Composition of the atmosphere
The atmosphere is the layer of air held around the Earth by gravity. It is a mix of gases:
- Nitrogen about 78%: needed by plants (through soil bacteria).
- Oxygen about 21%: we and animals breathe it.
- Argon about 0.9%, carbon dioxide about 0.04%: CO₂ is small but plants need it and it traps heat.
- Water vapour, dust and tiny particles: they help clouds and rain form.
Structure: the five layers
| Layer | Height (about) | Key fact |
|---|---|---|
| Troposphere | 0 – 8 to 18 km (13 km average) | All weather: clouds, rain, storms. Gets colder as you go up. |
| Stratosphere | up to ~50 km | Ozone layer blocks UV rays. Calm air, good for jet planes. |
| Mesosphere | up to ~80 km | Meteors burn up here. |
| Thermosphere | up to ~400 km+ | Ionosphere reflects radio waves; auroras glow. |
| Exosphere | top | Very thin air that fades into space. |
Elements of weather
Weather = the state of the air at one place and time. Its elements are:
- Temperature: how hot or cold (thermometer, °C).
- Air pressure: the weight of air pressing down (barometer). Hot air rises → low pressure.
- Wind: air moving from high pressure to low pressure (wind vane for direction).
- Humidity: water vapour in the air (hygrometer).
- Clouds and precipitation: rain, snow, hail (rain gauge).
Climate is the average weather of a place over 30 years or more. Example: "It is raining in Mumbai today" is weather; "Mumbai gets heavy rain every June–September" is climate.
Seasons of India and the monsoon
The word monsoon comes from an Arabic word for "season". Monsoon winds change direction with the season.
- Cold season (Dec–Feb): cool, dry winds blow from land to sea.
- Hot season (Mar–May): land heats up; loo winds in the north; duststorms.
- Rainy season / south-west monsoon (Jun–Sep): land is hot, low pressure forms; moist winds from the sea bring rain. Western Ghats and north-east hills get the most.
- Retreating monsoon (Oct–Nov): winds turn back; Tamil Nadu coast gets rain; cyclones may form.
India also marks six traditional ritus: vasanta, grishma, varsha, sharad, hemanta, shishira.
Climate change, floods and carbon footprint
Greenhouse gases like CO₂ and methane let sunlight in but trap some heat, like the glass of a greenhouse. This keeps Earth warm enough to live. But burning coal, petrol and forests adds extra CO₂, so Earth is warming. This is climate change.
Effects: glaciers melt, sea level rises, heatwaves get stronger, and rain comes in short, very heavy bursts, causing floods (cities also flood when drains are blocked and land is paved).
Your carbon footprint is the total greenhouse gas your activities add. Walk or cycle, use buses and trains, switch off lights, waste less food, and plant trees to lower it.
Key formulas and definitions
- Air ≈ 78% nitrogen + 21% oxygen + 1% others (argon, CO₂, …)
- Layers (bottom → top): Troposphere → Stratosphere → Mesosphere → Thermosphere → Exosphere
- Weather = state of air now; Climate = average weather over 30+ years
- Wind blows from high pressure to low pressure
- Carbon footprint ≈ Σ (activity amount × emission factor), e.g. 1 kWh ≈ 0.7 kg CO₂
Worked examples
1. How many out of 100 molecules of air are oxygen?
About 21.
2. "Delhi is cold in January every year." Weather or climate?
Climate, because it is the usual pattern over many years.
3. Why does wind blow from the sea to the land on a hot summer day?
Land heats faster, so air over land rises and makes low pressure. Cooler sea air (high pressure) moves in to fill it.
4. A family uses 6 units of electricity a day. At 0.7 kg CO₂ per unit, what is the daily footprint from electricity?
6 × 0.7 = 4.2 kg CO₂ per day.
5. You travel 20 km a day by car (0.15 kg/km). If you switch to a bus (0.05 kg/km), how much CO₂ do you save per day?
Car 20 × 0.15 = 3 kg; bus 20 × 0.05 = 1 kg; saving = 2 kg per day.
6. Temperature falls about 6.5 °C for every 1 km up in the troposphere. If it is 30 °C at the ground, what is it 4 km up?
30 − 4 × 6.5 = 30 − 26 = 4 °C.
Common mistakes
- Using weather and climate as the same word. Weather is short-term; climate is the long-term average.
- Thinking ozone is in the troposphere where we breathe. The protective ozone layer is in the stratosphere.
- Saying wind blows from low to high pressure. It always flows from high to low.
- Thinking CO₂ is bad in all amounts. Some is needed for plants and warmth; the problem is the extra amount we add.