- Layers of various gases / envelope of air spread around the Earth
| Gas | % (Volume) |
|---|
| Nitrogen | 78.08 |
| Oxygen | 20.95 |
| Argon | 0.934 |
| Carbon Dioxide | ~0.042 (OP reference, ~420 ppm) |
| Neon | 0.0018 |
| Helium | 0.0005 |
| Ozone | ~0.000007 (highly variable) |
Water Vapour
- Up to 0–4% of atmosphere
- Responsible for Clouds, Rain, Fog
- Responsible for Earth's heat retention
Dust Particles
- Aerosols act as condensation nuclei for cloud droplets.
- Blue sky is mainly Rayleigh scattering by air molecules; aerosols/dust cause haze and alter optical effects.
⚡ Quick Recall
Nitrogen (78%) + Oxygen (21%) together make up ~99% of the atmosphere's dry composition.
Troposphere0–18 km
StratosphereUp to 50 km
MesosphereUp to ~85 km
Thermosphere (Ionosphere Overlaps)Thermosphere ~85–600 km; ionosphere roughly ~48–965 km (variable)
ExosphereBegins near ~600 km; variable
- Height: 0–18 km (Pole: 8 km, Equator: 18 km, Average: 13 km)
- Lowest layer of atmosphere
- Almost all weather occurs here
- Normal Lapse Rate of temperature: 1 km → 6.5°C decrease
- Tropopause: boundary separating Troposphere and Stratosphere
- Up to 50 km
- Increase in temperature with height
- Temperature stable up to 20 km
- Suitable for airplane flight
- "Mother of Pearl" clouds present here
- Up to ~85 km
- Decrease in temperature with increasing height
- Mesopause: coldest atmospheric region
- Burning up of meteoroids occurs here
- Thermosphere ~85–600 km; ionosphere roughly ~48–965 km (variable)
- Increase in temperature with increasing height — hottest layer
- Reflection of radio waves
- Red glow of Aurora at poles
- Northern Lights (Aurora Borealis) and Southern Lights (Aurora Australis)
- From Thermosphere to about 10,000 km
- Very sparse atomic hydrogen (H) and helium (He)
📌 Karman Line (100 km)
Conventional 100 km boundary of space; the atmosphere thins gradually.
⚠ Exam Trap
Temperature
decreases with height in Troposphere and Mesosphere, but
increases with height in Stratosphere and Thermosphere — the pattern alternates layer by layer.
- Protects Earth from the Sun's ultraviolet rays
- Ozone concentration typically peaks at ~20–25 km, varying with latitude/season.
- Measurement unit – Dobson unit
- Maximum depletion – Over South Pole (Antarctica)
16 Sep
16 September commemorates the 1987 Montreal Protocol
~20–25 km
Typical Ozone Concentration Maximum (Variable)
- Depletion – Thinning / Hole in the ozone layer
- Montreal Protocol (Canada) → Aims to reduce CFCs (Chlorofluorocarbons)
- Kigali Amendment → Aims to reduce HFCs (Hydrofluorocarbons)
⚡ Quick Recall
Montreal Protocol → targets CFCs · Kigali Amendment (an amendment to Montreal Protocol) → targets HFCs
- Solar radiation energy received from the Sun
- Aphelion (4 July): Relatively receives less insolation
- Perihelion (3 January): Relatively receives more insolation
- Equator – comparatively less • Deserts – maximum
- Latitude
- Conditions of the atmosphere
- Day length
- Transparency of atmosphere
- Sunspots
- Distance between Earth and Sun
⚠ Exam Trap
Deserts receive
more insolation than the Equator, despite the Equator getting more direct sunlight overall — because deserts have clear skies and low humidity/cloud cover, allowing more solar radiation to reach the surface.
- Balance of temperature (Earth not getting excessively hot or cold)
- Average temperature of Earth – 15°C
- Incoming solar energy absorbed by Earth’s surface: ~48% in the OP/NASA budget.
- Amount of light reflected from any surface or object
- Earth’s average planetary albedo: ~30%
- Fresh snow has very high albedo, often 80–90%; bare glacier ice is generally lower.
- Albedo of Dry sand – 35–45%
⚡ Quick Recall
Higher Albedo = More reflection, less absorption → Ice/Snow (highest albedo) stays cooler.
| Process | Key Details |
|---|
| Conduction | Transfer of heat from higher to lower temperature • Heating of lower layers of atmosphere |
| Convection | Vertical transfer of heat • Vertical heating of atmosphere |
| Radiation | Heating through transmission of heat waves • Earth's heating from solar radiation |
| Advection | Horizontal transfer of heat • 'Loo' is the result of advection |
⚠ Exam Trap
Convection =
vertical heat transfer; Advection =
horizontal heat transfer — the hot "Loo" winds of North India are a classic example of Advection, not Convection.
- Causes: Latitude, Distance from sea, Ocean Currents, Altitude, Air circulation, Local factors
- Isotherms: Lines connecting places having equal temperature
- Temperature Range: Difference between maximum and minimum temperature
- Lower layer of air is cold and upper layer is warm (reversal of the normal pattern)
Favourable Conditions
- Dry and Calm air
- Long cold nights
- Cloudless and clear sky
- Stability in atmosphere
Effects
- Formation of toxic fog / smog in industrial areas
⚡ Quick Recall
Temperature Inversion is exactly the opposite of the normal Lapse Rate (normally temperature falls with height in the Troposphere).