Class 11 Geography - WEST-BENGAL
Composition and Structure of Atmosphere
This chapter explores the composition and vertical structure of Earth's atmosphere, which is vital for life and weather phenomena. Students will learn about the permanent and variable gases like nitrogen and oxygen, water vapour, and dust particles that make up the air. The chapter details the thermal layers of the atmosphere—troposphere, stratosphere, mesosphere, thermosphere, and exosphere—along with their temperature characteristics and significance. Understanding these concepts is essential for mastering climatology and scoring well in the West Bengal Council of Higher Secondary Education (WBCHSE) Class 11 Geography examinations.
Start Learning FreeKey Concepts
Composition of the Atmosphere
The atmosphere is a mixture of gases, water vapour, and dust particles. Nitrogen (78.08%) and oxygen (20.95%) make up the bulk of permanent gases, while variable gases like carbon dioxide and ozone play crucial roles in regulating temperature and absorbing harmful ultraviolet radiation.
Troposphere
The lowest layer of the atmosphere where temperature decreases with increasing altitude at a normal lapse rate of 6.5°C per 1,000 meters. All major weather and climatic phenomena occur in this layer.
Stratosphere
The layer above the troposphere that contains the ozone layer, which absorbs harmful UV rays from the sun. Unlike the troposphere, temperature increases with altitude in this calm layer, making it ideal for jet aircraft flight.
Normal Lapse Rate
The rate at which air temperature decreases with an increase in elevation in the troposphere, averaging approximately 6.5°C for every 1,000 meters of ascent.
Ionosphere
A sub-layer within the thermosphere containing electrically charged ions that reflect radio waves back to Earth, making long-distance wireless communication possible.
Important Formulas
Board Exam Info
In the West Bengal (WBBSE/WBCHSE) Class 11 Geography annual examination, this chapter typically carries around 6 to 8 marks. Questions frequently include short-answer questions (SAQs), multiple-choice questions (MCQs), and 3 to 5-mark descriptive questions explaining the different layers of the atmosphere or the significance of the ozone layer.
Frequently Asked Questions
Why does temperature decrease with height in the troposphere?
The troposphere is heated from below by terrestrial radiation (heat radiated from Earth's surface) rather than directly by the sun. As you move higher away from the heat source of the Earth's surface, the temperature drops.
What is the importance of the ozone layer in the stratosphere?
The ozone layer absorbs harmful ultraviolet (UV) radiation coming from the sun, protecting life on Earth from skin cancer, cataracts, and damage to ecosystems.
Why are dust particles important in the atmosphere?
Dust particles act as hygroscopic (water-absorbing) nuclei around which water vapour condenses to form clouds, fog, and precipitation.
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