📌 Key Points
- The atmosphere is the thin blanket of air surrounding the Earth, held by gravity, extending up to about 1,600 km
- Composition: Nitrogen (78%), Oxygen (21%), Other gases (1%) including CO₂, argon, ozone, and water vapour
- Carbon dioxide creates the greenhouse effect by trapping heat; used by plants for photosynthesis
- Ozone layer in the stratosphere protects Earth from harmful ultraviolet (UV) rays
- Five layers (bottom to top): Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere
- Troposphere (0–12 km): Lowest layer; all weather occurs here; temperature decreases with height
- Stratosphere (12–50 km): Contains ozone layer; temperature increases with height; jet aircraft fly here
- Mesosphere (50–80 km): Coldest layer (–100°C); meteorites burn up here (shooting stars)
- Thermosphere (80–400 km): Very hot; contains ionosphere for radio wave transmission; auroras occur here
- Exosphere (400+ km): Outermost layer; merges into outer space; satellites orbit here
- Weather = day-to-day condition (changes frequently); Climate = average weather over 25–30 years (stable)
- Temperature decreases with height at 6.5°C per 1,000 m (normal lapse rate) — hills are cooler than plains
- Temperature decreases from equator to poles — vertical rays heat more intensely than oblique rays
- Air pressure decreases with altitude; measured with a barometer; wind blows from high to low pressure
- Permanent winds: Trade winds, Westerlies, Polar easterlies — blow in same direction all year
- Seasonal winds: Monsoons — change direction with season; 'mausim' (Arabic) means season
- Local winds: Sea breeze (day, sea to land), Land breeze (night, land to sea), Loo (hot wind in N. India)
- Humidity = amount of water vapour in air; Evaporation = water to vapour; Condensation = vapour to droplets
- Precipitation = water falling from clouds as rain, snow, sleet, or hail
- Three types of rainfall: Convectional (hot air rises), Orographic (air over mountains), Cyclonic (warm meets cold air)
- Orographic: Windward side gets rain; Leeward side is rain shadow (dry). Example: Western Ghats
📘 Important Definitions
⚠️ Common Mistakes
✗ Wrong: Thinking oxygen is the most abundant gas in the atmosphere
✓ Correct: Nitrogen (78%) is the most abundant gas, not oxygen (21%). Oxygen is the second most abundant gas.
✗ Wrong: Confusing the location of the ozone layer
✓ Correct: The ozone layer is in the STRATOSPHERE (12–50 km), not in the troposphere. It protects Earth from UV rays.
✗ Wrong: Thinking the Sun directly heats the atmosphere
✓ Correct: The Sun first heats the Earth's surface, then the Earth heats the atmosphere from below. This is why temperature decreases with altitude.
✗ Wrong: Saying wind blows from low pressure to high pressure
✓ Correct: Wind always blows from HIGH pressure to LOW pressure. Remember: air moves from where there is more air (high pressure) to where there is less (low pressure).
✗ Wrong: Confusing land breeze and sea breeze timing
✓ Correct: Sea breeze = DAY (sea to land, because land is hotter). Land breeze = NIGHT (land to sea, because land is cooler). Remember: 'Sea' starts with S like 'Sun' (day).
✗ Wrong: Confusing windward and leeward in orographic rainfall
✓ Correct: Windward side = faces the wind, gets HEAVY rain. Leeward side = sheltered from wind, gets very LITTLE rain (rain shadow area).
📝 Exam Focus
These questions are frequently asked in CBSE exams:
🎯 Last-Minute Recall
Close your eyes and try to recall: Key definitions, formulas, and 3 common mistakes. If you can recall 80% without looking, you're exam-ready!