Composition and structure of the atmosphere: Geography UPSC

• The atmosphere is the envelope of gases, water vapour and suspended particles held around the Earth by gravity and rotating with it. It is the medium of weather and climate and the outer boundary of the biosphere.
• It is thin and bottom-heavy. Its density falls rapidly with height:
  • half of its mass lies below about 5.6 km
  • about 90% lies below about 16 km
  • about 99% lies below about 32 km
  • traces reach thousands of kilometres
• Air is colourless, odourless, mobile, elastic and compressible, and it has weight. The pressure it exerts averages 1013.25 hPa (millibars) at sea level.
• Its functions:
  • It filters ultraviolet radiation through the ozone layer.
  • It moderates temperature through the greenhouse effect: without it the surface would average about −18 °C instead of about 15 °C.
  • It burns up meteors, carries water in the hydrological cycle and redistributes heat from the tropics to the poles.

Composition of the Atmosphere

• The atmosphere is a mixture, not a compound. It has three components:
  • gases
  • water vapour
  • aerosols (solid and liquid particles)
• Without water vapour and aerosols, dry air has a nearly constant composition up to about 80–90 km, because turbulence keeps it well mixed.
• Gases fall into two groups:
  • Permanent gases keep fixed proportions: nitrogen, oxygen, argon and the other noble gases.
  • Variable gases change in time and space: water vapour, carbon dioxide, methane and ozone.
  • The variable gases are few in volume but decisive for climate.

Gases

• Nitrogen is the most abundant gas but the least reactive.
  • Its main role is to dilute oxygen, so that burning and oxidation stay controlled.
  • Nitrogen-fixing bacteria and lightning convert it into compounds that plants can use.
• Oxygen sustains respiration and combustion and combines with most elements to form oxides. Nearly all of it is the product of photosynthesis over the past 2.4 billion years.
• Carbon dioxide is small in volume but large in effect:
  • It is nearly transparent to incoming short-wave sunlight but absorbs and re-emits long-wave terrestrial radiation, warming the lower atmosphere.
  • Its concentration has risen from about 280 ppm before industrialisation to about 430 ppm (2025–26), growing by roughly 2–3 ppm a year.
  • Burning of coal, oil and gas and deforestation drive the rise. About half of the added CO₂ is taken up by oceans and plants; the rest stays in the air.
• Ozone is formed in the upper stratosphere when ultraviolet rays split O₂ and the free atoms join other O₂ molecules.
  • It is concentrated between 15 and 35 km, with the maximum at about 20–25 km.
  • At ground level it is a harmful secondary pollutant formed from vehicle exhaust in sunlight.

Water Vapour

• The most variable gas:
  • nearly 0% in cold deserts and polar air
  • up to about 4% by volume in the humid tropics
• It decreases with latitude and with height, because the air’s capacity to hold moisture depends on temperature:
  • about 90% lies below 6 km
  • less than 1% lies above 10 km
• Climatic roles:
  • It is the source of all clouds and precipitation.
  • It is the most important natural greenhouse gas: it absorbs long-wave earth radiation and part of the incoming infrared.
  • Its latent heat, released on condensation, is the main fuel of thunderstorms and cyclones and a major carrier of heat from the tropics to the poles.

Aerosols (Particulate Matter)

• Aerosols are the solid and liquid particles suspended in air (dust is the general term):
  • Natural: sea salt from breaking waves, desert dust, volcanic ash and sulphate, pollen, spores and meteoric dust.
  • Human-made: smoke, black carbon (soot), sulphate and nitrate from fuel burning, and crop-residue smoke.
• Most aerosols stay in the lower troposphere, but large volcanic eruptions inject sulphate into the stratosphere.
  • Mount Pinatubo (1991) cooled the global surface by about 0.5 °C for over a year.
• Roles:
  • Condensation nuclei: hygroscopic particles such as salt and sulphate are the nuclei on which water vapour condenses. Without them clouds and fog could hardly form.
  • Scattering of light:
    • Tiny molecules and fine particles scatter the short blue wavelengths more than red (Rayleigh scattering). This gives the blue sky and the red–orange sunrise and sunset, when light crosses a long path through the air.
    • Larger particles and cloud droplets scatter all wavelengths equally (Mie scattering or diffuse reflection). This makes clouds white and haze grey.
  • Climate effect: sulphates reflect sunlight and cool, while black carbon absorbs and warms, and particles alter cloud brightness and lifetime.
  • Twilight: scattering by particles prolongs dawn and dusk.
• India:
  • In winter, a dense aerosol layer forms over the Indo-Gangetic Plain, from crop-residue burning, vehicles, industry and household fuel. A shallow inversion traps it, producing smog and fog.
  • In the pre-monsoon months, dust from the Thar and Arabia raises the aerosol load over north-west India.

Structure of the Atmosphere

• The atmosphere is arranged in roughly spherical shells. Their boundaries are transition zones, not sharp surfaces, and they are classified by different properties:
  • chemical composition: homosphere and heterosphere
  • temperature: troposphere to exosphere
  • electrical state: ionosphere

Based on Composition: Homosphere and Heterosphere

• Homosphere (surface to about 85–90 km):
  • Turbulent mixing keeps the proportions of nitrogen, oxygen and argon uniform.
  • It includes the troposphere, stratosphere and mesosphere.
• Heterosphere (above about 90 km):
  • Collisions are rare, so gases separate by molecular weight (diffusive separation), with the heavier gases below.
  • It is arranged in four shells:
    • molecular nitrogen layer (about 90–200 km)
    • atomic oxygen layer (about 200–1,100 km)
    • helium layer (about 1,100–3,500 km)
    • hydrogen layer above, fading into space
• The boundary between the two is the homopause (turbopause).

Based on Temperature: The Thermal Layers

• Temperature does not fall steadily with height. It reverses three times:
  • It falls in the troposphere.
  • It rises in the stratosphere, where ozone absorbs ultraviolet.
  • It falls in the mesosphere.
  • It rises in the thermosphere, where oxygen and nitrogen absorb X-rays and extreme ultraviolet.
• Each turning point is a pause: the tropopause, stratopause and mesopause.

Troposphere

• Lowest layer:
  • about 8 km thick over the poles, 12–13 km in middle latitudes and 16–18 km over the equator
  • thicker in summer than in winter
  • thickest at the equator because strong heating and convection lift the air higher there
• Léon Teisserenc de Bort coined the name. Tropos means turning or mixing, and the layer is marked by turbulence and convection.
• It holds about 75–80% of the atmosphere’s mass and almost all water vapour and aerosols. It is therefore the “weather layer”: clouds, rain, storms, cyclones and anticyclones all form here.
• Temperature falls with height at an average environmental lapse rate of 6.5 °C per km:
  • The air is heated from below by the ground, not directly by the sun.
  • Rising air expands and cools.
• Local temperature inversions interrupt this fall, especially on clear winter nights and in valleys.
• Wind speed generally increases with height, reaching the jet streams near the top.
• Sub-layers near the ground:
  • The planetary boundary (friction) layer is up to about 1 km deep. Surface roughness controls wind speed and direction within it.
  • The surface layer is a few metres to tens of metres deep.
  • The laminar layer is only millimetres thick and clings to every surface.

Tropopause

• The tropopause is the transition zone (“where mixing stops”, a term introduced by Sir Napier Shaw). It is the upper limit of convection, clouds and storms:
  • Cumulonimbus anvils spread out against it.
  • Only the most violent thunderstorms overshoot into the stratosphere.
• It is coldest over the equator, not the poles:
  • about −80 °C at 16–18 km over the equator
  • about −45 to −55 °C at 8–9 km over the poles
  • The equatorial tropopause is so cold because it lies so high: air cools over a longer climb.
• Its height changes with season and weather. It is broken (tropopause gaps) near the subtropical and polar-front jet streams, where stratospheric and tropospheric air exchange.

Stratosphere

• The stratosphere extends from the tropopause to about 50 km:
  • The lower stratosphere is nearly isothermal, up to about 20 km.
  • Above 20 km temperature rises to about 0 °C at the stratopause.
• Ozone layer: ozone absorbs ultraviolet (UV-C and most UV-B) and converts it to heat, so the layer warms upward.
  • Ozone is concentrated at 15–35 km, densest at about 20–25 km.
  • Its thickness is measured in Dobson units: about 300 DU on average, with an “ozone hole” below 220 DU.
• Stable air (warm above cold) suppresses vertical motion:
  • The layer has few clouds and little turbulence, so long-haul jets cruise in the lower stratosphere and upper troposphere.
  • Strong horizontal winds blow in it.
  • Aerosols injected here linger for one to two years.
• Polar stratospheric (nacreous, mother-of-pearl) clouds form at about 15–25 km in the intense cold of the polar winter.
  • Chlorine chemistry on their surfaces destroys ozone each Antarctic spring, producing the ozone hole.
• Ozone depletion and recovery:
  • Chlorofluorocarbons (CFCs) and halons release chlorine and bromine, which destroy ozone. The Montreal Protocol (1987) phased them out, and the Kigali Amendment (2016) added the phase-down of HFCs.
  • The 2025 Antarctic ozone hole averaged about 18.7 million km² (7 September to 13 October), about 30% smaller than the 2006 record, and closed early on 1 December.
  • On current policies the ozone layer is projected to return to 1980 levels by about 2040 (most of the world), 2045 (Arctic) and 2066 (Antarctic).
• Stratospheric water vapour: the Hunga Tonga–Hunga Ha’apai eruption (January 2022) injected an unprecedented mass of water vapour into the stratosphere, raising its global stratospheric content by roughly 10%.

Mesosphere

• The mesosphere extends from about 50 km to 80–85 km. Temperature falls with height to about −90 °C at the mesopause, the coldest place in the atmosphere.
  • Ozone thins out above 50 km, and carbon dioxide radiates heat to space.
• Meteors burn up here by friction, leaving metal atoms that form thin layers.
• Noctilucent (polar mesospheric) clouds of ice crystals glow at about 76–85 km on summer nights in high latitudes. They are the highest clouds on Earth.
• The layer is too high for balloons and too low for satellites, so it is studied by rockets and radar. The MST (Mesosphere–Stratosphere–Troposphere) Radar at Gadanki, Andhra Pradesh is one of India’s main facilities.

Thermosphere

• The thermosphere extends from about 85 km to 500–1,000 km, depending on solar activity.
  • Temperature rises sharply because atomic oxygen and nitrogen absorb X-rays and extreme ultraviolet.
  • It can reach about 500 °C in a quiet sun and over 1,500 °C in an active sun.
• High temperature, little heat:
  • The gas is so thin that molecules carry very little energy.
  • An object there would not feel hot; it would lose heat by radiation.
• Features:
  • the ionosphere
  • aurorae at about 100–300 km
  • the Kármán line at 100 km, the conventional edge of space
  • the International Space Station (about 400 km) and low-orbit satellites, whose orbits decay faster when solar storms heat and expand this layer

Ionosphere

• The ionosphere is an electrical layer, not a separate thermal layer. It spans the upper mesosphere and thermosphere, from about 60 km to 1,000 km.
• Solar ultraviolet, X-rays and gamma rays knock electrons off atoms, creating ions and free electrons. These reflect radio waves and make long-distance radio communication possible.
• Its layers grow by day and partly vanish at night:
  • D layer (about 60–90 km):
    • exists only by day
    • absorbs medium- and high-frequency waves, which is why AM reception improves at night
  • E layer (about 90–150 km):
    • also called the Kennelly–Heaviside layer, after Arthur Kennelly and Oliver Heaviside, who predicted it in 1902
    • reflects medium-frequency waves
    • weakens at night
    • patches of sporadic E occur
  • F layer (about 150–500 km and above):
    • also called the Appleton layer, after Sir Edward Appleton (Nobel Prize, 1947)
    • splits into F1 and F2 by day and merges into one layer at night
    • F2 is the main reflector of high-frequency (shortwave) radio
• Space weather:
  • Solar flares and geomagnetic storms disturb the ionosphere and degrade GPS accuracy and HF radio. The May 2024 geomagnetic storm, the strongest in two decades, produced aurorae far into low latitudes.
  • The Thumba Equatorial Rocket Launching Station (1963) near Thiruvananthapuram sits close to the magnetic equator and was set up to study the equatorial electrojet, an intense current in the E layer.

Exosphere

• The exosphere is the outermost layer, beginning at about 500–1,000 km and fading into interplanetary space by about 10,000 km.
• It is dominated by hydrogen and helium. Particles are so far apart that they rarely collide, so the fastest ones escape the Earth’s gravity.
• Satellites in higher orbits move through it.

Thermal Layers at a Glance

Significance of Atmospheric Structure

• Weather and climate are confined to the troposphere, but the stratosphere steers them too:
  • Sudden stratospheric warmings weaken the polar vortex and can send cold outbreaks into the middle latitudes.
  • The tropopause height limits how tall storms can grow.
• Ozone layer: it shields life from ultraviolet. Its thinning raises the risk of skin cancer and cataracts and harms phytoplankton and crops.
• Ionosphere: it underpins radio communication, navigation and satellite operations.
• Heterosphere and exosphere: they mark the slow escape of light gases, which over geological time shapes the planet’s water budget.