Atmosphere (Earth)
The Earth's atmosphere is the layer of gases retained by Earth's gravity surrounding the planet, extending from the surface to approximately 10,000 km altitude where it gradually merges with the interplanetary medium. It consists primarily of nitrogen (78.09%), oxygen (20.95%), argon (0.93%), and carbon dioxide (0.04%), plus trace gases and variable amounts of water vapor. The atmosphere performs critical functions including regulating surface temperature through the greenhouse effect, protecting life from harmful ultraviolet radiation via the ozone layer, enabling weather and climate systems, and providing the oxygen and carbon dioxide essential for respiration and photosynthesis.
Key Formula
P = P0 × e^(−Mgh/RT)
LaTeX: P = P_0 \, e^{-Mgh/RT}
| Symbol | Meaning | Unit |
|---|---|---|
| P | Atmospheric pressure at altitude h | Pa |
| P_0 | Sea-level atmospheric pressure (101,325 Pa) | Pa |
| M | Molar mass of air (0.02896 kg/mol) | kg/mol |
| g | Gravitational acceleration (9.81 m/s²) | m/s² |
| h | Altitude above sea level | m |
| R | Universal gas constant (8.314 J/mol·K) | J/(mol·K) |
| T | Temperature | K |
Worked Example
Problem
Calculate the atmospheric pressure at an altitude of 5,000 m above sea level, assuming a constant temperature of 255 K (isothermal approximation). Use P₀ = 101,325 Pa, M = 0.02896 kg/mol, g = 9.81 m/s², R = 8.314 J/(mol·K).
Solution
Step 1: Calculate the exponent. Exponent = −Mgh/RT = −(0.02896 × 9.81 × 5000) / (8.314 × 255) = −(1421.4) / (2120.1) = −0.6705 Step 2: Evaluate the exponential. e^(−0.6705) ≈ 0.5114 Step 3: Calculate pressure. P = 101,325 × 0.5114 ≈ 51,807 Pa ≈ 51.8 kPa
Answer
Approximately 51.8 kPa (about 51% of sea-level pressure)
Composition of Earth's Dry Atmosphere
| Gas | Chemical Symbol | Volume Fraction (%) | Role | Trend |
|---|---|---|---|---|
| Nitrogen | N₂ | 78.09 | Diluent, nutrient cycle | Stable |
| Oxygen | O₂ | 20.95 | Respiration, combustion | Stable |
| Argon | Ar | 0.93 | Inert filler | Stable |
| Carbon dioxide | CO₂ | 0.042 | Greenhouse gas, photosynthesis | Increasing (~3 ppm/yr) |
| Methane | CH₄ | 0.000187 | Potent greenhouse gas | Increasing |
| Ozone | O₃ | 0.000006 | UV shield in stratosphere | Recovering |
Interactive Tools
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Related Terms
Troposphere
The troposphere is the lowest layer of Earth's atmosphere, extending from the surface to approximately 8–9 km at the poles and 16–18 km at the equator, and containing about 80% of the atmosphere's total mass and virtually all of its water vapor and weather. Temperature generally decreases with altitude at the environmental lapse rate of approximately 6.5°C per 1,000 m (the standard atmosphere value), until reaching the tropopause, a temperature inversion that caps the troposphere. All significant weather phenomena — clouds, precipitation, thunderstorms, cyclones, and jet streams — occur within the troposphere, making it the most meteorologically active layer.
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, extending from the tropopause (approximately 12 km at mid-latitudes) to the stratopause at about 50 km altitude. Unlike the troposphere, temperature in the stratosphere increases with altitude — from about −56°C at the tropopause to approximately 0°C at the stratopause — due to the absorption of ultraviolet radiation by the ozone layer concentrated within it. This temperature inversion creates very stable conditions that suppress vertical mixing, making the stratosphere nearly cloud-free and home to the polar vortex and stratospheric jet streams; it is also the layer traversed by high-altitude commercial aircraft.
Ozone Layer
The ozone layer is a region of Earth's stratosphere, concentrated approximately 15–35 km above the surface, where ozone (O₃) molecules are present at relatively high concentrations (2–8 ppm), absorbing 97–99% of the Sun's medium-frequency ultraviolet radiation. Ozone is continuously formed when UV radiation (wavelength < 240 nm) splits O₂ molecules into oxygen atoms that then react with other O₂ molecules, and destroyed by catalytic cycles involving chlorine, bromine, nitrogen, and hydrogen radicals. Depletion of the ozone layer by synthetic chlorofluorocarbons (CFCs) — culminating in the discovery of the Antarctic ozone hole in 1985 — led to the Montreal Protocol (1987), which has successfully reduced stratospheric chlorine loading and begun the layer's recovery.
From Greek "atmos" (vapor, steam) and "sphaira" (sphere, globe). The term was coined in the 17th century; it appears in scientific literature by 1638 (John Wilkins) and was used by Edmond Halley and others to describe the gaseous envelope of Earth. The concept that air has weight and pressure was established by Evangelista Torricelli in 1643.