropical westerly jet retreating north and the tropical easterly jet establishing; the Somali jet; and Himalayan–Tibetan snow cover (inverse relation with monsoon rainfall, Blanford's hypothesis).
• Monsoon features: onset over Kerala around 1 June (IMD's declared date, with its forecast error), progressive advance to cover India by mid-July, break monsoon (a monsoon trough shifting to the Himalayan foothills — dry plains, heavy foothill rain and Brahmaputra/Ganga floods), withdrawal from mid-September, and the NE monsoon retreating over Tamil Nadu (October–December, most of Chennai's rain).
• ENSO: El Niño — anomalous warming of the central and eastern equatorial Pacific with weakened trades and a weak Walker circulation, statistically associated with deficient Indian monsoon rainfall; La Niña the reverse; measured by the Southern Oscillation Index (Tahiti minus Darwin pressure anomaly). The Indian Ocean Dipole (IOD) — positive phase (warm western Indian Ocean) favours a good monsoon — can offset El Niño. The link is a tendency, not a certainty: several El Niño years have had normal Indian rainfall.
• IMD rainfall categories for the season: normal is ±10% of the long-period average; below normal −10 to −19%; deficient (drought) −20% or more; above normal +10 to +19%; excess +20% or more. (The LPA reference period is revised periodically.)
• Climate change: observed global mean surface warming of about 1.1–1.2 °C above 1850–1900; IPCC Sixth Assessment Report (2021–23) attributes it unequivocally to human greenhouse-gas emissions. Consequences for India — sea-level rise on the Sundarbans and Kerala coast, glacier retreat and GLOFs in the Himalaya, more heavy-rainfall days with fewer rainy days, longer heat waves, more intense Arabian Sea cyclones.
• Policy and institutions in India: IMD (founded 1875) for observation and forecasting; National Action Plan on Climate Change (NAPCC, 2008) with eight missions including the National Solar Mission and the National Mission for Sustaining the Himalayan Ecosystem; State Action Plans; India's Nationally Determined Contributions under the Paris Agreement (2015) — 2070 net-zero target, 50% non-fossil installed power capacity and a 45% cut in emissions intensity of GDP by 2030 against 2005; Carbon Credit Trading Scheme (2023) under the Energy Conservation (Amendment) Act, 2022; air quality governed by the Air (Prevention and Control of Pollution) Act, 1981, the Environment (Protection) Act, 1986, CPCB's National Ambient Air Quality Standards (2009) and the National Clean Air Programme (2019).
• Urban climate: the urban heat island — city centres 2–6 °C warmer than their rural surroundings because of low-albedo concrete and asphalt, waste heat, reduced evapotranspiration and a sky-view-factor effect; aggravated in Delhi, Ahmedabad and Chennai. Ahmedabad's Heat Action Plan (2013) was India's first.

Key points

• Climatology vs meteorology — long-term averages and causes of climate, against short-term weather forecasting.
• Climatic normals — averages over a standard 30-year period, as used by WMO and IMD.
• Troposphere — 0 to 8–18 km, almost all water vapour and weather; temperature falls with height.
• Normal lapse rate 6.5 °C/km; DALR 9.8 °C/km; SALR 4–6 °C/km — SALR is lower because latent heat is released.
• Temperature inversion — temperature rising with height; stable air, traps smoke and fog.
• Solar constant — about 1,361 W/m² at the top of the atmosphere; global average insolation about 340 W/m².
• Albedo — fraction of radiation reflected; Earth's mean about 30%, fresh snow 80–90%.
• Heat budget — radiation surplus between about 40°N and 40°S, deficit polewards; winds and currents close the gap.
• Coriolis force — deflection due to Earth's rotation; right in the Northern Hemisphere, zero at the equator.
• Geostrophic wind — pressure gradient balanced by Coriolis; wind parallel to isobars aloft.
• Tri-cellular circulation — Hadley, Ferrel and Polar cells; subsiding limbs at 30° give the world's hot deserts.
• Relative humidity — actual ÷ saturation vapour pressure; rises as air cools without adding moisture.
• Instability — ELR greater than the adiabatic rate; produces cumuliform cloud and showers.
• Rain shadow — dry leeward side of a barrier; the Deccan behind the Western Ghats.
• Air mass and front — uniform air body from a source region; sloping boundary between two of them.
• Tropical vs temperate cyclone — warm core, no fronts, small and violent, versus frontal, large and migratory.
• Köppen — empirical, letter-based classification; India chiefly Amw, Aw, As, BShw, BWhw, Cwg.
• ENSO and IOD — Pacific and Indian Ocean ocean-atmosphere oscillations that modulate the monsoon; influence, not control.

Formulas and examples

Example 1 — temperature at height using the lapse rate. Air temperature at Chennai (sea level) is 32 °C. Taking the normal lapse rate, find the temperature at Ooty, 2,200 m.

• Fall = 6.5 °C/km × 2.2 km = 14.3 °C.
• T = 32 − 14.3 = 17.7 °C. (Observed values are a little different because of local aspect and cloud, but this is the examined method.)

Example 2 — adiabatic cooling and cloud base. A parcel at 30 °C with dew point 18 °C is forced up the windward slope of the Western Ghats.

• Depression = 30 − 18 = 12 °C; cloud base h = 125 × 12 = 1,500 m.
• Using the DALR (9.8 °C/km) over 1,500 m the parcel cools 9.8 × 1.5 = 14.7 °C, so it reaches 15.3 °C at 1,500 m. Above the condensation level it cools at the SALR; taking 5 °C/km, at 2,000 m (a further 500 m) it is 15.3 − 2.5 = 12.8 °C.
• Comparison with the environment: if the ELR is 6.5 °C/km, surrounding air at 2,000 m is 30 − 13 = 17.0 °C. The parcel at 12.8 °C is colder and denser, so this parcel is stable once forced ascent stops.

Example 3 — Coriolis parameter. Compute f at Mumbai (19°N) and at Srinagar (34°N).

• At 19°: f = 2 × 7.292 × 10⁻⁵ × sin 19° = 1.4584 × 10⁻⁴ × 0.3256 = 4.75 × 10⁻⁵ s⁻¹.
• At 34°: f = 1.4584 × 10⁻⁴ × 0.5592 = 8.15 × 10⁻⁵ s⁻¹.
• Inference: for the same pressure gradient the geostrophic wind is weaker at higher latitude, and at the equator (sin 0 = 0) no geostrophic balance is possible — which is why tropical cyclones never form on the equator.

Example 4 — monsoon departure and drought category. A district's long-period average June–September rainfall is 850 mm; this year it received 646 mm.

• Departure = (646 − 850)/850 × 100 = −204/850 × 100 = −24.0%.
• Since the shortfall exceeds 20%, the season is deficient on IMD's scale — the threshold at which a meteorological drought is declared.

Example 5 — Köppen's B threshold. A station in western Rajasthan has mean annual temperature 27 °C and annual rainfall 290 mm, nearly all in summer.

• Threshold = 20 × 27 + 280 = 540 + 280 = 820 mm.
• Rainfall 290 mm is well below 820 mm, so the station belongs to group B (dry). Half the threshold is 410 mm; because 290 mm is below that it is a desert (BW) rather than a steppe (BS) — i.e. BWhw, "h" because the mean annual temperature exceeds 18 °C and "w" for a dry winter.

Where it is used

• India Meteorological Department, IITM Pune, NCMRWF and INCOIS: observation networks, numerical weather prediction, monsoon forecasting, cyclone warning and ocean-state forecasts — roles as scientific assistant, meteorologist and climate analyst via UPSC and IMD recruitment.
• Agriculture and agro-advisory: Gramin Krishi Mausam Sewa and Krishi Vigyan Kendras issue district block-level advisories; PMFBY crop-insurance payouts and weather-based index insurance use rainfall and temperature triggers.
• Disaster management: NDMA, state SDMAs and city heat action plans (Ahmedabad, Nagpur, Odisha) use climatological return periods for cyclones, floods and heat waves; IMD colour-coded warnings drive evacuation.
• Renewable energy and infrastructure: wind-resource assessment by the National Institute of Wind Energy, solar-insolation maps for solar parks, and design rainfall and wind speeds for structures taken from Indian Standards on wind loads and on drainage design.
• Urban planning and environment: urban heat island mapping, National Clean Air Programme city plans, ventilation and pollution-dispersion studies, environmental impact assessments.
• Research, teaching and consultancy: university and NET-qualified teaching posts, climate-risk and ESG reporting for banks and corporates, GIS and remote-sensing analysis of climate data using NRSC/Bhuvan and IMD gridded datasets.

Interview questions

• Q: How does climatology differ from meteorology? A: Meteorology studies the physics of day-to-day weather and forecasts it; climatology studies the long-term statistics of weather — usually 30-year normals — its spatial patterns and causes.
• Q: Why is the saturated adiabatic lapse rate lower than the dry adiabatic lapse rate? A: During saturated ascent condensation releases latent heat into the parcel, which partly offsets the cooling from expansion.
• Q: Why does the stratosphere's temperature increase with height? A: Ozone there absorbs ultraviolet radiation and heats the surrounding air, so the heat source is at the top of the layer rather than the bottom.
• Q: Why do tropical cyclones not form within about 5° of the equator? A: The Coriolis force, proportional to sin of the latitude, is too weak there to start the rotation needed for a cyclonic vortex.
• Q: What is the rain-shadow effect, with an Indian example? A: Air loses its moisture ascending a windward slope and descends dry and warming on the lee, giving low rainfall — the Deccan plateau and Pune behind the Western Ghats.
• Q: Why are the Southern Hemisphere westerlies stronger than those of the Northern? A: There is far less land to create friction and disturb the flow, so the "roaring forties" blow steadily.
• Q: What causes Delhi's winter smog? A: A strong radiation inversion with light winds caps the surface layer and traps vehicular, industrial and crop-residue emissions, and the fog droplets form on them.
• Q: Is El Niño always followed by an Indian monsoon failure? A: No — it raises the probability of deficient rainfall, but a positive Indian Ocean Dipole or other factors can offset it, and several El Niño years have had normal rainfall.
• Q: How does IMD define a meteorological drought over an area? A: When seasonal rainfall is 20% or more below the long-period average; departures within ±10% are called normal.
• Q: What are the limitations of Köppen's classification? A: It is purely empirical — it uses temperature and rainfall thresholds fitted to vegetation, ignores winds, air masses and causes, and draws sharp lines across gradual transitions.
• Q: What is a break in the monsoon and why does it matter? A: The monsoon trough shifts north to the Himalayan foothills, leaving the plains dry while the foothills get heavy rain; prolonged breaks damage the kharif crop while causing Himalayan-river floods.
• Q: What is an urban heat island and how can it be reduced? A: City cores are a few degrees warmer than their surroundings because of low-albedo surfaces, waste heat and little vegetation; cool/reflective roofs, urban greening, water bodies and better ventilation corridors reduce it.

Common mistakes

• Confusing the environmental lapse rate (an observed vertical temperature profile) with the adiabatic lapse rates (the cooling of a moving parcel) when judging stability.
• Treating relative humidity as the amount of moisture in the air — it is a ratio to capacity, so it changes with temperature alone.
• Saying subtropical highs at 30° are formed by heating; they are dynamically induced by subsidence of the Hadley cell, unlike the thermal equatorial low.
• Describing a tropical cyclone as having fronts, or giving a temperate cyclone a warm core and an eye.
• Writing the monsoon as purely a giant land–sea breeze; examiners expect the jet-stream, ITCZ and Tibetan-plateau arguments too.
• Mixing up Köppen's letters — the second letter is the rainfall or dryness character, the third is the temperature character.
• Quoting the solar constant as the average energy received at the surface; it is the value at the top of the atmosphere on a surface normal to the rays, and a quarter of it, further reduced by albedo and absorption, reaches the ground as a global average.

Related topics

Geomorphology · Oceanography · Biogeography · Climate change and environmental policy · Remote sensing and GIS in geography · Natural hazards and disaster management