Quick Summary
The atmosphere is the layer of gases surrounding the Earth. It protects life by blocking harmful UV rays, maintaining temperature, and providing oxygen. This chapter covers its composition, layers, pressure, wind systems, and humidity.
Key Terms
| Term | Meaning |
|---|---|
| Atmosphere | Layer of gases around Earth |
| Troposphere | Lowest layer; where weather occurs |
| Stratosphere | Contains ozone layer |
| Coriolis Effect | Deflection of winds due to Earth's rotation |
| Relative Humidity | Ratio of actual to maximum possible moisture |
Composition of the Atmosphere
| Gas | Percentage |
|---|---|
| Nitrogen (N₂) | 78.09% |
| Oxygen (O₂) | 20.95% |
| Argon (Ar) | 0.93% |
| Carbon dioxide (CO₂) | 0.04% |
| Other gases (Neon, Helium, etc.) | Trace |
Layers of the Atmosphere
| Layer | Height | Features |
|---|---|---|
| Troposphere | 0–12 km | Weather; temperature decreases with height |
| Stratosphere | 12–50 km | Ozone layer; jet aircrafts fly here |
| Mesosphere | 50–80 km | Meteors burn up here; coldest layer |
| Thermosphere | 80–700 km | Aurora; very high temperatures |
| Exosphere | Above 700 km | Transition to outer space; satellites orbit here |
Atmospheric Pressure
- Weight of air column above a given point
- Measured in millibars (mb) using a barometer
- Sea level: standard pressure = 1013.25 mb
- Pressure decreases with altitude (less air above)
- High pressure: cool, descending air → clear skies
- Low pressure: warm, rising air → clouds and rain
- Isobars: lines on map connecting points of equal pressure
Winds
- Air movement from high pressure to low pressure
- Permanent winds: Trade winds, Westerlies, Polar Easterlies
- Seasonal winds: Monsoon
- Local winds: Land breeze / Sea breeze; Valley breeze / Mountain breeze
- Coriolis Effect: in Northern Hemisphere winds deflect to the right; in Southern Hemisphere to the left
Pressure Belts of the World
| Belt | Location | Type |
|---|---|---|
| Equatorial Low | 0° | Low pressure (ITCZ) |
| Subtropical High | 30° N/S | High pressure |
| Sub-polar Low | 60° N/S | Low pressure |
| Polar High | 90° N/S | High pressure |
Humidity and Precipitation
- Humidity: water vapour in air
- Relative humidity: actual moisture ÷ maximum possible × 100%
- Dew point: temperature at which air becomes saturated
- Types of precipitation: Rain, Snow, Hail, Sleet, Drizzle, Dew, Fog
- Rainfall types: Convectional, Orographic (relief), Frontal (cyclonic)
Important Questions
One Mark
1. Name the layer of the atmosphere where weather occurs. 2. What percentage of atmosphere is nitrogen? 3. What is an isobar? 4. Define relative humidity. 5. What causes the Coriolis Effect?
Short Answer (3 marks)
1. Name the five layers of the atmosphere and state one feature of each. 2. How are land and sea breezes formed? 3. Explain the three types of rainfall.
Long Answer (5 marks)
1. Describe the structure of the atmosphere with a labelled diagram description. 2. Explain atmospheric pressure and its effect on winds and weather.
Common Mistakes
- Weather occurs in TROPOSPHERE, not stratosphere.
- Ozone layer is in STRATOSPHERE, not troposphere.
- Coriolis deflects right in NORTHERN hemisphere, left in Southern.
Revision Checklist
- I can list all 5 layers with their height range.
- I know the percentages of the 3 main gases.
- I understand why winds blow from high to low pressure.
- I can explain one type of rainfall with a diagram.
Frequently Asked Questions
The five layers of the atmosphere from bottom to top are: Troposphere (0–12 km), Stratosphere (12–50 km), Mesosphere (50–80 km), Thermosphere (80–700 km), and Exosphere (above 700 km). Weather occurs in the troposphere.