📑 Topics on this page (Jump to Topic):
📌 1. Composition of the Atmosphere
| Gas | Volume % |
|---|---|
| Nitrogen | 78.08 |
| Oxygen | 20.95 |
| Argon | 0.93 |
| Carbon dioxide | ≈ 0.04 (greenhouse gas) |
- Water vapour is variable (0–4%) and decreases with height; it drives all weather.
- Ozone (O₃) in the stratosphere (≈ 15–35 km) absorbs UV rays; unit Dobson Unit; main cause of depletion CFCs; Montreal Protocol (1987); 16 September — World Ozone Day.
📌 2. Layers of the Atmosphere
| Layer | Height (approx.) | Feature |
|---|---|---|
| Troposphere | ≈ 18 km at Equator, ≈ 8 km at poles | All weather; temperature falls ≈ 1 °C per 165 m (6.5 °C/km) |
| Stratosphere | Up to ≈ 50 km | Ozone layer; calm — ideal for jet aircraft |
| Mesosphere | Up to ≈ 80–85 km | Coldest layer (≈ −90 °C); meteors burn up here |
| Thermosphere / Ionosphere | ≈ 80–400+ km | Reflects radio waves; auroras |
| Exosphere | Outermost | Hydrogen, helium; satellites |
💡 Memory Tip: Order: Troposphere → Stratosphere → Mesosphere → Thermosphere → Exosphere ("The Strong Man Throws Eggs").
📌 3. Pressure Belts & Winds
- Wind always blows from high to low pressure; Earth's rotation (Coriolis force) deflects it to the right in the north and left in the south (Ferrel's law).
- Pressure belts: equatorial low (Doldrums, 0–5°), subtropical high (Horse latitudes, ≈ 30°), subpolar low (≈ 60°), polar high (90°).
- Planetary winds: Trade winds (30° → Equator, easterlies), Westerlies (30° → 60°; in the south "Roaring Forties, Furious Fifties, Shrieking Sixties"), Polar easterlies.
- Measurement: pressure — barometer (standard 1013.25 mb); wind speed — anemometer; direction — wind vane; lines of equal pressure — isobars.
| Local wind | Region | Nature |
|---|---|---|
| Loo | North India | Hot, dry (May-June) |
| Chinook ("snow eater") | Rockies (USA-Canada) | Warm, dry |
| Foehn | Alps (Switzerland) | Warm, dry |
| Sirocco | Sahara → Italy | Hot, dusty |
| Harmattan ("Doctor wind") | West Africa | Hot, dry |
| Mistral | France (Rhône valley) | Cold |
| Bora | Adriatic coast | Cold |
| Blizzard | Polar/Canada | Snowstorm |
📌 4. Cyclones & Rainfall
- Cyclone = low pressure at the centre; anticlockwise in the north, clockwise in the south. Anticyclone = high pressure, clear weather.
- Names of tropical cyclones: Indian Ocean — cyclone; Atlantic/Caribbean — hurricane; W. Pacific — typhoon; Australia — willy-willy; violent land vortex in USA — tornado. The calm centre — the "eye".
- Types of rain: convectional (daily afternoon rain at the Equator), orographic (relief) — heavy on windward slope, leeward slope is the rain shadow; cyclonic/frontal.
- Humidity: relative humidity in % — measured by a hygrometer; rain — rain gauge; lines of equal rainfall — isohyets; equal temperature — isotherms.
| Climate region | Key features |
|---|---|
| Equatorial | Hot-humid all year, daily rain, evergreen forests (Amazon, Congo) |
| Mediterranean | Winter rain, dry summer; grapes-citrus ("orchards of the world") |
| Savanna | Tall grass, summer rain; Africa |
| Tundra | Very cold, mosses-lichens; Inuit |
| Monsoon | Summer rain, dry winter; India, SE Asia |
⚠️ Exam Traps:
- Mediterranean climate gets rain in winter, not summer.
- Weather is in the troposphere; ozone in the stratosphere; the coldest layer is the mesosphere.
- A cyclone rotates anticlockwise in the Northern Hemisphere.
📌 5. Practice MCQs
📝 Practice MCQs (prepared by SF Prep — not PYQs)
Q1. Which gas is most abundant in the atmosphere? Practice
Correct answer: (B)
Nitrogen is ≈ 78%.
Nitrogen is ≈ 78%.
Q2. The ozone layer is mainly found in the — Practice
Correct answer: (C)
Ozone is concentrated in the stratosphere at ≈ 15–35 km.
Ozone is concentrated in the stratosphere at ≈ 15–35 km.
Q3. All weather phenomena occur in the — Practice
Correct answer: (D)
Almost all water vapour and dust lie in the troposphere.
Almost all water vapour and dust lie in the troposphere.
Q4. The coldest layer of the atmosphere is the — Practice
Correct answer: (A)
Temperature drops to ≈ −90 °C at the top of the mesosphere.
Temperature drops to ≈ −90 °C at the top of the mesosphere.
Q5. Which layer reflects radio waves back to Earth? Practice
Correct answer: (B)
Charged particles in the ionosphere reflect radio waves.
Charged particles in the ionosphere reflect radio waves.
Q6. Which is mainly responsible for ozone depletion? Practice
Correct answer: (C)
Chlorine released from CFCs breaks ozone; the Montreal Protocol addresses this.
Chlorine released from CFCs breaks ozone; the Montreal Protocol addresses this.
Q7. Why are winds deflected to the right in the Northern Hemisphere? Practice
Correct answer: (D)
The Coriolis force from Earth's rotation (Ferrel's law).
The Coriolis force from Earth's rotation (Ferrel's law).
Q8. "Doldrums" refers to the — Practice
Correct answer: (A)
The calm low-pressure zone near the Equator with little wind.
The calm low-pressure zone near the Equator with little wind.
Q9. The "Horse latitudes" lie around — Practice
Correct answer: (B)
Subtropical highs lie at ≈ 30–35°.
Subtropical highs lie at ≈ 30–35°.
Q10. The "Roaring Forties" are — Practice
Correct answer: (C)
With more ocean in the south, westerlies blow very strongly at 40°.
With more ocean in the south, westerlies blow very strongly at 40°.
Q11. Which local wind is called the "snow eater"? Practice
Correct answer: (D)
The warm dry Chinook on the eastern Rockies melts snow.
The warm dry Chinook on the eastern Rockies melts snow.
Q12. The "Doctor wind" of West Africa is the — Practice
Correct answer: (A)
The dry Harmattan brings relief from humid coastal weather.
The dry Harmattan brings relief from humid coastal weather.
Q13. Tropical cyclones in the West Pacific (China, Japan) are called — Practice
Correct answer: (B)
Hurricanes in the Atlantic, willy-willies in Australia.
Hurricanes in the Atlantic, willy-willies in Australia.
Q14. In the Northern Hemisphere, winds in a cyclone rotate — Practice
Correct answer: (C)
Winds converging on low pressure turn anticlockwise due to Coriolis.
Winds converging on low pressure turn anticlockwise due to Coriolis.
Q15. Which climate region gets most rain in winter? Practice
Correct answer: (D)
Westerlies bring winter rain; summers are dry.
Westerlies bring winter rain; summers are dry.
Q16. The low-rainfall area on the leeward side of a mountain is the — Practice
Correct answer: (A)
Descending air warms and becomes dry.
Descending air warms and becomes dry.
Q17. Daily afternoon rain in equatorial regions is — Practice
Correct answer: (B)
Intense heating makes air rise, cool and rain.
Intense heating makes air rise, cool and rain.
Q18. Lines joining places of equal pressure on a map are — Practice
Correct answer: (C)
Isobar = equal pressure; isohyet = equal rain; isotherm = equal temperature.
Isobar = equal pressure; isohyet = equal rain; isotherm = equal temperature.
Q19. In the troposphere, temperature falls with height at about — Practice
Correct answer: (D)
Normal lapse rate ≈ 6.5 °C/km (≈ 1 °C per 165 m).
Normal lapse rate ≈ 6.5 °C/km (≈ 1 °C per 165 m).
Q20. The Montreal Protocol (1987) relates to — Practice
Correct answer: (A)
It phases out CFCs etc.; Kyoto/Paris deal with GHGs.
It phases out CFCs etc.; Kyoto/Paris deal with GHGs.