Atmosphere — Class 9 Social Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 9 Social Science, Geography — Chapter 3. Earth's blanket of air — composition, layers, weather, and climate.
1. About this lesson
This chapter covers the atmosphere — the thin layer of gases surrounding the Earth. You will learn about its composition, structure (layers), weather and climate, temperature, atmospheric pressure, winds, humidity, clouds, and precipitation.
2. Composition of the Atmosphere
| Gas | Percentage |
|---|---|
| Nitrogen (N₂) | ~78% |
| Oxygen (O₂) | ~21% |
| Argon | ~0.93% |
| Carbon dioxide (CO₂) | ~0.04% |
| Other gases (neon, helium, methane, ozone, etc.) | Trace |
- Water vapour — variable (0–4%); important for weather and the water cycle.
- Dust particles — act as nuclei for condensation.
- Ozone (O₃) — found in the stratosphere; absorbs harmful UV radiation.
3. Structure of the Atmosphere (Layers)
| Layer | Altitude | Key Features |
|---|---|---|
| Troposphere | 0–13 km (varies: 8 km at poles, 18 km at equator) | Weather occurs here; temperature decreases with height (normal lapse rate: 6.5°C/km); contains ~75% of atmospheric mass |
| Stratosphere | 13–50 km | Ozone layer (absorbs UV); temperature increases with height (due to ozone absorbing UV); jet aircraft fly here |
| Mesosphere | 50–80 km | Temperature decreases with height (coldest layer); meteors burn up here |
| Thermosphere (Ionosphere) | 80–400+ km | Temperature increases sharply; radio waves reflected; auroras occur here |
| Exosphere | 400+ km | Outermost layer; merges with outer space; very thin air |
Tropopause, Stratopause, Mesopause: Boundaries between layers where temperature stops changing.
4. Weather and Climate
| Term | Definition |
|---|---|
| Weather | Short-term atmospheric conditions of a place at a given time (temperature, humidity, wind, rainfall) |
| Climate | Average weather conditions of a place over a long period (at least 30 years) |
Elements of Weather and Climate
- Temperature, atmospheric pressure, wind, humidity, clouds, precipitation.
5. Temperature
- Insolation: Incoming solar radiation received by the Earth.
- Terrestrial radiation: Heat radiated back by the Earth.
- Temperature decreases from the equator towards the poles.
- Factors affecting temperature distribution: latitude, altitude, distance from the sea, ocean currents, winds, cloud cover.
Heat Zones of the Earth
| Zone | Latitude Range |
|---|---|
| Torrid Zone | 0° – 23½° N/S (tropics) |
| Temperate Zone | 23½° – 66½° N/S |
| Frigid Zone | 66½° – 90° N/S (poles) |
6. Atmospheric Pressure
- Pressure = weight of air on a unit area.
- Measured with a barometer; unit: millibar (mb).
- Normal sea-level pressure: 1013.25 mb.
- Pressure decreases with altitude.
- Isobars: Lines on a map joining places of equal pressure.
Pressure Belts
| Pressure Belt | Latitude | Type |
|---|---|---|
| Equatorial Low | 0°–5° N/S | Thermal low (heating) |
| Sub-tropical High | 30°–35° N/S | Dynamic high |
| Sub-polar Low | 60°–65° N/S | Dynamic low |
| Polar High | 85°–90° N/S | Thermal high (cooling) |
7. Winds
Wind = air moving from high pressure to low pressure.
Types of Winds
A. Planetary (Permanent) Winds:
| Wind | Direction (N Hemi) | From → To |
|---|---|---|
| Trade Winds | NE → SW | Sub-tropical High → Equatorial Low |
| Westerlies | SW → NE | Sub-tropical High → Sub-polar Low |
| Polar Easterlies | NE → SW | Polar High → Sub-polar Low |
B. Seasonal Winds:
- Monsoons: Seasonal reversal of wind direction; crucial for Indian climate.
C. Local Winds:
| Wind | Region | Type |
|---|---|---|
| Loo | North India (summer) | Hot, dry |
| Chinook | Rockies (USA) | Warm, dry (snow-eater) |
| Foehn | Alps (Europe) | Warm, dry |
| Mistral | Southern France | Cold, dry |
8. Humidity and Clouds
- Humidity: Amount of water vapour in the air.
- Relative humidity: Ratio of actual water vapour to maximum possible at that temperature (%).
- Condensation: Water vapour → liquid; occurs when air cools below dew point.
- Clouds: Visible mass of water droplets/ice crystals.
Cloud Types
| Cloud | Altitude | Appearance |
|---|---|---|
| Cirrus | High | Thin, wispy, feathery |
| Cumulus | Medium | Puffy, cotton-like; fair weather |
| Stratus | Low | Layered, sheet-like; overcast |
| Nimbus | All levels | Rain-bearing; dark |
9. Precipitation (Rainfall)
Types of Rainfall
| Type | Mechanism | Where |
|---|---|---|
| Convectional | Heated air rises, cools, condenses | Equatorial regions; summer thunderstorms |
| Orographic (Relief) | Moist air forced to rise over mountains; windward side gets rain, leeward side is dry (rain shadow) | Mountain ranges (Western Ghats, Himalayas) |
| Cyclonic (Frontal) | Warm and cold air masses meet; warm air rises over cold air | Temperate regions |
Forms of Precipitation
Rain, snow, sleet, hail, drizzle.
10. Worked examples
Example 1. Which layer contains the ozone layer? Stratosphere.
Example 2. In which layer does all weather occur? Troposphere.
Example 3. What is the normal lapse rate? Temperature decreases by 6.5°C per 1,000 metres of ascent in the troposphere.
11. Book-back questions (Samacheer Kalvi)
I. Choose the correct answer
-
The lowermost layer of the atmosphere is — (a) Stratosphere / (b) Troposphere / (c) Mesosphere / (d) Thermosphere. Ans: (b) Troposphere.
-
The gas which constitutes about 78% of the atmosphere — (a) Oxygen / (b) Nitrogen / (c) Carbon dioxide / (d) Argon. Ans: (b) Nitrogen.
-
The ozone layer is found in the — (a) Troposphere / (b) Stratosphere / (c) Mesosphere / (d) Thermosphere. Ans: (b) Stratosphere.
-
The instrument used to measure atmospheric pressure — (a) Thermometer / (b) Barometer / (c) Anemometer / (d) Hygrometer. Ans: (b) Barometer.
-
Trade winds blow from — (a) Equatorial low to sub-tropical high / (b) Sub-tropical high to equatorial low / (c) Sub-polar low to polar high / (d) Polar high to sub-polar low. Ans: (b) Sub-tropical high to equatorial low.
II. Fill in the blanks
- The average weather condition over 30+ years is called climate.
- The boundary between troposphere and stratosphere is tropopause.
- Lines joining places of equal pressure are isobars.
- Convectional rainfall occurs in equatorial regions due to heating.
- The hot, dry local wind in North India is called Loo.
III. Answer briefly
-
Layers of atmosphere — Troposphere (weather, temp ↓), Stratosphere (ozone, temp ↑), Mesosphere (meteors, temp ↓), Thermosphere (radio, auroras, temp ↑), Exosphere (outermost).
-
Weather vs Climate — Weather = short-term atmospheric conditions; Climate = long-term (30+ year) average.
-
Pressure belts — Equatorial Low (0°), Sub-tropical High (30°), Sub-polar Low (60°), Polar High (90°).
-
Types of rainfall — Convectional (heated air rises), Orographic (air rises over mountains → windward rain, leeward rain shadow), Cyclonic (warm + cold air masses meet).
12. Common mistakes
- Mistake: Ozone layer is in the troposphere. Fix: The ozone layer is in the stratosphere.
- Mistake: Oxygen is the most abundant gas. Fix: Nitrogen (~78%) is most abundant; oxygen is ~21%.
- Mistake: Climate = day-to-day weather. Fix: Climate is the long-term (30+ year) average of weather.
- Mistake: Temperature increases with height in the troposphere. Fix: Temperature decreases with height in the troposphere (normal lapse rate: 6.5°C/km).
13. Quick revision
- Geography Ch 3 · Atmosphere.
- Composition: Nitrogen 78%, Oxygen 21%, others (CO₂, argon, ozone, water vapour).
- Layers: Troposphere (weather, ↓temp) → Stratosphere (ozone, ↑temp) → Mesosphere (meteors, ↓temp) → Thermosphere (radio, auroras, ↑temp) → Exosphere.
- Pressure belts: Eq Low (0°) → Sub-tr High (30°) → Sub-polar Low (60°) → Polar High (90°).
- Winds: Trade (Sub-tr High → Eq Low), Westerlies, Polar Easterlies. Local winds: Loo, Chinook, Foehn.
- Humidity → condensation → clouds (Cirrus, Cumulus, Stratus, Nimbus) → precipitation.
- Rainfall types: Convectional, Orographic (relief), Cyclonic (frontal).
