Chapter 4: Composition of the Atmosphere

  • The air in earth’s atmosphere is made up of approximately 78 % Nitrogen and 21 % Oxygen.

  • Nitrogen and Oxygen are two gases which make up the bulk of the atmosphere.

  • Carbon dioxide, Helium, Ozone, Argon and Hydrogen are found in lesser quantities.

Some Important Gases

  • Nitrogen is the most plentiful gas in the air. When we inhale, we take some amount of nitrogen into our lungs and exhale it.

  • But plants need nitrogen for their survival. They can not take nitrogen directly from the air.

  • Bacteria, that live in the soil and roots of some plants, take nitrogen from the air and change its form so that plants can use it

Some Important Gases

  • Oxygen is the second most plentiful gas in the air Humans and animals take oxygen from the air as they breathe.

  • Green plants produce oxygen during photosynthesis.

  •  In this way oxygen content in the air remains constant. If we cut trees then this balance gets disturbed.

Gases Affecting The Atmosphere

  • Carbon dioxide is another important gas. Green plants use carbon dioxide to make their food and release oxygen.

  • Humans or animals release carbon dioxide. The amount of carbon dioxide released by humans or animals seems to be equal to the amount used  by the plants which make a perfect balance.

  • However, the balance is upset by burning of fuels, such as coal and oil.

  • They add billions of tons of carbon dioxide into the atmosphere each year  As a result, the increased volume of carbon dioxide is affecting the earth’s weather and climate.

Green House

  • The atmosphere creates a green house effect by trapping the heat radiated from the earth.

  • It is therefore called a greenhouse gas and without it the earth  would have been too cold to live in.

  • When its level in the atmosphere increases due to factory smoke or car fumes the heat retained, increases the temperature of the earth. This is called global warming.

Layers Of  The Atmosphere

  • Our atmosphere is divided into five layers starting from the earth surface.

  • The five layers are Troposphere, Stratosphere, Mesosphere, Thermosphere and Exosphere


  • 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.


  • It is the second layer of the atmosphere as you go upward. Above the troposphere.

  • It extends up to a height of 50 km.

  • The ozone layer is found within the stratosphere, this layer is almost free from clouds and associated weather phenomenon, making conditions most ideal for flying aero planes.


  • This is the third layer of the atmosphere above the stratosphere and below the thermosphere

  • It extends up to the altitude of 80 km.

  • The term near space is also sometimes used to refer to altitudes within the mesosphere. This term does not have a technical definition, but typically refers to the region of the atmosphere up to 100 km


  • The thermosphere is the layer in the Earth’s atmosphere directly above the mesosphere and below the exosphere.

  • Ionosphere is a part of this layer.

  • It extends   between 80-400 km.

  • This layer helps in radio transmission.


  • Exosphere is the outermost region of a planet’s atmosphere.

  • The exosphere is a thin, atmosphere-like volume surrounding a planet or natural satellite where the density is too low for them to behave as a gas by colliding with each other.

  • Light gases like helium and hydrogen float into the space from here.


  • The degree or intensity of heat present in a substance or object, especially as expressed according to a comparative scale and shown by a thermometer or perceived by touch.

  • As you rise up through the atmosphere, the temperature can vary greatly.


  • An important factor that influences the distribution of temperature is  insolation

  • Insolation  is the solar radiation that reaches the earth’s surface.

  • The amount of insolation  decreases from the equator towards pole therefore the temperature decreases in the same manner.

  • Thermometer measures the temperature

Air Pressure

  • Air pressure or atmospheric  pressure  is the pressure within the atmosphere of earth or the pressure exerted by the weight of air on the earth’s surface.

  • The air presses us from all directions and our body exerts a counter pressure.

  • Pressure on Earth varies with the altitude of the surface so air pressure on mountains is usually lower than air pressure at sea level.

  • Atmospheric pressure is measured by Barometer


  • Wind is the flow of gases on a large scale on the surface of the Earth, wind consists of the bulk movement of air from high pressure area to low pressure area.

  • The three types of winds.

  • Permanent /  Planetary winds

  • Periodic / seasonal winds

  • Local winds

  • Permanent winds –

  • Permanent winds are those which blow continuously in the same belt all through the year. They blow from high pressure to low pressure area.

  • Trade winds, westerlies and easterlies are the permanent winds 

Seasonal winds

  • These winds blow according to the season.

  • These are the winds blowing at a much smaller scale in comparison to the planetary winds.

  • Local winds

  •  They occur over a much smaller land scale, blow during a particular period of the day in a small area.

  • It is called Loo in northern planes of India


  • A cyclone is a large scale air mass that rotates around a strong center of low atmospheric pressure.

  • Astorm or system of winds that rotates about a center of low atmospheric pressure, advances at a speed of 20 to 30 miles (about 30 to 50 kilometers) an hour, and often brings heavy rain.

Types of rainfall

  • There are three main types of rainfall.
  • Convectional rainfall
  • Cyclonic/Frontal rainfall
  • Orographic /Relief rainfall

Convectional rainfall

  • Convectional rainfall occurs when   the energy of the sun heats the surface of the Earth, causing  water to evaporate to form Water  vapour.     
  • When the land heats up, it warms the  air above it. This causes the air to expand and rise
  • As the air rises, it cools and becomes saturated and dew point temperature is attained and then clouds will form. By further cooling, precipitation  takes place.

Cyclonic rainfall

  • Cyclonic  rainfall is caused by cyclonic  activity and it occurs along the  fronts of the cyclone.

  •  It is formed when two masses of air of different temperature, humidity and  density meets.

  • A layer separating them is called the front, this front has two parts , the warm front and the cold front.

  • At the warm front, the warm lighter wind rises gently over the heavier cold air, which being heavy stays close to the ground.

  • As the warm air rises, it cools, and the moisture present in it condenses to form clouds.

Relief rainfall

  • Relief or Orographic rain is formed when air is forced to cool when it rises over relief features in the landscape  such as hills or mountains. As it rises it cools, condenses and forms rain.
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