Chapter 8 — Composition And Structure Of Atmosphere

Introduction
The atmosphere is the blanket of gases that surrounds Earth. It is a mixture of permanent gases (mainly nitrogen and oxygen), variable gases (water vapour, carbon dioxide, ozone) and suspended particles (aerosols). The atmosphere extends from the ground up to several hundred kilometres, with most mass concentrated in the lowest 50 km. It is usually divided vertically into layers (troposphere, stratosphere, mesosphere, thermosphere and exosphere) based on temperature variations with altitude.

Composition (by volume, approximate)

• Nitrogen (N2): ~78.08%
• Oxygen (O2): ~20.95%
• Argon (Ar): ~0.93%
• Carbon dioxide (CO2): ~0.041% (≈417 ppm, rising)
• Water vapour: variable, 0–4% depending on location and weather

Why the atmosphere is important

• Support of life: Provides oxygen for respiration and carbon dioxide for photosynthesis; maintains pressure and temperature conditions compatible with life.
• Temperature regulation: Traps outgoing longwave radiation (greenhouse effect) and moderates daily and seasonal temperature extremes.
• Protection from solar radiation: The ozone layer in the stratosphere absorbs most of the Sun’s harmful ultraviolet (UV-B and UV-C) radiation.
• Weather and climate: The troposphere contains water vapour and aerosols; its dynamics produce weather (winds, clouds, precipitation) and long-term climate patterns.
• Water cycle and agriculture: Controls evaporation, cloud formation, and precipitation—essential for crops and freshwater supply.
• Transport medium: Carries dust, pollen, pollutants, and seeds; facilitates aircraft flight and long-distance movement of airborne materials.
• Communication and navigation: Ionosphere (part of upper atmosphere) reflects or modifies radio waves enabling long-range radio communication and affects GPS signals.
• Human activities and hazards: Weather extremes (storms, heatwaves) and air pollution impact health, economy and infrastructure.

Connection to basic physical principles
To understand atmospheric behaviour, a few physical relations are essential: the ideal gas law links pressure, density and temperature; hydrostatic balance explains how pressure decreases with altitude; lapse rates describe vertical temperature change. These relations help explain why temperature, pressure and density vary with height and how weather systems develop.

Summary
The atmosphere is both a life-supporting system and an active physical medium. Its composition and layered structure determine climate, weather, protection from radiation and many aspects of human activity. Studying the atmosphere combines chemistry, physics and geography to explain everyday phenomena—from breathing and weather forecasts to climate change and aviation.

The atmosphere is a mixture of gases, water vapour, and suspended particles that surrounds Earth. Its composition is usually separated into:

• Permanent (major) gases — present in nearly constant proportions by volume: Nitrogen (~78.08%), Oxygen (~20.95%), Argon (~0.93%), and small, well-mixed amounts of other noble/trace gases.
• Variable gases — concentrations vary with time and place: Water vapour (0–4% by volume), Carbon dioxide (~420 ppm or ~0.042% and rising), Ozone (trace, but concentrated in the stratosphere), Methane, Nitrous oxide, etc.
• Aerosols and particulates — dust, sea salt, pollen, soot, volcanic ash; highly variable in space and time and important for weather, visibility and health.

Two useful ways to view composition are by volume fraction at the surface (pie-chart view) and by altitude. In the homosphere (surface to ~80 km) turbulent mixing keeps the major gases nearly uniform in composition. Above that, in the heterosphere, gases begin to stratify by molecular weight.

Why composition matters: Oxygen supports aerobic life; nitrogen is chemically inert in the air but central to the nitrogen cycle; water vapour controls humidity, clouds and weather; carbon dioxide and methane are greenhouse gases that regulate Earth’s temperature; ozone in the stratosphere blocks harmful ultraviolet radiation; aerosols affect air quality, cloud formation and radiative balance.

Human activities alter the variable components (e.g., rising CO2 and methane, aerosols from combustion, ozone-depleting CFCs), producing climate change, air pollution and changes to radiative forcing.

Definition: Permanent gases are those atmospheric constituents whose relative proportions remain nearly constant (well‑mixed) through the lower atmosphere (the homosphere). They do not condense under normal Earth surface conditions and have long residence times compared with atmospheric mixing times.

Main components (by volume % / ppm): Nitrogen (N2) ≈ 78.08%, Oxygen (O2) ≈ 20.95%, Argon (Ar) ≈ 0.93%, and trace noble gases (Ne, He, Kr, Xe) at ppm or sub‑ppm levels. (Carbon dioxide and water vapour are variable gases.)

Why they are ‘permanent’: Turbulent mixing in the homosphere (to roughly 80 km altitude) distributes these gases uniformly so their mole fractions remain effectively constant with altitude. Their chemical inertness or long residence times mean they are not removed quickly by physical processes that affect water vapour or aerosols.

Importance and roles:

• Nitrogen: an inert buffer gas for the atmosphere, essential for the nitrogen cycle (biological fixation) and used industrially (e.g., inert atmospheres, fertilizer production).
• Oxygen: essential for respiration and combustion; its constant fraction means the percentage remains ~21% though its partial pressure falls with altitude.
• Argon and noble gases: chemically inert; used in welding, lighting, cryogenics (He) and scientific instruments.

Vertical behaviour (key idea): Mole fractions of permanent gases remain nearly constant through the homosphere. However, as total atmospheric pressure decreases with altitude, the partial pressures (and hence number densities) of these gases fall exponentially — this explains effects such as altitude sickness even though oxygen remains ~21% by volume.

Connection to structure of atmosphere: The homosphere (surface → ~80 km) is the well‑mixed region of permanent gases. Above it, in the heterosphere, diffusive separation causes lighter gases (H2, He) to dominate at high altitudes and mole fractions are no longer constant.

Definition: Variable gases are components of the atmosphere whose concentrations vary significantly in time and space (unlike the nearly constant permanent gases N2 and O2). Major variable gases include water vapor (H2O), carbon dioxide (CO2), ozone (O3), methane (CH4), nitrous oxide (N2O) and trace pollutants (NOx, SO2, CO).

Main characteristics:

• Water vapor — most abundant variable gas (0–4% by volume; global mean ~1%). Strongly controls humidity, cloud formation and latent heat release. Varies rapidly with temperature, season and location (high in tropics, low in polar regions).
• Carbon dioxide (CO2) — well-mixed but increasing due to fossil fuel burning and land-use change (~420 ppm in recent years). Important greenhouse gas with long-term climate impact.
• Ozone — two roles: stratospheric ozone (protects life by absorbing UV) and tropospheric ozone (a pollutant and greenhouse gas formed by photochemical reactions).
• Methane and nitrous oxide — potent greenhouse gases with smaller concentrations (methane ~1.9 ppm; N2O ~0.33 ppm) but high radiative efficiency and important sources/sinks (wetlands, agriculture, fossil fuels).
• Anthropogenic pollutants (NOx, SO2, CO) — highly variable near sources (cities, industries); affect air quality, health, visibility and chemistry (acid rain, smog).

Sources and sinks: Variable gases originate from natural processes (evaporation, respiration, volcanic emissions, wetlands) and human activities (combustion, agriculture, industry). Sinks are chemical reactions, uptake by oceans and biosphere, photodissociation, and precipitation (for soluble species).

Spatial and temporal variability: Concentrations change with altitude (e.g., water vapor decreases rapidly with height; ozone peaks in the stratosphere), latitude (higher humidity in tropics), season (CO2 has a seasonal cycle due to vegetation), and time of day (tropospheric ozone forms during daytime).

Importance: Variable gases control weather (water vapor, clouds), climate (greenhouse gases), protection from UV (stratospheric ozone), and air quality (tropospheric ozone, NOx, SO2). They participate in atmospheric chemistry and radiative balance, so even small concentration changes can have large effects.

Measurement & units: Concentrations are reported as volume mixing ratio (ppm, ppb), partial pressure (hPa or Pa), mass mixing ratio (g/kg), or percent by volume. Monitoring is done by ground stations, balloons, aircraft and satellites (e.g., Mauna Loa CO2 record).

Key practical points for students:

• Water vapor feedback: warmer air holds more water vapor, amplifying warming (positive feedback).
• CO2 lifetime and radiative forcing: CO2 accumulates over decades to centuries making it central to long-term climate change.
• Ozone distinction: stratospheric ozone is protective, tropospheric ozone is harmful — know their sources and effects.

Definition: Aerosols are tiny solid or liquid particles suspended in the atmosphere. Particulates (particulate matter, PM) are airborne particles that vary in chemical composition, size and origin.

Size classes: Particle behaviour and impact are governed mainly by size. Common size categories are:

• Ultrafine / nanoparticles: < 0.1 μm (100 nm) — high number concentration, long atmospheric lifetime, important for health and some optical effects.
• Fine particles (PM2.5): 0.1–2.5 μm — penetrate deep into lungs, long residence time (days–weeks), strongly affect climate and health.
• Coarse particles (PM10): 2.5–10 μm — settle faster (hours–days), primarily from mechanical processes (dust, sea spray, pollen).

Sources: Natural — sea salt, mineral dust (e.g., desert dust), volcanic ash, biogenic particles (pollen, spores), organic emissions from vegetation. Anthropogenic — combustion (soot/black carbon), industrial emissions, vehicle exhaust, construction dust, biomass burning, secondary aerosols formed chemically from gaseous precursors (sulphates, nitrates, secondary organic aerosols).

Removal processes and lifetime: Removal occurs by dry deposition (gravitational settling, impaction) and wet deposition (scavenging by raindrops). Coarse particles settle quickly; fine and ultrafine can remain aloft for days to weeks and be transported far from their sources.

Climate and radiation effects: Aerosols interact with solar and terrestrial radiation in two principal ways: direct effect (scattering and absorption of sunlight) and indirect effect (acting as cloud condensation nuclei or ice nuclei, altering cloud reflectivity, lifetime and precipitation). The net climatic effect depends on composition — e.g., sulfate aerosols scatter sunlight (cooling), black carbon absorbs sunlight (warming).

Health and environmental impacts: PM2.5 and smaller particles can penetrate the respiratory tract and bloodstream, causing cardiovascular and respiratory diseases. High particulate loads reduce visibility (haze), contribute to soil and water acidification (when chemically active), and damage crops and materials.

Optical properties and measurement: Aerosol Optical Depth (AOD) quantifies columnar aerosol extinction (scattering + absorption). Spectral behaviour is often expressed with the Ångström relationship, linking AOD to wavelength to infer size distribution. Ground-based networks (e.g., sunphotometers) and satellite sensors measure AOD; in-situ samplers measure mass concentration (μg/m3) and size-resolved number concentrations.

Why size matters: Size controls deposition velocity, transport distance, optical interactions and health effects. Policy and monitoring therefore often use PM10 and PM2.5 as regulatory indicators.

Overview
The atmosphere is divided into layers based on how temperature changes with altitude. Temperature gradients (lapse rates and inversions) determine the vertical structure and the boundaries between layers. The main temperature-defined layers are the troposphere, stratosphere, mesosphere and thermosphere, separated by the tropopause, stratopause and mesopause respectively.

Troposphere (0 — ~11 km)
Temperature generally decreases with height (environmental lapse rate ≈ 6.5°C/km on average). The troposphere contains most of the atmospheric mass, weather systems, clouds and vertical mixing. The top of this layer, the tropopause, is a temperature minimum (≈ −56.5°C at mid-latitudes) that acts as a lid on strong convection. Jet streams occur near the tropopause.

Stratosphere (~11 — 50 km)
Temperature increases with height in the stratosphere because ozone in the upper stratosphere absorbs incoming ultraviolet radiation and warms the air. This temperature inversion produces a stable layer (suppresses vertical mixing). Commercial jet aircraft commonly cruise in the lower stratosphere to take advantage of stable air and strong winds.

Mesosphere (~50 — 85 km)
Temperature again decreases with altitude in the mesosphere, reaching the coldest temperatures in the atmosphere (≈ −90°C near the mesopause at ~85 km). This layer is where noctilucent clouds form and most meteors burn up.

Thermosphere (~85 km upward)
In the thermosphere temperature increases rapidly with height due to absorption of extreme ultraviolet (EUV) and X‑ray radiation by sparse gas molecules; temperatures can rise to several hundred or thousand °C (but air density is extremely low, so heat content is small). Phenomena such as auroras occur here; the International Space Station orbits within the lower thermosphere.

Important concepts
- Lapse rate: rate of temperature change with altitude (negative when temperature falls with height).
- Temperature inversion: a layer where temperature increases with height (e.g., stratospheric inversion, or a near-surface inversion that can trap pollutants).
- Boundaries: tropopause, stratopause, mesopause mark changes in the vertical temperature trend and dynamic behavior.

Typical temperature profile (mid-latitude standard atmosphere)
Surface ≈ +15°C → troposphere falls to ≈ −56.5°C at ~11 km (tropopause) → stratosphere rises to ≈ 0°C by ~50 km (stratopause) → mesosphere falls to ≈ −90°C by ~85 km (mesopause) → thermosphere rises rapidly above ~85 km.

Besides the main thermal layers (troposphere, stratosphere, mesosphere, thermosphere and exosphere), the atmosphere is also divided vertically by composition and by ionisation. These "other divisions" are important for chemistry, radio propagation, and life on Earth. The main divisions are the homosphere and heterosphere, the ozonosphere (ozone layer) and the ionosphere.

• Homosphere (surface to ~80–100 km): The lowest region where atmospheric gases are well mixed by turbulence. Composition is nearly uniform (≈78% N2, 21% O2, ~1% Ar plus variable water vapour and trace gases). The top of this region is the homopause (or turbopause).
• Heterosphere (above the homopause): Above the homopause molecular diffusion dominates, so gases stratify by molecular weight. Lighter gases (He, H) become relatively more abundant at very high altitudes; heavier gases decline. Chemical composition thus changes with height.
• Ozonosphere (ozone layer, mainly 15–35 km): Concentrated within the lower stratosphere, ozone (O3) has a strong peak roughly 20–30 km above the surface. Ozone forms by photochemical reactions: O2 + UV (<240 nm) → 2O, then O + O2 + M → O3 + M (where M is any third body). The ozonosphere absorbs most harmful UV-B and UV-C radiation, protecting life. Human-released chemicals (CFCs) can catalytically destroy ozone, creating depletion (e.g., Antarctic ozone hole).
• Ionosphere (roughly 60–400+ km): A region where solar extreme-UV and X-ray radiation ionise atoms and molecules producing free electrons and ions. Structurally it is described by D, E and F layers (D ~60–90 km, E ~90–120 km, F >120 km; F splits into F1 and F2 by day). The ionosphere:
• Reflects and refracts radio waves (shortwave/AM) enabling long-distance communication.
• Is where auroras occur (charged solar particles interacting with atmospheric constituents), producing visible light at high latitudes.
• Causes propagation delays and scintillation for satellite signals (GPS/communication).
• Practical boundaries/pauses: The tropopause, stratopause and mesopause remain important markers because chemistry and dynamics change across them. The homopause and the base of the ionosphere are other useful boundaries for composition and electrical properties.

Why these divisions matter:

• Biological protection: The ozonosphere filters UV radiation; depletion raises surface UV and health/ecosystem risks.
• Communications and navigation: The ionosphere enables (and sometimes disrupts) long-range radio and affects satellite communications.
• Space operations: Heterosphere composition and the thermosphere determine satellite drag and atmospheric re-entry conditions.

Key physical relations (used to describe how pressure, density and temperature change with height) are given below. These equations connect the compositional/ionisation divisions to measurable atmospheric behaviour.

Atmospheric pressure is the weight of the column of air above a unit area at a given level. It is measured as force per unit area (SI unit: pascal, Pa). Standard sea-level pressure is 1013.25 hPa (hectopascal) = 101325 Pa.

Air density (ρ) is the mass of air per unit volume (kg m⁻³). At sea level under standard conditions (15°C, dry air) density ≈ 1.225 kg m⁻³.

Why pressure and density change with height: gravity pulls air molecules toward Earth so air is densest at the surface. As altitude increases, there is less air above, so pressure and density decrease. The decrease is rapid near the surface and becomes slower at higher altitudes; the variation is approximately exponential if temperature is constant.

Basic relations and physical laws:

• Hydrostatic equilibrium: vertical balance between pressure gradient and gravity — dP/dz = −ρ g. This states that pressure decreases with height because each layer supports the weight of the air above it.
• Ideal gas law (per unit mass): P = ρ R T, where R is the specific gas constant for dry air (R ≈ 287 J kg⁻¹ K⁻¹) and T is absolute temperature in kelvin. This links pressure, density and temperature.

Combining hydrostatic equilibrium and the ideal gas law gives the barometric formula. For an isothermal layer (constant T):

P(z) = P₀ exp(−z/H), where H = RT/g is the scale height. For typical Earth conditions (T ≈ 288 K) H ≈ 8 km. Similarly, ρ(z) = ρ₀ exp(−z/H).

Factors affecting atmospheric pressure and density:

• Altitude: pressure and density decrease with height.
• Temperature: warmer air expands (lower density) and raises pressure aloft; at the surface warm air often leads to low surface pressure (because air rises).
• Humidity: moist air is slightly less dense than dry air at the same temperature and pressure because water vapour has lower molecular weight than dry air.
• Weather systems: high- and low-pressure systems produce horizontal pressure gradients that drive winds.

Instruments and units: barometer measures atmospheric pressure (mercury or aneroid barometers). Common units: pascal (Pa), hectopascal (hPa), millibar (mb) (1 mb = 1 hPa = 100 Pa), atm (1 atm = 101325 Pa).

Geographical relevance (Class 11 context): Knowledge of how pressure and density vary helps explain wind formation (pressure gradients), climate and weather patterns, effects of altitude on human physiology (breathing and boiling point), and why aircraft and balloons behave as they do.

What is temperature variation with height? Temperature in the atmosphere usually changes with height. The rate at which temperature decreases (or increases) with altitude is called a lapse rate.

Key types of lapse rates

• Environmental lapse rate (Γe): the actual rate of temperature change observed in the atmosphere at a particular time and place. Not constant—varies with weather and location.
• Normal lapse rate: a long-term average environmental lapse rate of about 6.5 °C per km in the troposphere.
• Dry adiabatic lapse rate (DALR, Γd): the rate at which an unsaturated (dry) air parcel cools as it rises adiabatically. Γd ≈ 9.8 °C per km (commonly rounded to 10 °C/km).
• Moist (saturated) adiabatic lapse rate (MALR or Γm): the rate at which a saturated (condensing) air parcel cools as it rises. It is variable (≈4–7 °C per km) because latent heat release during condensation reduces cooling.

Sign convention & formula

The lapse rate is defined as Γ = −dT/dz, where T is temperature and z is height. By this convention a positive Γ means temperature decreases with height.

Why different values? When unsaturated air rises it expands and cools at the DALR because no heat is exchanged with the environment but no latent heat is released. If the rising air is saturated, condensation releases latent heat and the cooling slows; hence MALR < DALR.

Atmospheric stability — how lapse rates control weather

• Compare Γe (actual atmosphere) with Γd and Γm to determine stability:
• Absolutely stable: Γe < Γm — rising parcels cool faster than environment and sink back.
• Conditionally unstable: Γm < Γe < Γd — instability only if air is lifted to saturation; otherwise stable.
• Absolutely unstable: Γe > Γd — rising parcels remain warmer than environment and continue to rise (promotes convection and thunderstorms).

Temperature variation through atmospheric layers

• Troposphere: temperature generally decreases with height (average ~6.5 °C/km) until the tropopause.
• Stratosphere: temperature increases with height (temperature inversion) because ozone absorbs UV radiation—this inversion inhibits vertical mixing (why jet aircraft fly here).
• Mesosphere: temperature again decreases with height.
• Thermosphere: temperature increases strongly with height due to absorption of high-energy solar radiation.

Temperature inversions

An inversion occurs when temperature increases with height. Inversions trap pollutants and moisture near the surface and suppress convection. Examples include nocturnal (radiation) inversions on clear calm nights and subsidence inversions beneath high-pressure systems.

Practical consequences: lapse rates and stability explain cloud formation, fog, thunderstorms, mountain climate (cooler with height), frost in valley bottoms (cold air pooling), and pollution episodes under inversion.

Simple sample calculation

If surface temperature = 30 °C, temperature at 2 km using DALR: T = 30 − 9.8×2 ≈ 10.4 °C. Using the normal lapse rate (6.5 °C/km): T ≈ 30 − 6.5×2 = 17 °C.

What is the ozone layer?

The ozone layer is a region of relatively high ozone (O3) concentration in the stratosphere, roughly between 15 km and 35 km above the surface. It absorbs most of the Sun’s harmful ultraviolet-B (UV-B, 280–315 nm) and a portion of ultraviolet-C (UV-C, < 280 nm), protecting life on Earth.

Formation and natural destruction (Chapman mechanism)

• O2 photolysis: O2 + hν (< 242 nm) → 2 O (atomic oxygen)
• Ozone formation: O + O2 + M → O3 + M (M = any third body that carries away excess energy)
• O3 photolysis: O3 + hν (< 320 nm) → O2 + O
• Recombination: O + O3 → 2 O2

These reactions, proposed by Chapman, establish a natural steady-state ozone concentration controlled by solar radiation and temperature.

Catalytic destruction: why ozone is sensitive to trace gases

Certain radicals (X = Cl, Br, OH, NO) destroy ozone via catalytic cycles — a single radical can destroy many O3 molecules. Example (chlorine catalytic cycle):

• Cl + O3 → ClO + O2
• ClO + O → Cl + O2
• Net: O3 + O → 2 O2

CFCs (chlorofluorocarbons) release Cl atoms in the stratosphere after photodissociation and are the primary anthropogenic cause of large-scale ozone depletion.

Antarctic ozone hole: special conditions

• Polar winter: strong, stable polar vortex isolates air; extremely cold temperatures lead to formation of polar stratospheric clouds (PSCs).
• PSCs provide surfaces for heterogeneous reactions that convert reservoir chlorine (HCl, ClONO2) into active Cl2. When sunlight returns in spring, Cl2 photolyzes to release Cl radicals leading to rapid ozone destruction.
• Result: seasonal “ozone hole” over Antarctica (deep reduction in total column ozone) each austral spring (September–November).

Consequences of ozone depletion

• Higher surface UV-B: increased risk of skin cancers, cataracts, immune suppression in humans.
• Ecological impacts: reduced primary productivity (phytoplankton), crop yield reductions, harm to terrestrial and aquatic ecosystems.
• Material damage: accelerated degradation of polymers and paints.

Policy response and recovery

The role of CFCs in ozone depletion was predicted (Molina & Rowland, 1974) and confirmed by observations (Farman et al., 1985). The Montreal Protocol (1987) phased out many ozone-depleting substances (ODS). Since then global ozone columns have shown signs of stabilization and slow recovery; full recovery is expected during the mid- to late-21st century if controls remain effective.

Key facts & typical values

• Ozone layer peak: ~20–30 km altitude (stratosphere).
• Typical total column ozone: ~300 Dobson Units (DU) globally; an ozone hole is commonly defined when column < 220 DU.

Definition: The greenhouse effect is the process by which certain atmospheric gases trap outgoing longwave (infrared) radiation emitted by Earth's surface, warming the lower atmosphere and surface. Radiative balance (or Earth's energy balance) describes the equilibrium between incoming solar (shortwave) radiation and outgoing terrestrial (longwave) radiation.

How the natural greenhouse effect works (stepwise):

• Sunlight (shortwave radiation) reaches Earth; much passes through the atmosphere and is absorbed by the surface.
• The warmed surface emits energy as longwave (infrared) radiation.
• Greenhouse gases (water vapour, carbon dioxide, methane, nitrous oxide, ozone, CFCs) absorb much of this outgoing IR and re-radiate it in all directions.
• Some of the re-radiated energy returns to the surface, raising surface temperature above what it would be with no greenhouse gases.

Key components and terms:

• Greenhouse gases (GHGs): H₂O, CO₂, CH₄, N₂O, O₃, CFCs.
• Atmospheric window: Wavelength range (about 8–14 μm) where the atmosphere is relatively transparent to IR; important for direct loss of heat to space.
• Effective emission level: The altitude from which Earth effectively radiates to space; if this level rises (due to more GHGs), the surface must warm to restore balance.

Radiative balance — basic formulation:

• Solar constant S₀ ≈ 1361 W m⁻² (incoming solar radiation at top of atmosphere).
• Average solar energy received by Earth (accounting for geometry): S₀/4.
• Net absorbed by Earth: (S₀/4) × (1 − A), where A = planetary albedo (≈ 0.30).
• At radiative equilibrium, outgoing longwave radiation = absorbed shortwave: σT_e⁴ = (S₀/4)(1 − A), where σ is the Stefan–Boltzmann constant.

Numerical illustration:

• With S₀ ≈ 1361 W m⁻² and A ≈ 0.30, the absorbed flux ≈ 240 W m⁻².
• Thus σT_e⁴ ≈ 240 W m⁻² → T_e ≈ 255 K (about −18 °C). This is Earth’s effective radiating temperature without atmospheric greenhouse warming.
• Observed global mean surface temperature ≈ 288 K (≈ 15 °C). The ~33 °C difference is due to the natural greenhouse effect.

Anthropogenic (enhanced) greenhouse effect: Human activities (fossil-fuel burning, deforestation, agriculture) increase concentrations of GHGs, reducing outgoing IR at certain wavelengths and forcing the climate system out of radiative balance. The system responds by warming until a new balance is achieved.

Radiative forcing and climate sensitivity:

• Radiative forcing (ΔF): Change in net (down minus up) radiative flux at the tropopause after perturbation (units W m⁻²).
• Approximate forcing from CO₂ increase: ΔF ≈ 5.35 ln(C/C₀) W m⁻² (Myhre et al.).
• Climate sensitivity: Equilibrium global mean surface temperature change for a doubling of CO₂ — likely around 3 °C (uncertainty range ≈ 1.5–4.5 °C).

Consequences of an enhanced greenhouse effect: Global warming, melting ice, sea-level rise, altered precipitation patterns, more frequent/extreme heat events, ecological and agricultural impacts.

Simple conceptual model: Consider a one-layer atmosphere that is transparent to solar radiation but absorbs a fraction of terrestrial IR. If the atmosphere absorbs all outgoing IR, the surface must emit more (and thus be warmer) to supply the same outgoing flux to space from the atmospheric top layer. This illustrates why increased absorption by GHGs warms the surface.

Takeaway: The greenhouse effect is essential to sustain habitable surface temperatures, but increases in greenhouse gases from human activities cause additional warming by disturbing Earth’s radiative balance.

Overview
Water vapour is the gaseous form of water present in the atmosphere. It is invisible and variable in amount, and it plays a central role in weather and climate because it carries latent heat and forms clouds and precipitation when it condenses.

Water vapour in the atmosphere

• Sources: evaporation from oceans, lakes, rivers, transpiration from plants (evapotranspiration), sublimation from ice.
• Vertical and horizontal distribution: greatest near the surface and in the tropics; decreases with height and toward poles.
• Measurement: hygrometers, psychrometers, radiosondes, and remote sensing (satellite sensors).

Measures of humidity

• Absolute humidity: mass of water vapour per unit volume of air (g·m⁻3). Varies with pressure and temperature.
• Specific humidity: mass of water vapour per unit mass of moist air (g·kg⁻1). Roughly conserved in adiabatic vertical motion.
• Mixing ratio (w): mass of water vapour per unit mass of dry air (g·kg⁻1). Often used in thermodynamic calculations.
• Relative humidity (RH): ratio of actual vapour pressure to saturation vapour pressure at the same temperature, expressed as a percentage. RH is strongly temperature dependent.
• Dew point: the temperature to which air must be cooled (at constant pressure) for saturation to occur and condensation to begin. Dew point is a direct measure of moisture content.

Saturation and condensation
Saturation occurs when the air contains the maximum water vapour possible at a given temperature and pressure; any further cooling or addition of vapour causes condensation. Condensation requires: (1) cooling of air to the dew point (often by adiabatic expansion when air rises) or addition of moisture, and (2) condensation nuclei (aerosols) on which droplets can form.

Processes that cause condensation

• Adiabatic cooling of rising air (convection, frontal uplift, orographic uplift). When rising air cools to its dew point, clouds form.
• Radiative cooling of the ground and the air near the surface at night (produces dew or frost).
• Mixing of two air masses with different temperatures and humidities (can produce fog or clouds).

Forms of condensation (near-surface and atmospheric)

• Dew: liquid water on surfaces when temperature falls to or below the dew point (above 0°C).
• Frost: deposition of ice directly from vapour when surface temperature is below freezing.
• Fog and mist: clouds in contact with the ground; fog has higher water content and visibility <1 km.
• Clouds and precipitation: larger-scale condensation aloft; growth of droplets or ice crystals leads to rain, snow, sleet, etc.

Thermodynamic importance
Condensation releases latent heat (latent heat of condensation/vaporisation), which warms the surrounding air and influences atmospheric stability and development of storms. Latent heat release is a key energy source for convection and cyclogenesis.

Key instruments & practical notes

• Sling (wet-and-dry) psychrometer measures dry-bulb and wet-bulb temperatures; their difference gives an estimate of vapour pressure and RH.
• Hygrometers (capacitive, resistive), dew-point hygrometers, and radiosondes provide vertical humidity profiles.

Summary: Water vapour is variable and critical for weather. Humidity expresses how much vapour is present; saturation and condensation (to form clouds, fog, dew) depend strongly on temperature, pressure and available nuclei. Understanding humidity and condensation explains everyday phenomena (fog, dew, cloud formation) and large-scale weather systems.

What is the Standard Atmosphere? The standard atmosphere (often called the International Standard Atmosphere, ISA) is an idealized, average vertical distribution of atmospheric properties — principally pressure, temperature and density — used as a reference for scientific calculations, aviation and meteorology. It defines mean sea-level values and how those values change with altitude under typical conditions.

Key standard sea-level values (ISA): pressure = 1013.25 hPa (or mb), temperature = 15 °C (288.15 K), density = 1.225 kg m^-3.

Vertical structure by temperature (major layers relevant to Class 11):

• Troposphere (surface to ~11 km): Temperature decreases with height at the environmental lapse rate (standard value about -6.5 °C km^-1). Weather and most clouds occur here. Tropopause at ~11 km (temperature about -56.5 °C) acts as a cap on vertical mixing.
• Stratosphere (~11 km to ~50 km): Temperature increases with height because of ozone absorption of ultraviolet radiation; this creates a stable layer and little vertical mixing. The stratopause is near ~50 km.
• Mesosphere (~50 km to ~85 km): Temperature again decreases with height; the coldest region (mesopause ~ -90 °C) occurs near ~85 km.
• Thermosphere (above ~85 km): Temperature increases strongly with height due to absorption of high-energy solar radiation; densities are very low.

How pressure and density change with height — Pressure and density decrease rapidly with altitude because of the weight of the air above. The decrease is approximately exponential for an isothermal layer; with a changing temperature the change follows a power-law form derived from the hydrostatic equation combined with the ideal gas law.

Why vertical profiles matter (practical importance): Vertical profiles control weather, cloud formation, aircraft performance, buoyancy of balloons, radio propagation, and the distribution of atmospheric gases (e.g., ozone in the stratosphere). Standard atmosphere models provide baseline values for design and analysis (aircraft, rockets, radiosondes).

Limitations: The standard atmosphere is an average model — real atmosphere shows variations with latitude, season, weather and local conditions. Actual lapse rates and tropopause heights differ day-to-day and regionally.

The atmosphere is the envelope of gases surrounding Earth. Beyond being a mixture of gases, it performs multiple vital functions and provides services that sustain life, regulate climate and weather, support human activities, and protect the planet from external hazards.

Main functions and services

• Provision of essential gases: Supplies oxygen for respiration, carbon dioxide for photosynthesis, and nitrogen for many biological and chemical processes.
• Protection from harmful solar and space radiation: The ozone layer in the stratosphere absorbs most of the Sun's harmful ultraviolet (UV‑B and UV‑C) radiation. The atmosphere also reduces the impact of cosmic rays and small meteoroids (which burn up on entry).
• Thermal regulation and climate control: The atmosphere retains heat through the greenhouse effect (water vapour, CO2, methane, etc.), moderates day–night temperature differences, and helps maintain a climate suitable for life.
• Weather generation and the hydrological cycle: Evaporation, cloud formation, precipitation and atmospheric transport distribute water around the globe—providing freshwater and determining local weather patterns (rain, snow, storms, monsoons).
• Energy and momentum transport: Winds, driven by pressure and temperature gradients, transport heat and moisture, driving ocean–atmosphere interaction, weather systems and large‑scale circulation patterns (trade winds, westerlies, jet streams).
• Filtering, recycling and chemical processing: The atmosphere chemically transforms gases (e.g., ozone chemistry, nitrogen fixation in lightning), disperses and dilutes pollutants, and is a medium for the global carbon and nitrogen cycles.
• Medium for life and human activities: Enables aviation and wind energy, transports pollen and seeds (affecting agriculture), carries sound, and supports radio and communication via the ionosphere.
• Protection from impacts: Frictional heating in the upper atmosphere burns up many meteoroids, preventing frequent surface impacts.

Why these functions matter (brief impacts):

• Without the ozone layer, biological systems would be heavily damaged by UV radiation.
• Without greenhouse gases, Earth would be much colder (approx. −18 °C planetary equilibrium vs. +15 °C observed average).
• Atmospheric circulation distributes heat poleward; disruptions (e.g., changes in circulation) alter climate and weather extremes.
• Atmospheric chemistry controls air quality—pollution (smog, acid rain) directly affects human health, crops and ecosystems.

Link to structure and composition: Each of these services depends on vertical layers and gas composition: the troposphere drives weather and contains most water vapour; the stratosphere contains the ozone layer; the ionosphere/thermosphere affects radio waves and space weather.

Human influence and vulnerability: Anthropogenic emissions (CO2, CH4, CFCs) change atmospheric composition, weakening services (e.g., climate regulation, ozone protection) and creating problems like global warming, ozone depletion, and urban air pollution. Conservation and emissions control help preserve atmospheric services.

Overview
Human activities alter the composition and properties of the atmosphere by emitting gases and particles (aerosols). These changes cause air pollution, modify climate (greenhouse effect), deplete stratospheric ozone, change radiative balance and affect human health, ecosystems and infrastructure.

Main categories of pollutants

• Primary pollutants — emitted directly: carbon monoxide (CO), sulphur dioxide (SO2), nitrogen oxides (NOx), volatile organic compounds (VOCs), particulate matter (PM10, PM2.5), ammonia (NH3), black carbon.
• Secondary pollutants — formed in the atmosphere by reactions: ozone (O3 at ground level), sulphates and nitrates (components of acid rain and fine particulates), secondary organic aerosols.

Sources

• Combustion of fossil fuels (power plants, industry, transport) — primary source of CO2, NOx, SO2, PM and VOCs.
• Agriculture and livestock — methane (CH4), ammonia (NH3), nitrous oxide (N2O).
• Industrial processes and solvents — VOCs, fluorinated gases (CFCs historically, HFCs, SF6).
• Biomass burning and deforestation — CO, PM, VOCs, increased CO2.
• Urban activities — construction dust, domestic biofuel burning.

Chemical and physical processes

• Photochemical smog (sunlight-driven): NO2 + hv → NO + O; O + O2 → O3. VOCs and NOx produce ozone and oxidants that form secondary aerosols.
• Acid rain formation: SO2 and NOx oxidize to H2SO4 and HNO3 (sulphuric and nitric acids) which acidify precipitation and soils.
• Ozone depletion: halogenated compounds (e.g., CFCs) release Cl atoms in the stratosphere which catalytically destroy O3.
• Greenhouse warming: increased concentrations of greenhouse gases (CO2, CH4, N2O, some fluorinated gases) trap outgoing longwave radiation and raise global temperatures.

Impacts

• Health: PM2.5, ozone and NO2 cause respiratory and cardiovascular disease; CO reduces oxygen delivery; long-term exposure increases mortality.
• Environment: acidification of soils and lakes, eutrophication from nutrient deposition (N), crop yield losses from ozone, altered rainfall and cloud patterns from aerosols.
• Climate: increased greenhouse gases drive warming; aerosols can cool (reflect sunlight) or warm (black carbon) regionally and affect monsoon and precipitation patterns.
• Materials & visibility: corrosion of buildings and reduced visibility (haze).

Mitigation and policy responses

• Emission controls: flue-gas desulphurization (scrubbers), catalytic converters, particulate filters.
• Fuel & technology shift: renewables, electrification of transport, energy efficiency.
• Agricultural practices: optimized fertilizer use, manure management to reduce N2O and NH3.
• Regulation & international agreements: Clean Air Acts, Montreal Protocol (CFCs), Paris Agreement (GHG reductions).
• Local measures: air quality monitoring, emission zoning, public transport and urban green spaces.

Key measurements & indicators
Common indicators include concentrations (µg/m3 or ppb) of PM2.5/PM10, NO2, SO2, O3 and CO; greenhouse gas mixing ratios in ppm/ppb (CO2, CH4, N2O); and Air Quality Index (AQI) summaries used for public advisories.

Overview
The atmosphere constantly exchanges energy, mass and momentum with the lithosphere (land), hydrosphere (oceans, lakes, rivers), biosphere (plants, animals, microbes), cryosphere (ice and snow) and the anthroposphere (human systems). These interactions determine weather, climate, air composition and many environmental processes.

Main pathways of interaction

• Energy exchange: Radiation (shortwave solar, longwave terrestrial), conduction at surfaces, and convection in the air redistribute heat between spheres. Surface albedo and heat capacity (land vs. water) control how much energy is absorbed and released.
• Mass exchange (water): Evaporation from water bodies and transpiration from plants move water into the atmosphere as vapor; condensation and precipitation return it to the surface.
• Gas exchange and chemistry: Photosynthesis, respiration and combustion exchange O2 and CO2; oceans absorb and emit CO2; chemical reactions in the atmosphere create or remove pollutants and aerosols.
• Particle and nutrient transfer: Dust, sea salt, volcanic ash and pollutants move between ground, ocean and air, affecting cloud formation, radiation and soil/sea nutrient cycles.
• Dynamic forcing: Topography (mountains) forces air upwards causing orographic rainfall; ocean–atmosphere coupling drives large-scale climate modes (e.g., El Niño).

Why these interactions matter
They control local weather (sea breezes, valley winds), seasonal and global climate (greenhouse effect, ocean heat uptake), ecosystem productivity (carbon and water cycles), and human impacts (air quality, extreme events).

Processes and typical behavior

• Surface temperature response: Land warms and cools faster than ocean because of lower heat capacity — this drives diurnal and seasonal pressure differences and circulations (sea/land breezes, monsoons).
• Hydrological coupling: Warmer air holds more moisture (see Clausius–Clapeyron), intensifying the water cycle — more evaporation, heavier precipitation extremes.
• Biological feedbacks: Vegetation modifies albedo, transpiration and roughness; deforestation alters local climate and carbon balance.
• Cryosphere feedbacks: Melting ice lowers albedo and releases freshwater into oceans, altering circulation and atmospheric humidity.

Scale and examples
Interactions occur across scales: microscale (soil-atmosphere moisture exchange), mesoscale (sea breezes, lake effect snow), synoptic and global scale (ocean–atmosphere coupled modes, greenhouse warming).

The atmosphere is the envelope of gases surrounding Earth. Key concepts and terminology help describe its composition, vertical structure, and physical behaviour. This section summarizes the fundamental terms, their meanings and how they relate to everyday phenomena.

Composition: The atmosphere consists of permanent gases (those with nearly constant concentrations) and variable gases (those whose concentrations vary in time and space). Major permanent gases by volume are nitrogen (~78.08%), oxygen (~20.95%), argon (~0.93%), and trace gases such as carbon dioxide (~0.04 or ~400 ppm) and neon. Variable constituents include water vapour (0–4% by volume locally), ozone, and aerosols (tiny solid or liquid particles).

Structure (vertical layers): The atmosphere is divided into layers based on temperature trends with altitude:

• Troposphere (surface to ~8–18 km): Temperature generally decreases with altitude (environmental lapse rate ~6.5°C/km). Most weather, clouds and life are here. The top boundary is the tropopause.
• Stratosphere (~18–50 km): Temperature increases with altitude because of ozone absorption of ultraviolet radiation; stable layer where commercial jets often cruise near the lower stratosphere.
• Mesosphere (~50–85 km): Temperature decreases with altitude; meteors burn up here. The top is the mesopause (coldest region).
• Thermosphere (~85–600 km): Temperature increases with altitude due to absorption of high-energy solar radiation. Contains the ionosphere, where charged particles influence radio propagation and auroras occur.
• Exosphere (above ~600 km): Extremely low density; molecules may escape to space; satellites orbit within this region.

Homo­sphere and Heterosphere: The homosphere (surface to ~80–100 km) has well-mixed gases with nearly uniform composition. Above this lies the heterosphere where lighter gases (H, He) dominate at higher altitudes due to molecular diffusion.

Key physical ideas: Air pressure and density decrease with height approximately exponentially; temperature profiles define atmospheric stability; the ionosphere (part of the thermosphere) contains charged ions important for radio-wave reflection. The ozone layer (peak concentration around 20–30 km) absorbs harmful ultraviolet radiation and warms the stratosphere.

Why these terms matter: Knowing these concepts explains why weather is confined to the troposphere, why aircraft choose certain flight levels, how greenhouse gases trap heat, and why ozone depletion raises UV exposure at the surface.