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Atmosphere: Composition, Structure, and Temperature Atmosphere: Composition, Structure, and Temperature
Introduction
The atmosphere is the gaseous envelope surrounding the Earth, held by gravity. It plays a critical role in sustaining life, regulating climate, and protecting the Earth from harmful solar radiation. Its composition, vertical structure, and thermal characteristics form the foundation of climatology and weather studies.
Composition of the Atmosphere
Major Constituents (by volume)
Gas Formula Percentage Key Role Nitrogen N₂ 78.08% Diluent; essential for plant nutrition (via fixation) Oxygen O₂ 20.95% Respiration, combustion Argon Ar 0.93% Inert gas Carbon Dioxide CO₂ 0.04% (415 ppm) Greenhouse gas; photosynthesis Neon Ne 0.0018% Trace
Agricultural Revolutions in India: Green, White, Blue, and Golden
Biogeography: Biomes, Biodiversity, and Distribution
Coastal and Oceanic Features: Gulfs, Straits, and Island Groups
India's coastline spans 7,517 km and borders multiple critical maritime features. This note covers India's coastal geography — gulfs (Kutch, Khambhat, Mannar), straits (Palk Strait, Malacca, Hormuz, Bab-el-Mandeb), and island groups (Andaman & Nicobar, Lakshadweep) — their strategic significance for trade, energy security, and defence.
Methane CH₄ ~1.9 ppm Potent greenhouse gas
Ozone O₃ Variable UV radiation absorption
Water Vapour H₂O 0-4% (variable) Weather, greenhouse effect
Variable Components
Water vapour : 0-4% — highest in tropics, lowest in polar/desert regionsCO₂ : Highest near industrial areas; varies seasonallyOzone : Concentrated in stratospheric ozone layerAerosols : Dust, sea salt, smoke, pollutants — influence radiation and cloud formationParticulate matter : PM2.5, PM10 — health and climate impacts
Significance of Gases
Nitrogen : Essential for amino acids, proteins — biological cycle depends on nitrogen fixationOxygen : Supports life; ozone (O₃) protects from UV-B radiationCO₂ : Key greenhouse gas; regulates Earth's temperature through the greenhouse effectWater vapour : Most abundant greenhouse gas; drives weather processes
Structure of the Atmosphere
Vertical Layers Based on temperature variation with height, the atmosphere is divided into five layers:
Layer Altitude Range Temperature Trend Key Features Troposphere 0-8 km (poles) to 18 km (equator) Decreases (~6.5°C/km) Weather phenomena; turbulent; 75% of mass Stratosphere Troposphere to ~50 km Increases (ozone absorption) Ozone layer; stable; jet aircraft fly here Mesosphere 50-80 km Decreases Coldest layer (~-90°C); meteors burn here Thermosphere 80-700 km Increases (solar radiation) Aurora; International Space Station; high temp but low sensible heat Exosphere 700+ km Gradual escape to space Atoms escape Earth's gravity; outermost layer
Special Features
Tropopause
Boundary between troposphere and stratosphere
Temperature inversion at this levelHeight varies: ~16 km at equator, ~8 km at poles
Stratospheric Ozone Layer
Concentrated at 15-35 km altitude
Absorbs 97-99% of harmful UV-B radiation
Ozone depletion : CFCs caused thinning — Montreal Protocol (1987) successfully addressed itOzone hole : Over Antarctica, seasonal (spring)
Ionosphere
Part of thermosphere (60-500 km)
Contains electrically charged particles (ions)
Reflects radio waves — enables long-distance communicationAuroras occur here (aurora borealis/australis)
Temperature of the Atmosphere
Factors Affecting Temperature Distribution
Solar radiation (insolation) : Primary energy source; varies with latitudeAngle of incidence : Lower angle → more atmosphere to pass through → less heatingDuration of sunshine : Longer days → more heatingSurface characteristics : Land heats faster than water; albedo effectAltitude : Temperature decreases with height in troposphere (lapse rate)Cloud cover : Clouds reflect insolation (cooling) and trap terrestrial radiation (warming)Ocean currents : Warm currents increase coastal temperatures; cold currents decreaseVegetation : Forests moderate temperature; deserts extreme
Lapse Rate Type Description Rate Environmental Lapse Rate (ELR) Actual temperature decrease with height ~6.5°C/km (global average) Dry Adiabatic Lapse Rate (DALR) Rate for unsaturated air rising adiabatically ~9.8°C/km Wet/Moist Adiabatic Lapse Rate (WALR) Rate for saturated air rising ~4-6°C/km (variable)
Inversion of Temperature
Condition : Temperature increases with height (opposite of normal)Types :
Ground inversion : Clear night, cold air near groundSubsidence inversion : Air descends from high pressure, compresses, warmsFrontal inversion : Cold and warm air masses meet
Effects : Traps pollutants (smog); fog formation; aviation hazards
Heat Budget and Greenhouse Effect
Incoming solar radiation (100 units) :
35 units reflected back (albedo)
15 units absorbed by atmosphere
50 units reach Earth's surface
Outgoing terrestrial radiation : Earth emits longwave (infrared) radiationGreenhouse gases (CO₂, H₂O, CH₄, N₂O) trap outgoing radiation → greenhouse effect Natural greenhouse effect : Keeps Earth ~15°C warmer (essential for life)Enhanced greenhouse effect : Anthropogenic emissions → global warming
Conclusion The atmosphere is a complex, dynamic system that makes life possible on Earth. Its composition, layered structure, and thermal properties determine weather, climate, and habitability. Understanding the atmosphere — from the troposphere's weather systems to the ozone layer's protection and the greenhouse effect's regulation — is essential for climatology and environmental science.
Practice Questions
Describe the composition of the atmosphere. Why are variable gases like CO₂ and water vapour significant?
Explain the vertical structure of the atmosphere. Why is the stratosphere important for life on Earth?
What is the greenhouse effect? Distinguish between natural and enhanced greenhouse effects.