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ICSE Class 7 Geography Question 7 of 16

Atmosphere — Question 7

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Question 4(v)

Where is ozone layer? Why is it getting depleted?

Answer

The ozone layer lies in the stratosphere, between 16 and 50 km above the earth’s surface.
Certain chemicals like CFCs (chlorofluorocarbons) cause ozone break down and thinning of the layer. Another cause of ozone depletion is the emission of green house gases due to burning of fossil fuels.

ICSE Class VII Geography — Chapter 3: Atmosphere

Bright Tutorials | ICSE Class VII | Geography | Chapter 3

Composition of the Atmosphere

The atmosphere is the blanket of air surrounding the earth, held by gravity. It is composed primarily of nitrogen (78%) and oxygen (21%), with argon (0.93%), carbon dioxide (0.04%), and traces of water vapour and other gases. Nitrogen dilutes oxygen and supports plant growth, oxygen is essential for breathing, and carbon dioxide helps trap heat through the greenhouse effect.

Five Layers of the Atmosphere

The atmosphere has five layers: the Troposphere (0-12 km, where all weather occurs), Stratosphere (12-50 km, contains the ozone layer), Mesosphere (50-80 km, coldest layer where meteors burn), Thermosphere (80-700 km, auroras and ISS orbit here), and Exosphere (700+ km, merges with space). Temperature decreases in the troposphere and mesosphere but increases in the stratosphere and thermosphere.

Atmospheric Pressure and Pressure Belts

Atmospheric pressure is the weight of air pressing down on the earth's surface, measured by a barometer. Normal pressure at sea level is 1,013.25 mb. Pressure decreases with altitude. The earth has four major pressure belts: Equatorial Low (0°), Subtropical High (30°), Subpolar Low (60°), and Polar High (90°). Wind flows from high pressure to low pressure areas.

Types of Winds

Planetary winds blow constantly: Trade Winds (towards equator), Westerlies (towards poles from 30°), and Polar Easterlies (from poles). Periodic winds change direction: monsoons change seasonally, while land and sea breezes change daily. Sea breeze blows during the day (land heats faster), land breeze at night (land cools faster). Local winds like the Loo (India), Chinook (Rockies), and Mistral (France) are caused by local conditions.

Key Takeaways for ICSE Class VII Students

  • Memorise: Nitrogen 78%, Oxygen 21% — these two make up 99% of the atmosphere
  • Use the mnemonic "The Sun Makes Things Exciting" for layer order
  • Wind always blows from HIGH pressure to LOW pressure
  • Sea breeze = daytime, Land breeze = nighttime
  • Draw and label the atmosphere layers diagram for exam preparation