Class 7 Social Science Notes Chapter 4 (Air) – Our Environment Book
Alright class, let's focus on Chapter 4, 'Air', from our Class 7 Geography book. This chapter is crucial not just for understanding our environment but also forms a base for many questions in competitive government exams. Pay close attention as we break down the key concepts.
Chapter 4: Air - Detailed Notes for Exam Preparation
1. Introduction: What is Atmosphere?
- The huge blanket of air surrounding the Earth is called the Atmosphere.
- It's essential for life: provides the air we breathe, protects us from harmful sun rays (UV radiation), and regulates Earth's temperature, making life possible. Without this blanket, we'd bake during the day and freeze at night.
2. Composition of the Atmosphere
The air we breathe is a mixture of many gases. The main components are:
- Nitrogen (N2) - 78%:
- Most plentiful gas.
- Essential for plant growth (plants cannot take it directly from the air; bacteria in soil and roots convert it into usable forms).
- We inhale nitrogen, but our lungs don't use it; we exhale it.
- Oxygen (O2) - 21%:
- Second most plentiful gas.
- Essential for respiration (breathing) for humans and animals.
- Green plants produce oxygen during photosynthesis, maintaining the balance.
- Argon (Ar) - 0.93%: An inert gas.
- Carbon Dioxide (CO2) - 0.03% (approx., but increasing):
- Essential for plants (used in photosynthesis).
- Absorbs heat radiated by the Earth (greenhouse effect), keeping the planet warm.
- However, increased levels (due to burning fossil fuels, deforestation) enhance the greenhouse effect, leading to global warming and climate change.
- Other Gases - 0.04%: Includes Helium, Ozone, Hydrogen, etc., in trace amounts.
- Dust Particles & Water Vapour: Also present in the lower atmosphere. Dust particles act as nuclei for condensation (cloud formation). Water vapour leads to precipitation.
3. Structure of the Atmosphere
The atmosphere is divided into five distinct layers based on temperature variations:
- (i) Troposphere:
- Lowest and densest layer.
- Average height: ~13 km (higher at the equator, lower at the poles).
- All weather phenomena (rain, fog, clouds, storms, hail) occur here.
- Temperature decreases with increasing altitude (approx. 1°C for every 165m ascent – known as the Normal Lapse Rate).
- Contains almost all the water vapour and dust particles.
- The air we breathe exists here.
- (ii) Stratosphere:
- Extends up to ~50 km above the Troposphere.
- Almost free from clouds and weather phenomena, making it ideal for flying jet aircraft.
- Contains the Ozone Layer (O3), which absorbs harmful Ultraviolet (UV) radiation from the sun. Ozone depletion (due to CFCs etc.) is a major concern.
- Temperature increases with increasing altitude due to UV absorption by ozone.
- (iii) Mesosphere:
- Extends up to ~80 km above the Stratosphere.
- Meteorites burn up in this layer upon entering from space.
- Temperature decreases with increasing altitude, reaching the coldest temperatures in the atmosphere (around -90°C or lower).
- (iv) Thermosphere:
- Extends from ~80 km up to ~400 km (variable upper limit).
- Temperature rises very rapidly with increasing altitude due to absorption of X-rays and UV radiation.
- Contains the Ionosphere: A sub-layer within the Thermosphere containing electrically charged particles (ions). These ions reflect radio waves back to Earth, enabling radio communication.
- (v) Exosphere:
- Uppermost layer, gradually merging with outer space.
- Air is extremely thin (very low density).
- Light gases like Helium and Hydrogen float into space from here.
- Satellites orbit the Earth in this layer and the upper Thermosphere.
4. Weather and Climate
- Weather: The hour-to-hour, day-to-day condition of the atmosphere at a particular place. It can change dramatically and quickly (e.g., sunny morning, rainy afternoon).
- Climate: The average weather condition of a place over a long period (typically 25 years or more). It represents the long-term pattern.
- Elements of Weather & Climate: Temperature, Air Pressure, Wind, Humidity, Precipitation.
5. Temperature
- The degree of hotness or coldness of the air.
- Measured using a Thermometer. Standard unit: Degrees Celsius (°C). (Fahrenheit is also used).
- Insolation (Incoming Solar Radiation): The solar energy intercepted by the Earth. It's the primary factor influencing temperature distribution.
- Amount of insolation decreases from the equator towards the poles. This is why poles are cold and covered in ice, while equatorial regions are hot.
- Factors affecting Temperature Distribution:
- Latitude: Higher latitude = less direct sunlight = lower temperature.
- Altitude: Higher altitude = thinner air = lower temperature (in Troposphere).
- Distance from the Sea: Coastal areas have moderate temperatures (maritime effect), while inland areas have extreme temperatures (continental effect).
- Ocean Currents: Warm currents raise coastal temperatures; cold currents lower them.
- Winds: Winds blowing from colder regions lower temperature; winds from hotter regions raise it.
6. Air Pressure
- The pressure exerted by the weight of air on the Earth's surface.
- Measured using a Barometer. Standard unit: millibars (mb).
- Air pressure is highest at sea level and decreases with increasing altitude (as there is less air above).
- Temperature Influence:
- Hot air is lighter, rises, creating Low-Pressure areas. Associated with cloudy skies and wet weather.
- Cold air is denser, sinks, creating High-Pressure areas. Associated with clear and sunny skies.
- Air always moves from High-Pressure areas to Low-Pressure areas. This movement causes wind.
7. Wind
- The movement of air from a high-pressure area to a low-pressure area.
- Named after the direction from which it blows (e.g., a Westerly wind blows from the west).
- Measured using an Anemometer (speed) and Wind Vane (direction).
- Types of Winds:
- (i) Permanent Winds (Planetary Winds): Blow constantly throughout the year in a particular direction. Examples:
- Trade Winds: Blow from subtropical high-pressure belts towards the equatorial low-pressure belt. (NE Trades in Northern Hemisphere, SE Trades in Southern Hemisphere).
- Westerlies: Blow from subtropical high-pressure belts towards sub-polar low-pressure belts. (SW in NH, NW in SH).
- Polar Easterlies: Blow from polar high-pressure belts towards sub-polar low-pressure belts. (NE in NH, SE in SH).
- (ii) Seasonal Winds: Change their direction in different seasons. Example:
- Monsoons in India (Sea to land in summer, Land to sea in winter).
- (iii) Local Winds: Blow only during a particular period of the day or year in a small area. Examples:
- Land and Sea Breezes: Coastal phenomena due to differential heating.
- Loo: Hot and dry local wind blowing in the northern plains of India during summer afternoons.
- Fohn/Chinook: Warm, dry winds on the leeward side of mountains.
- (i) Permanent Winds (Planetary Winds): Blow constantly throughout the year in a particular direction. Examples:
8. Moisture
- Humidity: The amount of water vapour present in the air. Measured by a Hygrometer.
- When air is full of water vapour, we call it a humid day. Warm air can hold more moisture than cold air.
- Water Cycle: Evaporation -> Condensation -> Precipitation.
- Evaporation: Water turns into vapour due to heat.
- Condensation: Water vapour rises, cools, and condenses into tiny water droplets, forming Clouds. Clouds are masses of such water droplets.
- Precipitation: When these water droplets become too heavy to float, they fall down as precipitation.
- Can be liquid (Rain) or solid (Snow, Sleet, Hail).
- Types of Rainfall:
- (i) Convectional Rainfall: Caused by intense heating of the surface, leading to rising warm, moist air which cools, condenses, and causes heavy rain, often with thunder and lightning. Common in equatorial regions.
- (ii) Orographic (Relief) Rainfall: Occurs when moist air is forced to rise over a mountain barrier. Air cools, condenses, causing rain on the windward side. The leeward side (rain shadow area) receives very little rain.
- (iii) Cyclonic (Frontal) Rainfall: Occurs when warm and cold air masses meet. The warmer, lighter air is forced to rise over the colder, denser air, leading to condensation and precipitation along the front. Common in temperate regions.
Multiple Choice Questions (MCQs)
-
Which gas is the most abundant in the Earth's atmosphere?
a) Oxygen
b) Carbon Dioxide
c) Nitrogen
d) Argon -
In which layer of the atmosphere do all weather phenomena occur?
a) Stratosphere
b) Troposphere
c) Mesosphere
d) Thermosphere -
The Ozone layer, which protects us from harmful UV radiation, is found in the:
a) Troposphere
b) Mesosphere
c) Stratosphere
d) Exosphere -
The degree of hotness and coldness of the air is known as:
a) Air Pressure
b) Humidity
c) Wind
d) Temperature -
Air pressure is highest at:
a) Mountain tops
b) Sea Level
c) Mesosphere
d) Stratosphere -
Which instrument is used to measure atmospheric pressure?
a) Thermometer
b) Barometer
c) Anemometer
d) Hygrometer -
Winds that blow constantly throughout the year in a particular direction are called:
a) Local Winds
b) Seasonal Winds
c) Permanent Winds
d) Cyclonic Winds -
'Loo' is an example of a:
a) Permanent Wind
b) Seasonal Wind
c) Local Wind
d) Trade Wind -
Which type of rainfall is common in equatorial regions, often occurring in the afternoon?
a) Orographic Rainfall
b) Cyclonic Rainfall
c) Convectional Rainfall
d) Frontal Rainfall -
Which gas in the atmosphere is essential for photosynthesis in plants?
a) Oxygen
b) Nitrogen
c) Carbon Dioxide
d) Ozone
Answer Key for MCQs:
- (c) Nitrogen
- (b) Troposphere
- (c) Stratosphere
- (d) Temperature
- (b) Sea Level
- (b) Barometer
- (c) Permanent Winds
- (c) Local Wind
- (c) Convectional Rainfall
- (c) Carbon Dioxide
Make sure you revise these notes thoroughly. Understanding the composition, structure, and dynamics of the atmosphere is fundamental. Good luck with your preparation!