# Atmosphere of Earth The atmosphere of [[Earth]] is a 5.15×10¹⁸ kg shell of gas — about a millionth of the planet's mass — bound by [[Gravity]], stirred by sunlight, and held at a mean sea-level pressure of 101.325 kPa. It is mostly [[Nitrogen]] (78.08% by volume of dry air) and [[Oxygen]] (20.95%), with [[Argon]], CO₂, [[Water]] vapor, and [[Noble_gas]] traces making up the rest, and it does three jobs at once: it is the radiative blanket that lifts the surface 33 K above the airless temperature, the fluid in which all weather and [[Turbulence]] happens, and a leaky container that bleeds [[Hydrogen]] and [[Helium]] to space continuously. Its numbers — a 6.5 K/km tropospheric lapse rate, an 8.4 km scale height, 340 W/m² of incoming sunlight averaged over the sphere — are the boundary conditions for every flying machine, every climate model, and every breath. ## Composition: a nitrogen–oxygen mixture with telltale traces | Gas | Dry-air fraction | Where it comes from | | --- | --- | --- | | [[Nitrogen]] N₂ | 78.084% | primordial, biologically cycled | | [[Oxygen]] O₂ | 20.946% | photosynthesis (the [[Oxygen-evolving_complex]]) | | [[Argon]] | 0.934% | radiogenic ⁴⁰Ar from [[Potassium]]-40 [[Radioactive_decay]] | | CO₂ | ≈0.043% (≈425 ppm, rising) | volcanism, respiration, combustion | | [[Neon]], [[Helium]], [[Krypton]], [[Xenon]] | 18.2, 5.2, 1.1, 0.09 ppm | primordial and radiogenic traces | [[Water]] vapor is excluded from those figures because it varies from nearly zero over polar ice to ≈4% in tropical air — the atmosphere's most variable major constituent and its dominant greenhouse absorber. Free O₂ is a biological artifact: before photosynthesis oxygenated the air in the Great Oxidation Event (~2.4 billion years ago), the atmosphere was essentially anoxic, and without continuous resupply reactive O₂ would rust and rot its way out of the air. The composition is uniform up to ≈100 km (the homosphere) because turbulent mixing beats diffusion; above that turbopause, gases stratify by mass, grading upward through atomic [[Oxygen]] to [[Helium]] and finally [[Hydrogen]]. ## Pressure and the 8.4 km scale height Hydrostatic balance plus the ideal gas law gives the barometric law: pressure falls as p = p₀·exp(−z/H), with scale height H = RT/(Mg) ≈ 8.4 km for mean molar mass 28.97 g/mol near 288 K. Pressure halves roughly every 5.5 km — at Everest's summit it is about a third of sea level — and ≈90% of the atmosphere's mass lies below 16 km, a skin thinner, relative to Earth, than the shell of an egg. Sea-level air has density 1.225 kg/m³, enough for wings, [[Balloon]]s filled with any [[Lifting_gas]] lighter than 29 g/mol, and a speed of [[Sound]] of 343 m/s at 20 °C. The same density is what [[Drag_(physics)]] works on: meteors ablate at 70–100 km, and satellites below ~500 km slowly spiral down against thermospheric drag. ## Vertical structure and lapse rates The troposphere (surface to 8–15 km) holds ~80% of the mass and nearly all [[Water]]; sunlight heats the ground, so temperature falls with height at an environmental average of 6.5 K/km, against the dry-adiabatic 9.8 K/km (= g/c_p) that rising parcels follow — the gap sets convective stability, thunderstorms, and the inversions that trap [[Air_pollution]] in valleys. Jet aircraft cruise near the tropopause, scribbling [[Contrail]]s. Above lies the [[Stratosphere]] (to ~50 km), warmed from above because the [[Ozone_layer]] absorbs solar ultraviolet: temperature *rises* with height, convection shuts off, and the air stratifies — hence the name and the layer's decades-long residence times. The mesosphere cools again to the coldest natural temperatures on Earth (≈−90 °C near 85 km); the thermosphere above is technically hot (500–2,000 K) but so rarefied it would feel cold, and its ionized fraction — [[Plasma_(physics)]] maintained by solar X-rays and UV — reflects radio and hosts aurorae. The 100 km Kármán line, where aerodynamic flight gives way to orbital mechanics, is the conventional edge of space; the exosphere's hydrogen geocorona extends far beyond. ## Radiative balance: 340 in, 240 kept, 33 K of greenhouse The [[Sun]] delivers 1,361 W/m² at Earth's distance; spread over the rotating sphere that is 340 W/m². About 30% reflects straight back (albedo from clouds, ice, deserts), leaving ≈240 W/m² absorbed, which a blackbody would radiate at an effective 255 K. The observed surface mean is 288 K: the 33 K difference is the greenhouse effect, mostly [[Water]] vapor and clouds, with CO₂ the controlling *non-condensing* knob — raise it and warming lets the air hold more vapor, a fast [[Positive_feedback]] that roughly doubles the direct effect. Satellite radiometry now measures a persistent imbalance near +1 W/m², the running heat gain of oceans and ice. Because outgoing radiation, convection, and latent-heat [[Heat_transfer]] all interlock, quantitative prediction needs a full [[Atmospheric_model]]; modern [[Weather_forecasting]] is the daily, verifiable success of that [[Fluid_dynamics]]-plus-radiation machinery. ## The leaky top: hydrogen and helium escape At the exobase (~500–600 km) collisions become rare, and any particle in the thermal tail moving faster than the 11.2 km/s escape speed simply leaves — Jeans escape, which discriminates ruthlessly by mass. Earth loses [[Hydrogen]] at roughly 3 kg/s and [[Helium]] at tens of grams per second (charge-exchange and polar-wind channels included); heavy gases stay. That is why atmospheric helium sits at a steady 5.2 ppm no matter how much [[Alpha_decay]] in the crust generates: [[Helium-4]] percolates up, accumulates only where trapped with [[Natural_gas]], and once vented reaches space within about a million years — the physics behind [[Helium_storage_and_conservation]] and the [[National_Helium_Reserve]], and behind the extreme scarcity of primordial [[Helium-3]]. Over geologic time the slow hemorrhage of hydrogen — much of it originally [[Water]] — has irreversibly oxidized the planet: the sky, quite literally, dried and rusted the Earth. **On the spine:** [[Oxygen]] · [[Hydrogen]] · [[Helium]] · [[Water]] · [[Ozone_layer]] · [[Earth]]. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Atmosphere_of_Earth) : [Wikitube](https://en.wikitube.io/wiki/Atmosphere_of_Earth) ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Thury_Hydrodynamics_Apex_Spine]], [[PORTAL_Hydrogen]], [[PORTAL_Oxygen]], [[PORTAL_Helium-3]], [[PORTAL_Helium]]. --- *Repopulated 2026-08-12 · redlink fill · 0 deletions.*