Atmospheric Structure and Composition
~10 min read
- Composition: Nitrogen 78%, Oxygen 21%, Argon 0.93%, CO₂ 0.04%, plus water vapour and trace gases.
- Five layers: Troposphere (0-12 km) → Stratosphere (12-50, ozone) → Mesosphere (50-80, meteor burn-up) → Thermosphere (80-700, aurora, ISS) → Exosphere.
- Ozone: In stratosphere; absorbs UV; thinned by CFCs; depleted most over Antarctica.
The atmosphere is the thin gaseous envelope around Earth, vital for life and weather. NDA tests its layered structure, composition percentages, and the ozone layer's role.
Composition
| Gas | % |
|---|---|
| Nitrogen (N₂) | 78.09 |
| Oxygen (O₂) | 20.95 |
| Argon (Ar) | 0.93 |
| Carbon dioxide (CO₂) | ~0.04 |
| Trace (Ne, He, CH₄, Kr, N₂O) | 0.003 |
Variable components: water vapour (0–4%), dust, pollutants. CO₂ has risen from ~280 ppm (pre-industrial) to ~420 ppm today.
Vertical Structure
| Layer | Altitude | Key feature |
|---|---|---|
| Troposphere | 0–12 km (varies 8 polar, 18 equatorial) | Weather happens here; temperature decreases with height; contains ~75% of mass |
| Stratosphere | 12–50 km | Contains ozone layer (15–35 km); temperature INCREASES with height; jet aircraft fly here |
| Mesosphere | 50–80 km | Coldest layer (-90 °C); meteors burn up here |
| Thermosphere | 80–700 km | Temperature rises sharply; ISS orbits here (~400 km); auroras occur here; ionosphere overlaps |
| Exosphere | >700 km | Outermost layer, gradually merging with space |
The Ozone Layer
Located in the stratosphere (15–35 km). Ozone (O₃) absorbs harmful UV-B and UV-C radiation, protecting life on Earth.
- Depletion cause: CFCs (chlorofluorocarbons) from refrigerants, aerosols, foam blowing agents. One Cl atom can destroy ~100,000 ozone molecules.
- Ozone hole: Discovered over Antarctica in 1985; biggest in October.
- Montreal Protocol (1987): International treaty to phase out CFCs. Has been successful; ozone layer is healing.
Ionosphere and Aurora
Part of the upper atmosphere (60–1000 km) where solar radiation ionises gases. Reflects radio waves — historically vital for long-distance radio communication.
Auroras (Northern Lights = Aurora Borealis; Southern Lights = Aurora Australis) occur in the ionosphere/thermosphere when charged solar particles interact with Earth's magnetic field near the poles.
NDA PYQ Examples
Q: Most of Earth's weather occurs in which layer?
(a) Stratosphere (b) Troposphere (c) Mesosphere (d) Thermosphere
Answer: (b) Troposphere — contains ~75% of atmospheric mass and water vapour.
Q: The ozone layer is located in:
(a) Troposphere (b) Stratosphere (c) Mesosphere (d) Exosphere
Answer: (b) Stratosphere — 15–35 km altitude.
Q: The major constituent of Earth's atmosphere by volume is:
(a) Oxygen (b) Nitrogen (c) Carbon dioxide (d) Argon
Answer: (b) Nitrogen — 78%.
Drill Atmospheric Structure and Composition for NDA
NDA-pattern items on Atmospheric Structure and Composition with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why does temperature decrease with altitude in the troposphere but increase in the stratosphere?
Troposphere is heated from below (by Earth's surface); higher = farther from heat source = colder. Stratosphere is heated from within by ozone absorbing UV; higher in the stratosphere = more ozone heating = warmer.
Why is the sky blue?
Rayleigh scattering — air molecules scatter shorter (blue) wavelengths of sunlight more than longer (red) wavelengths. At sunrise/sunset, sunlight passes through more atmosphere, so reds and oranges dominate.
What causes auroras?
Solar wind particles (electrons and protons) channelled by Earth's magnetic field collide with gas atoms in the ionosphere. Excited atoms emit light — green from oxygen, red and blue from nitrogen.
What is the Montreal Protocol?
A 1987 international treaty to phase out ozone-depleting substances (CFCs, halons). Considered the most successful environmental treaty ever — ozone layer is now recovering, expected to fully heal by ~2065.
Why is the mesosphere so cold?
No significant heat source — too high for Earth's surface heating to matter, too low for direct solar absorption. The coldest temperatures in the atmosphere (-90 °C) occur near the top of the mesosphere.