Air Summary Class 7 Geography Chapter 4

Structure of Atmosphere

Atmosphere is divided into five layers starting from the earth’s surface:

• Troposphere
• Stratosphere
• Mesosphere
• Thermosphere 
• Exosphere

1. Troposphere

• Most important layer as the air we breathe exists here and almost all the weather phenomena like rainfall, fog and hailstorm occur in this layer.
• Its average height is 13 km.

2. Stratosphere

• It lies above the troposphere.
• 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.
• It contains a layer of ozone gas which protects us from the harmful effect of the sun rays.

3. Mesosphere

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

4. Thermosphere

• Temperature rises very rapidly with increasing height in this layer.
• Ionosphere is a part of this layer. 
• It extends between 80-400 km. 
• This layer helps in radio transmission.

5. Exosphere

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

Weather and Climate

•  Weather is this hour-to-hour, day to day condition of the atmosphere.
  • It can change dramatically from day to day.
• The average weather condition of a place for a longer period of time represents the climate of a place.

Temperature

• The degree of hotness and coldness of the air is known as temperature.
• The  temperature  of  the atmosphere changes not only between day and night but also from season to season.
• Insolation is an important factor that influences the distribution of temperature.
  • Insolation is the incoming solar energy intercepted by the earth.
  • The amount of insolation decreases from the equator towards the poles. Therefore, the temperature decreases in the same manner.
• Temperature in cities is much higher than that of villages.
  • The concrete and metals in buildings and the asaphalt of roads get heated up during the day which is released during the night.

Air Pressure

• 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 temperature of air at a given place.
• Where temperature is high the air gets heated and rises which creates a low-pressure area.
  • Low pressure is associated with cloudy skies and wet weather.
• Where temperature is low, the air is cold and therefore, heavy. Heavy air sinks and creates a high pressure area.
  • High pressure is associated with clear and sunny skies.
• The air always moves from high pressure areas to low pressure areas.

Wind

• The movement of air from high pressure area to low pressure areas is called wind.
• 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.
• The hot and dry local wind of northern planes of India is called loo.

Moisture

• When water evaporates from land and different water bodies, it becomes water vapour.
• Moisture in the air at any time, is known as humidity.→ When the air is full of water vapour we call it a humid day.
• As the air gets warmer, its capacity to hold the water vapour increases and so it becomes more and more humid.
• When the water vapour rises, it starts cooling. 
  • The water vapour condenses causing formation of droplets of water. Clouds are just masses of  such water droplets. 
  • When these droplets of water become too heavy to float in air, then they come down as precipitation.
  • Precipitation that comes down to the earth in liquid form is called rain.
• There are three types of rainfall:
  • the convectional rainfall
  • the orographic rainfall
  • the cyclonic rainfall
• Importance of Rainfall:
  • It is very important for the survival of plants and animals.
  • It brings fresh water to the earth’s surface. 
• If rainfall is less – water scarcity and drought occur. On the other hand if it is more, floods take place.