Atmosphere: Composition, Structure and Elements of Weather

1. The atmosphere and its gases

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• The atmosphere is a blanket of gases, water vapour and dust around the Earth. Gravity holds it in place. Air has no colour or smell, and we feel it only as wind (Class 11).
• 99 per cent of its mass lies within 32 km of the surface. It gives oxygen to animals and carbon dioxide to plants. It blocks ultraviolet (UV) rays and keeps the Earth warm by trapping part of the Sun's energy (Class 9).
• Composition of the atmosphere (dry air, by volume): nitrogen about 78.08%, oxygen 20.95%, argon 0.93%, carbon dioxide about 0.04%. Neon, helium, krypton, xenon, hydrogen and ozone are present in traces.
• The mix changes with height. Oxygen is almost nil by 120 km. CO2 and water vapour are found only up to 90 km.
• Carbon dioxide lets incoming sunlight (short-wave) pass through but blocks outgoing heat from the Earth (long-wave). This drives the greenhouse effect. Burning fossil fuels has raised it from about 280 ppm before industry to over 420 ppm (Mauna Loa annual mean, 2024; verify current).
• Water vapour is a variable gas. It falls with height and from the equator to the poles. It is up to 4% of air in the warm, wet tropics and under 1% in deserts and polar areas. It soaks up part of the sunlight and holds in the Earth's heat like a blanket. It also controls whether air is stable or unstable.
• NCERT conflict: Class 9 gives water vapour as "0.1 to 0.4 per cent". That is a typical average, not the full range. Class 11's 4% is the tropical maximum.

2. Aerosols, black carbon and particulate matter

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• Aerosols ("dust particles" in Class 11) are tiny bits of sea salt, fine soil, smoke-soot, ash, pollen, dust and broken meteors. Most stay in the lower layers, but rising air currents (convection) can lift them high.
• Where they gather: most in subtropical and temperate regions, where winds are dry. There are fewer in equatorial and polar regions.
• Why they matter: dust and salt are hygroscopic nuclei (specks that attract water). Water vapour condenses on them, so without them clouds would hardly form. Aerosols also scatter and absorb sunlight.
• Black carbon (beyond NCERT) is soot from incomplete burning of diesel, coal, wood and crop residue.
• It absorbs sunlight and warms the air.
• When it settles on Himalayan snow and glaciers, it darkens them and speeds up melting.

• It is a short-lived climate pollutant that leaves the air in days to weeks, so cutting it brings quick gains. Main Indian sources are household biomass stoves, brick kilns, diesel vehicles and stubble burning.

• Particulate matter is classed by size: PM10 (10 micrometres or smaller) and PM2.5 (2.5 micrometres or smaller, small enough to reach deep into the lungs and blood).

PM2.5 / PM10 limits (µg/m³)	Annual	24-hour
India NAAQS (2009)	40 / 60	60 / 100
WHO guidelines (2021), PM2.5	5	15

• North India's winter smog
• Stubble smoke, vehicle exhaust and factory emissions build up in the air.
• A temperature inversion (cold air trapped near the ground under warmer air) and calm winds stop them from spreading out.
• PM2.5 levels shoot up. The National Clean Air Programme (2019) targets a 40% cut in PM10 by 2025-26 (verify current).

3. Structure of the atmosphere: the troposphere and stratosphere

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• Structure of the atmosphere: there are five layers, defined by how temperature changes with height: troposphere, stratosphere, mesosphere, thermosphere, exosphere. Air density is highest at the surface and falls with height.
• Trap: temperature falls with height only in the troposphere and mesosphere (Class 9).
• Troposphere: the lowest layer.
• Its average height is 13 km (Class 11). Class 9 says about 12 km, and Class 7 says 6–18 km.
• It is about 8 km at the poles and 18 km at the equator, because strong convection carries heat higher at the equator.
• It holds nearly all the water vapour and dust, and all weather happens here. It is the most important layer for life.

• Temperature falls about 1°C per 165 m (the normal lapse rate, about 6.5°C per km).

• Tropopause: the boundary zone above the troposphere, where temperature stays nearly constant. It is about −80°C over the equator and −45°C over the poles. The equatorial tropopause is colder because it is much higher. Jet streams (fast, narrow bands of high-level wind) blow near it.

• Stratosphere: from the tropopause up to about 50 km.
• Temperature rises with height, because the ozone here absorbs UV rays.
• It contains the ozone layer.
• It is calm, dry and nearly cloud-free, which is why Class 9 calls it ideal for aeroplanes. Long-haul jets actually cruise at about 10–12 km, near the tropopause.
• Big volcanic eruptions push sulphate particles up here and cool the surface for a year or two (Mount Pinatubo, Philippines, 1991).

4. The upper atmosphere: mesosphere, thermosphere, ionosphere and exosphere

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• Mesosphere: from about 50 to 80 km. Temperature falls again, to about −100°C at 80 km, the coldest point in the atmosphere. Its top is the mesopause. Most meteors burn up here. Beyond NCERT: noctilucent clouds, the highest clouds of all, form near the mesopause in polar summer.
• Thermosphere: above the mesopause. Class 9 gives it as 80–700 km. Temperature rises sharply with height because the thin gas absorbs X-rays and short-wave UV. The International Space Station orbits here, at about 400 km.
• Ionosphere: about 80–400 km (Class 11), inside the thermosphere. It holds electrically charged particles called ions, made by solar radiation.
• It reflects radio waves back to the Earth, which makes long-distance (short-wave) radio possible.

• Solar storms disturb it, which upsets radio and GPS signals.

• Aurora: coloured light in the night sky near the poles. It forms in the thermosphere, not the stratosphere.

• The solar wind (charged particles thrown out by the Sun) reaches the Earth.
• The Earth's magnetic field steers these particles towards the magnetic poles.
• The particles excite gases, which glow: oxygen green and red, nitrogen blue and purple.

• It is called Aurora Borealis in the north and Aurora Australis in the south. In the geomagnetic storm of May 2024, an aurora was seen from Hanle, Ladakh.

• Exosphere: the topmost layer. Its very thin hydrogen and helium escape to space because gravity is weak there. It slowly merges into outer space, and little is known about it.

• Geographers study mainly the first two layers (Class 11).

5. The ozone layer, its depletion and the Montreal Protocol

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• Ozone layer: about 90% of ozone (O3) is in the stratosphere, mostly at about 15–35 km. Class 11 gives the range as 10–50 km. It absorbs harmful UV-B and UV-C rays, which protects us from skin cancer and cataracts and protects crops and plankton. Ozone near the ground, by contrast, is a pollutant made from vehicle fumes in sunlight.
• Dobson unit (DU): 1 DU is a layer of pure ozone 0.01 mm thick at standard temperature and pressure. The global average is about 300 DU.
• Ozone hole: an area where ozone falls below 220 DU. It forms over Antarctica every spring (September–October). The British Antarctic Survey (Farman, Gardiner and Shanklin) reported it in 1985.
• How the hole forms
• Chlorine and bromine come from CFCs (refrigerants, sprays, foams), halons (fire extinguishers), carbon tetrachloride and methyl bromide.
• In the polar winter, below about −78°C, polar stratospheric clouds form. Reactions on their surfaces free the chlorine.

• When sunlight returns in spring, the chlorine destroys ozone over and over. The Antarctic hole is bigger than the Arctic one because the southern polar vortex (a ring of strong winds around the pole) is colder and steadier.

• The global response

• Vienna Convention: 1985.
• Montreal Protocol: adopted 16 September 1987 (now World Ozone Day) to phase out ozone-depleting substances. In 2009 it became the first UN treaty ratified by every country. India joined in 1992 and phased out CFCs by 2010. HCFCs are being phased out (verify current schedule).

• Kigali Amendment: agreed in 2016, in force from 2019. It phases down HFCs, which do not harm ozone but are strong greenhouse gases. India ratified it in 2021. India's baseline is 2024–26, with a freeze in 2028 and four cuts from 2032, reaching 85% by 2047. The India Cooling Action Plan (2019) supports this.

• Recovery: the WMO/UNEP 2022 assessment expects ozone to return to 1980 values by about 2066 over Antarctica, 2045 over the Arctic and 2040 elsewhere.

• Stratospheric aerosol injection (beyond NCERT) is a geoengineering idea: spray reflective sulphate particles into the stratosphere to copy volcanic cooling. It is untested at scale. It could slow ozone recovery, upset monsoon rain and cause sudden "termination shock" warming if stopped.

6. Weather, climate and the elements of weather

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• Weather is the state of the atmosphere at a given place and time. It changes from hour to hour and day to day.
• Climate is the average weather over a large area and a long period, usually 30 years or more. The current WMO standard period ("normal") is 1991–2020.

Weather and climate	Weather	Climate
Time scale	Hours to days	30+ years
Change	Fast	Very slow
Example	"It is raining in Delhi today"	"Delhi has a hot semi-arid to humid subtropical climate"

• Seasons sit between the two. IMD recognises four seasons.
• Elements of weather: temperature, pressure, wind (speed and direction), humidity, clouds and precipitation. Class 11 lists all six; Classes 7 and 9 list five, leaving out clouds.
• One element often dominates (Class 7): rain in July, temperature in May and December, pressure when a cyclone moves, wind when the loo (hot, dusty summer wind of north India) blows.
• Simple links (Class 9):
• Sunlight and temperature fall from the equator to the poles.
• Warm air rises and makes low pressure (cloudy, wet). Cold air sinks and makes high pressure (clear).
• Wind blows from high to low pressure and is named after the direction it comes from.

• Class 9's wind-speed table (calm 0–1 km/h up to storm 103–117 km/h) is a simplified Beaufort scale.

• Wet-bulb temperature (beyond NCERT): the lowest temperature air can reach by evaporating water into it, so it measures heat and humidity together.

• About 35°C for a few hours was proposed as the limit for human survival, because sweating can no longer cool the body (Sherwood and Huber, 2010).
• Later studies put the real limit lower, at about 31°C.
• Humid coasts and the Indo-Gangetic plain are most at risk.

7. Measuring the weather: instruments and weather stations

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• Traditional weather prediction: people read signs in nature, such as ants moving eggs to higher ground, frogs croaking, birds flying low and pine cones closing in humid air.
• Ancient texts used the sky. Krishi-Parashara used the positions of the Sun and Moon, and Varahamihira's Brihat Samhita used the nakshatras (27 lunar mansions).
• Kalidasa's Meghadutam traces the path of monsoon clouds, and the Arthashastra records rainfall measurement.
• Meteorology is the systematic, measured study of weather.

Instrument	Measures	Unit / note
Thermometer (max–min)	Temperature	Range = max − min; mean = (max + min)/2
Rain gauge	Precipitation	mm
Barometer	Pressure	millibar (mb)
Wind vane, wind sock	Wind direction	—
Anemometer	Wind speed	km/h
Hygrometer	Humidity	relative humidity, %

• Barometer types:
• The mercury barometer (Torricelli, 1643) shows air pressure as the height of a mercury column, about 760 mm at sea level.
• The aneroid barometer uses a sealed metal box that flexes as pressure changes.

• A barograph records pressure continuously.

• Pressure values: normal sea-level pressure is about 1,013 mb, and below 1,000 mb indicates a depression. Pressure at Khardung La, Ladakh is about 650 mb. NCERT says the pass is over 5,600 m; the surveyed height is lower (verify).

• Thermometers are kept in a white, airy box called a Stevenson screen.
• Weather stations bring the instruments together and read them at fixed times.
• Automatic weather station (AWS): sensors record temperature, humidity, wind, rain and pressure and send the data without people. In September 2023, NDMA-led teams set up AWS at Sikkim's South Lhonak and Shako Cho glacial lakes as a flood early warning (Class 7: above 4,800 m; the lakes lie above 5,000 m).
• Radiosondes on weather balloons measure the upper air.

8. Forecasting the weather: the IMD and India's forecasting system

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• India Meteorological Department (IMD)
• It was set up in 1875, first at Calcutta and now at Mausam Bhawan, New Delhi, under the Ministry of Earth Sciences (MoES, 2006).
• Its motto is adityat jayate vrishti, "from the sun arises rain" (Manusmriti).
• It issued its first monsoon forecast in 1886 (Henry Blanford).
• It is the WMO Regional Specialised Meteorological Centre (RSMC), New Delhi, for North Indian Ocean cyclones.

• Sister bodies under MoES: IITM Pune, NCMRWF Noida and INCOIS Hyderabad.

• Weather forecasting

• Data come from surface stations, AWS, radiosondes, Doppler weather radars, ships, buoys and satellites (INSAT-3D, 3DR and 3DS, launched February 2024).
• The data are fed into computer models on supercomputers: Pratyush and Mihir (2018), then Arka and Arunika (2024) (verify current).
• Forecasts cover different time ranges: nowcast (hours), short range (1–3 days), medium range (3–10 days), extended range (up to 4 weeks) and seasonal.

• Forecasts are used to warn fishermen, clear coasts before cyclones, and prepare for heat waves, floods and droughts.

• IMD colour-coded weather warnings are impact-based and issued district by district:

• green: no warning
• yellow: be aware
• orange: be prepared
• red: take action

• Class 7's map of 19 May 2024 shows a heat wave in the north-west and thunderstorms in Tripura and Lakshadweep.

• National Monsoon Mission (2012): built monsoon prediction models.

• Mission Mausam
• It was approved in September 2024 with about Rs 2,000 crore for two years.
• It was launched in January 2025, on IMD's 150th foundation day.

• It aims to make India "weather-ready and climate-smart" through more radars and AWS, better models, and research on cloud seeding (spraying clouds with particles to make them rain).

• Bharat Forecast System: a weather model built by IITM Pune and launched in 2025. It forecasts at about 6 km resolution, among the finest of any global model (verify current).

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Exam angles

Prelims — high-yield facts and traps

• Composition: nitrogen 78%, oxygen 21%, argon 0.93%, CO2 about 0.04% (over 420 ppm, verify current). Oxygen is negligible by 120 km. CO2 and water vapour reach only 90 km. 99% of the mass lies within 32 km.
• CO2 lets short-wave sunlight in and blocks long-wave heat from the Earth. Water vapour is up to 4% in the tropics and falls with height and latitude.
• Layer order: troposphere → tropopause → stratosphere (ozone) → mesosphere (coldest, −100°C, meteors burn) → thermosphere/ionosphere (radio reflection, auroras, ISS) → exosphere (hydrogen and helium escape).
• Troposphere: 8 km at the poles, 18 km at the equator, 13 km on average; temperature falls 1°C per 165 m. Tropopause: −80°C at the equator, −45°C at the poles.
• "Temperature falls with height in the stratosphere": FALSE. It falls only in the troposphere and mesosphere.
• "The tropopause is warmer over the equator": FALSE. It is higher and colder there.
• "Auroras occur in the stratosphere": FALSE. They occur in the thermosphere.
• "HFCs deplete ozone": FALSE. They are greenhouse gases controlled by Kigali (2016).
• Ozone: average 300 DU, hole below 220 DU; polar stratospheric clouds form below −78°C. Vienna 1985 → Montreal 1987 (16 September, World Ozone Day) → Kigali 2016. Recovery over Antarctica by about 2066.
• Instruments: barometer–pressure (mb), anemometer–wind speed, wind vane–direction, hygrometer–humidity, rain gauge–rainfall (mm). Normal pressure is 1,013 mb; a depression is below 1,000 mb.
• IMD: 1875, MoES, "adityat jayate vrishti", RSMC New Delhi; warning colours green, yellow, orange, red. Mission Mausam (2024). Bharat Forecast System (6 km). INSAT-3DS (2024).
• PM2.5 vs PM10; NAAQS PM2.5 annual limit 40 vs WHO 5 µg/m³. Black carbon is a short-lived climate pollutant.
• Climate is averaged over 30 years; the current WMO normal is 1991–2020.

Mains — GS-I themes

1. Why is the troposphere the most important layer for life, and why is it thickest at the equator?
2. The Montreal Protocol is often called the most successful environmental treaty. What made it work, and what lessons does it hold for climate agreements, with Kigali as a bridge?
3. Black carbon and aerosols over the Himalaya and the Indo-Gangetic plain: their effects on glaciers, the monsoon and health.
4. How has India's weather-forecasting capacity grown, and how does impact-based forecasting save lives? Use cyclone and heat-wave examples.
5. Weather vs climate: why one cold winter does not disprove global warming.
6. Heat stress beyond temperature: the case for wet-bulb and heat-index warnings in Indian heat action plans.

Current-affairs hooks

• Yearly CO2 records from Mauna Loa and the WMO Greenhouse Gas Bulletin (verify current).
• Ozone-hole size each September–October, WMO/UNEP assessments, and India's HCFC phase-out and Kigali milestones.
• The Mission Mausam rollout: new Doppler radars and AWS, the Bharat Forecast System and cloud-seeding trials (verify current).
• INSAT-3DS and future weather satellites; geomagnetic storms and auroras seen far from the poles (Ladakh, May 2024).
• Delhi-NCR winter smog, NCAP progress and stubble-burning data.
• Heat waves, wet-bulb extremes, IMD heat-index products and heat action plans.