Composition and Structure of Atmosphere

In this article, we will learn about the Composition and Structure of Atmosphere. But before we do so, let’s understand what do we mean by Atmosphere.

What is an Atmosphere?

Atmosphere is all around us. It can be traced upto a height of 10,000 km, but 90% of its mass is confined within 30 km from earth’s surface.

Atmosphere is a mixture of different gases around the earth, held to the earth by the force of gravity.

This mixture is called air. It is colourless and odourles.

Composition of Atmosphere

The atmosphere is mainly composed of:

• Gases
• Water vapour
• Dust particles

Gases

Nitrogen

Nitrogen (N₂) is the most abundant gas in Earth’s atmosphere (around 78%).

Oxygen

Oxygen (O₂) is around 21% of all the gases in the atmosphere. It’s almost in negligible quantity after the height of 120 km. Humans and animals need O₂.

Carbon di-oxide

Carbon di-oxide (CO₂) is found only up to 90 km from the surface of the earth. Plants need CO₂.

Whenever we talk about Carbon di-oxide, we often also talk about Green house effect.

Green house effect – CO₂ is transparent to the incoming solar radiation but opaque to the outgoing terrestrial radiation.

It is rising in volume - Mainly because of the burning of fossil fuels. So, we are witnessing enhanced green house effect and rising global temperature.

Note

Question: Which is the hottest planet in our solar system?

Answer: It’s Venus, and not Mercury (even though Mercury is closer to the Sun than Venus).

Do you know the reason? It’s because of excessive levels of CO₂ in its atmosphere.

Scientists think that at one point of time, Venus used to be just like Earth. It had atmosphere and oceans. But then the temperature rose and oceans vaporised. Due to excessive levels of water vapour and CO₂ in its atmosphere, greenhouse effect further raised the temperature levels and Venus fell into this vicious trap of ever-increasing temperatures. So much so that, scientists believe that there is rain of liquid metals on Venus.

Ozone

Ozone (O₃) is found between 10 - 50 km above the earth’s surface.

UV filter - absorbs the ultra-violet rays coming from the sun, preventing them from reaching the surface of the earth.

Water Vapour

Amount of water vapour (H₂O in gaseous state) decreases with altitude - Found only up to 90 km from the surface of the earth. (just like CO₂)

Amount decreases from the equator towards the poles – Upto 4% of the air by volume in the warm and wet tropics, to less than 1% in the dry and cold areas of desert and polar regions.

Green house effect – Water Vapour absorbs parts of the insolation from the sun and preserves the earth’s radiated heat. This causes Green house effect.

Water Vapour causes great contrasts in weather and climate over different parts of the world.

Dust Particles

Origin of Dust Particles - Dust Particles come from various sources, such as sea salts, fine soil, smoke-soot, ash, pollen, meteors etc.

Amount decreases with altitude - Dust is generally concentrated in the lower layers of the atmosphere.

Dust is found in higher concentration in subtropical and temperate regions (in comparison to equatorial and polar regions) - due to dry winds.

Cloud Formation - Dust particles act as hygroscopic nuclei, around which water vapour condenses.

Structure of Atmosphere

Atmosphere consists of different layers. The density and temperature of these layers differ a lot.

• Density - It’s highest near the surface of the earth. It decreases with increasing altitude.

• Temperature - We divide the column of atmosphere into five different layers depending majorly upon the temperature conditions:
  
  These layers are:

• troposphere

• stratosphere

• mesosphere

• thermosphere and

• exosphere

Let us study these layers one by one:

Troposphere

Features of Troposphere

• Position - lowermost layer of the atmosphere.
  
  So, it can be considered the most important layer, as far as biological activity is concerned.

• Thickness - Its average height is 13 km.
  
  Thickness is greatest at the equator. (8 km near the poles and about 18 km at the equator)

• Weather & Climate - This layer contains dust particles and water vapour. Most of the climatic and weather related activities are confined to this layer. So, in Climatology we mostly confine ourselves to Troposphere (and to some extent Stratosphere).

Note

Environmental Lapse Rate (ELR)

Actual rate of decline of temperature with increase in height of atmosphere at a particular place and a particular point of time.

Normal Lapse Rate (NLR)

Average of ELR - a decline of about 6.4°C per km rise in the atmosphere OR a decline of 1°C for every 165 m of height.

Tropopause

The zone separating the troposphere from stratosphere.

The temperature here is nearly constant (so called the tropopause). It is about - 80°C over the equator and about - 45°C over the poles.

Why lower temperature over the equator than the poles?

Troposphere has greater height over the equator than the poles. Higher the height, lower will be the temperature.

Stratosphere

Features of Stratosphere

• Position - It is above the tropopause and extends up to a height of 50 km.
  
  It contains the ozone layer.
  

• Temperature – This layer is very cold. The temperature increases with height or remains constant (but there are two contradicting theories on this).

• Here the air is extremely thin, without dust, smoke or water vapour. This layer is cloudless.

Mesosphere

Features of Mesosphere

• Position - It lies above the stratosphere and extends up to a height of 80 km.

• Temperature – decreases with height (reaches up to - 100°C at the height of 80 km).

Mesopause - The upper limit of mesosphere.

Thermosphere

Features of Thermosphere

• Position - located between 80 and 400 km above the mesopause.

• Temperature – increases with height (reaches up to 1700°C).

Exosphere

Features of Thermosphere

• Position - uppermost layer of the atmosphere above the thermosphere. It gradually merges with the outer space.

Ionosphere

Ionosphere is a special layer that overlaps the mesosphere, thermosphere, and exosphere.

It’s very active part of the atmosphere, and grows and shrinks as it absorbs energy from the sun. Gases in these layers are excited by solar radiation to form electrically charged particles known as ions. Hence, this is called ionosphere.

Features of Ionosphere

• Auroras - In ionosphere, ions are affected by the magnetic fields of both Earth and the sun. This is where auroras (bright, beautiful bands of light) happen. They are caused by high-energy particles from the sun interacting with the atoms in ionosphere.

• Radio transmission - Radio waves that are transmitted from the earth are reflected back to the earth by this layer. So, this layer makes short-wave radio transmission possible over long distances.