The Earth’s Atmosphere

The Earth’s atmosphere is a blanket of gases that surrounds the planet and is retained by the Earth’s gravity. It is most commonly referred to as air, extending to about 10000km from the Earth surface. The atmosphere is a mixture of several gases, the major gases of which are nitrogen and oxygen accounting for about 99% of its volume. The remaining 1% comprises of trace gases which are carbon dioxide, methane, argon, neon, ozone along with water vapour. These trace gases (excluding argon) are called greenhouse gases. The greenhouse gases absorbs heat from the sun and spread the warmth across the planet thereby maintaining the Earth’s temperature so that it is neither too hot nor too cold. This is called the greenhouse effect. However, when the percentage of the greenhouse gases in the atmosphere increases, it leads to the Earth receiving excessive warming which results to a concept known as global warming.

The atmosphere is regarded as the Earth’s protective ceiling. The protective layer of gases that make up the Earth’s atmosphere is what makes it the only planet in our solar system that can sustain life. This is because it supplies optimally the essential gases (oxygen and carbon dioxide) needed for living things to survive. It protects the Earth against harmful ultraviolet radiation from the Sun which are capable causing severe damages to the skin and eyes.  The weather in the Earth is created by pressure systems in the atmosphere. Changes in the atmospheric temperature and pressure sets air in motion resulting to wind.

The Atmospheric layers

The Earth’s atmosphere is said to be stratified. It is divided into five major layers which are based upon the difference in temperature and density.

The five major layers includes

  • Troposphere
  • Stratosphere
  • Mesosphere
  • Thermosphere
  • Exosphere
The Atmosphere of the Earth

The atmospheric profile showing the variations of temperature with increasing altitude.

Troposphere

The troposphere is the lowest layer whose depth varies among regions. It has a depth of approximately 8 – 16km and is affected by the altitude of a place, being higher near the equator and lower near the poles. It is also affect by seasons and time of the day. The troposphere controls to a great extent the weather condition of a place. It contains 99% of the water vapour in the Earth’s entire atmosphere and approximately 80% of its mass.

This region of the atmosphere is not static. It is constantly changing in temperature, pressure and density. Temperature decreases with increasing altitude. This is because the troposphere is heated through transfer of energy from the Earth surface. Most weather associated clouds are formed in the troposphere.

Stratosphere

This is the second lowest layer lying just above the troposphere. The transition zone between the troposphere and the stratosphere is called tropopause. The stratosphere has a depth of about 40 – 45km and it extends to about 50-55km from the surface of the Earth. The rise in temperature in the stratosphere remains constant with altitude for some distances and at roughly half the depth, temperature begins to increase with increasing altitude. This rise in temperature is caused by a layer within the stratosphere known as ozone layer which absorbs and scatters ultraviolet radiation from the Sun. Hence the top of the stratosphere is much warmer than the parts closer to the tropopause.

The temperature in the stratosphere creates very stable atmospheric conditions. Therefore, there are almost no weather phenomena and almost no clouds in this layer making it the perfect layer for aviation. It is the highest layer that can be accessed by jet-powered aircrafts.

Mesosphere

This is the third layer of the atmosphere lying just above the stratosphere. The transition zone between the stratosphere and the mesosphere is called stratopause. The mesosphere extends up to about 80km from the surface of the Earth, spanning a height of about 30km.

Temperature decreases with increasing altitude in this layer. The top of the mesosphere (called mesopause) is the coldest part of the Earth’s atmosphere with an average temperature of -85 degrees Celsius.

The mesosphere is the region where most meteors burn up completely upon entrance in the atmosphere. The very little water vapour found in this region is sublimated under very low temperature to form the highest clouds in the atmosphere. Only rocket-propelled aircrafts can access this layer.

Thermosphere

This is the fourth layer of the atmosphere occupying the area above the mesosphere and below the exosphere. It starts from about 90km from the surface of the Earth and extends to about 420 to 1000km from the surface of the Earth. Temperature in this region gradually increases with altitude and density is low. The International Space Station orbits in this layer at a height of350km to 420km.

The lowest part of the thermosphere is known as ionosphere. This is where electrically charged ions flow. Charged particles from space collide with atoms and molecules in the thermosphere, making it excited into higher states of energy. These atoms shed the excess energy by emitting photons of light which is seen as Auroras in the thermosphere. Auroras are polar lights which may be Auroras Borealis (Northern lights) or Auroras Australis (Southern lights). Radio broadcasting is possible in this region because the radio waves are reflected back to the Earth by the ionosphere.

Exosphere

This is the fifth layer of the atmosphere and it starts from the thermopause (which is the transition zone between the exosphere and the thermosphere) at a height of about 1000km from the surface of the Earth and extends up to 10000km where it merges with the outer space. It is composed of very widely dispersed particles of hydrogen and helium (though the thermopause contains gases like oxygen, nitrogen and carbon dioxide). The exosphere is so far away from the Earth that meteorological phenomena may not be possible.

Due to lack of gravitational pull, It has very low density and molecules of the gases found can travel easily without any collision and escape into space. The part close to the thermosphere may also experience Auroras

 


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