📖 PART 1: Meaning and Composition
The Atmosphere is a blanket of air (gases, water vapour, and dust particles) that surrounds
the Earth. It is held in place by Earth's gravity. It protects life by absorbing UV radiation, warming the
surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night.
Composition of the Atmosphere
| Gas |
Percentage by Volume |
Importance |
| Nitrogen (N₂) |
78% |
Dilutes oxygen (preventing rapid burning); essential for plant growth (via nitrogen fixation).
|
| Oxygen (O₂) |
21% |
Essential for respiration in plants and animals; required for combustion (burning). |
| Argon (Ar) |
0.93% |
Inert (unreactive) gas; used in light bulbs. |
| Carbon Dioxide (CO₂) |
0.03% – 0.04% |
Essential for photosynthesis; acts as a greenhouse gas (absorbs terrestrial radiation, keeping
Earth warm). |
| Other Gases |
Trace amounts |
Ozone, Helium, Neon, Hydrogen, Methane. |
- Water Vapour: Varies from 0–4%. Crucial for weather phenomena (clouds, rain, snow).
Absorbs heat. Concentrated in lower atmosphere.
- Dust Particles: Act as hygroscopic nuclei — water vapour condenses around them to form
clouds and fog. They scatter sunlight, making the sky look blue and causing red sunsets.
☁️ PART 2: Structure of the Atmosphere
The atmosphere is divided into concentric layers based on temperature changes with altitude.
| Layer |
Altitude |
Key Characteristics |
| Troposphere |
0 to ~12 km (18 km at equator; 8 km at poles) |
The Weather Layer: Contains 90% of atmospheric mass, all water vapour &
dust. All weather events (clouds, rain, storms) happen here.
Normal Lapse
Rate: Temperature decreases with height (appx. 1°C per 165m or 6.4°C per
km).
Upper boundary is the Tropopause. |
| Stratosphere |
~12 km to 50 km |
The Flight Layer: Clear, cloudless, no weather → ideal for jet aircraft to
fly.
Ozone Layer (Ozonosphere): Found here (20-30 km). Absorbs harmful UV
radiation from the Sun.
Temperature increases with height due to UV absorption by
ozone.
Upper boundary is the Stratopause. |
| Mesosphere |
50 km to 80 km |
The Cold Layer: Temperature drops rapidly, reaching the coldest temperatures in
the atmosphere (-90°C) at the top.
Meteor burn-up: Most meteors burn up here
due to friction with gas molecules.
Upper boundary is the Mesopause. |
| Thermosphere (Ionosphere) |
80 km to 400 km |
The Radio Layer: Contains electrically charged particles (ions) that reflect
radio waves back to Earth (enabling wireless communication
distance).
Auroras: Northern/Southern Lights occur here.
Temperature
rises rapidly with height (can reach 1500°C+). |
| Exosphere |
Above 400 km |
The Outer Layer: Extremely thin air; gradually merges into the vacuum of space.
Contains light gases like hydrogen and helium. Satellites orbit here. |
🌍 PART 3: Global Warming and Greenhouse Effect
The Greenhouse Effect is the natural process by which certain gases in the atmosphere (CO₂,
methane, water vapour) trap the heat radiating from Earth’s surface, keeping the planet warm enough to
sustain life.
Global Warming is the abnormal, rapid increase in Earth's average surface
temperature due to human activities (burning fossil fuels, deforestation) releasing excess greenhouse gases.
Impact of Global Warming
- Melting of ice caps & glaciers: Loss of fresh water reserves; rising sea levels
threatening coastal cities and islands (e.g., Maldives).
- Climate abnormalities: More frequent and intense heatwaves, droughts, cyclones, and
floods. Disruption of ocean currents.
- Ecosystems & Agriculture: Loss of biodiversity (coral reefs bleaching); disruption
of crop cycles leading to food shortages.
- Spread of diseases: Warmer climates allow tropical diseases (like malaria and dengue
fever) to spread to new altitudes and latitudes.
🛡️ PART 4: Ozone Depletion
The Ozone Layer acts as a protective shield in the stratosphere, preventing lethal
Ultraviolet (UV) radiation from reaching Earth.
- Causes of Depletion: The release of Chlorofluorocarbons (CFCs) used in
refrigerators, ACs, aerosol sprays, and foam. In the stratosphere, UV breaks down CFCs, releasing
chlorine atoms that destroy ozone molecules. Over Antarctica, an "ozone hole" was discovered in the
1980s.
Impacts of Ozone Depletion
- Humans: Increased UV-B radiation causes skin cancer (melanoma), cataracts, and weakens
the immune system.
- Environment: Damages phytoplankton in oceans (the base of the marine food web) and
reduces crop yields.
✅ Countermeasures
The
Montreal Protocol (1987) is an international treaty that phased out the production of
CFCs. It has been highly successful, and the ozone layer is now slowly healing.
📌 Chapter Summary
- Atmosphere: 78% Nitrogen, 21% Oxygen, 0.03% CO₂. Contains water vapour and dust (cloud formation).
- Structure: Troposphere (weather, normal lapse rate), Stratosphere (jets, ozone layer), Mesosphere
(cold, meteors), Thermosphere/Ionosphere (radio waves, auroras), Exosphere (space).
- Global Warming: Caused by excess greenhouse gases (CO₂, methane). Impacts: sea level rise, extreme
weather, biodiversity loss.
- Ozone Depletion: Caused by CFCs destroying O₃ in stratosphere. Lets cancer-causing UV rays reach
surface. Stopped by Montreal Protocol.