NCERT Summary: Air | Geography (Prelims) by Valor Academy - UPSC PDF Download

Composition of the Atmosphere

Composition of Atmosphere

• Carbon dioxide released in the atmosphere creates a greenhouse effect by trapping the heat radiated from the earth. It is therefore called a greenhouse gas.
• Nitrogen and oxygen are two gases that make up the bulk of the atmosphere. Carbon dioxide, helium, ozone, argon, and hydrogen are found in lesser quantities.
• Nitrogen is the most plentiful gas in the air. Oxygen is the second most plentiful gas in the air. Green plants produce oxygen during photosynthesis.
• Carbon dioxide is another important gas. Green plants use carbon dioxide to make their food and release oxygen. Humans or animals release carbon dioxide.

Structure of the Atmosphere

• Our atmosphere is divided into five layers starting from the earth’s surface.
• These are Troposphere, Stratosphere, Mesosphere, Thermosphere, and Exosphere.

➢ Troposphere

• This layer is the most important layer of the atmosphere.
• Its average height is 13 km.
• The air we breathe exists here.
• Almost all the weather phenomena like rainfall, fog, and hailstorm occur in this layer.

➢ Stratosphere

• Above the troposphere lies the stratosphere.
• It extends up to a height of 50 km.
• This layer is almost free from clouds and associated weather phenomenon, making conditions most ideal for flying aeroplanes.
• One important feature of the stratosphere is that it contains a layer of ozone gas, which protects us from the harmful effect of the sun's rays.

➢ Mesosphere

• This is the third layer of the atmosphere.
• It lies above the stratosphere.
• It extends up to a height of 80 km.
• Meteorites burn up in this layer on entering from space.

➢ Thermosphere

• In the thermosphere, the temperature rises very rapidly with increasing height.
• The ionosphere is a part of this layer.
• It extends between 80-400 km.
• This layer helps in radio transmission.
• In fact, radio waves transmitted from the earth are reflected back to the earth by this layer.
  Question for NCERT Summary: Air

  Try yourself:The layer of the atmosphere which contains dust particles and water vapour is called 

  • A.

    Stratosphere
  • B.

    Troposphere
  • C.

    Ionosphere
  • D.

    Mesosphere

  View Solution

➢ Exosphere

• The uppermost layer of the atmosphere is known as the exosphere.
• This layer has very thin air.
• Light gases like helium and hydrogen float into space from here.

➢ Weather and Climate

• The average weather condition of a place for a longer period of time represents the climate of a place.
• The degree of hotness and coldness of the air is known as temperature.
• An important factor that influences the distribution of temperature is insolation.
• Insolation is the incoming solar energy intercepted by the earth.
• The amount of insolation decreases from the equator towards the poles.
  
• Air pressure is defined as the pressure exerted by the weight of air on the earth’s surface.
• The air pressure is highest at sea level and decreases with height.
• Horizontally the distribution of air pressure is influenced by the temperature of the air at a given place. In areas where the temperature is high the air gets heated and rises.
• This creates a low-pressure area. Low pressure is associated with cloudy skies and wet weather.
• High pressure is associated with clear and sunny skies.
• The movement of air from the high-pressure areas to low-pressure areas is called wind.
  Question for NCERT Summary: Air

  Try yourself:Which of the following does not affect the distribution of insolation? 

  • A.

    Altitude
  • B.

    The angle of sun rays
  • C.

    Length of the day
  • D.

    Ocean Currents

  View Solution

➢ Winds can be broadly divided into three types.

• Permanent winds - The trade winds, westerlies, and easterlies are the permanent winds. These blow constantly throughout the year in a particular direction.
• Seasonal winds - These winds change their direction in different seasons. For example monsoons in India.
• Local winds - These blow only during a particular period of the day or year in a small area. For example, land and sea breeze. Do you recall the hot and dry local wind of northern planes of India? It is called a loo.
• On the basis of mechanism, there are three types of rainfall: convectional rainfall, Orographic rainfall, and the cyclonic rainfall
  

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