ICSE Class 7 Answered

The atmosphere is divided into five layers beginning from the surface of the Earth. These are:

Troposphere

• It is the most important layer of the atmosphere and extends up to 13 km above the surface of the Earth.
• Oxygen required for respiration and survival of life exists in this layer of the Earth.
• The temperature decreases in this layer with an increase in the height.
• All the weather phenomena like rainfall, hail and fog occur in this layer as clouds are formed in this layer.
• The upper limit of the troposphere is known as the tropopause. Temperatures may be as low as -58°C at this level.

Stratosphere

• This layer of atmosphere lies above the troposphere and extends up to a height of 50 km.
• Airplanes fly in this layer, as this layer is free from clouds and other weather phenomena. This layer is also free from the water vapour and dust particles.
• Stratosphere contains a layer of ozone gas which protects us from the harmful ultraviolet rays of the Sun.
• In the higher levels of the stratosphere, the temperature increases with height due to the absorption of UV rays by the ozone layer.
• The upper part of stratosphere is known as stratopause.

 Mesosphere

• This is the third layer of the atmosphere and lies above the stratosphere up to a height of 80 km.
• It is in this layer that the meteorites burn when they enter into the atmosphere from space.
• The upper layer of mesosphere is known as the mesopause.

Thermosphere or Ionosphere

• This layer extends from 80-400 km.
• Temperature increases drastically in this layer and can go up to 1,480°C. It is because the molecules in this layer absorb the Sun’s UV rays.
• The thermosphere helps in the transmission of radio waves as the latter is reflected back to the Earth by this layer.

Exosphere

• It is the uppermost layer of the Earth and has a very thin layer of air.
• Light gases like helium and hydrogen float into the space from this layer due to lack of gravity.
• This layer extends from 480 km to 960 km above the Earth.
• Temperature increases with height in this layer. The temperature may well go beyond 1,600°C.