Composition And Structure Of Atmosphere

The atmosphere is the gaseous envelope surrounding the Earth.

It is composed of:

• Gases
• Water vapour
• Dust particles

The composition of the atmosphere is not exactly the same at all heights. The proportion of gases changes in the higher layers of the atmosphere.

On the basis of residence time and composition, atmospheric gases are divided into:

• Permanent gases
• Variable gases

Composition Of The Atmosphere

The atmosphere contains several gases, but a few gases form most of its volume.

Major Atmospheric Gases

Constituent	Percentage By Volume
Nitrogen (N₂)	78.08
Oxygen (O₂)	20.95
Argon (Ar)	0.93
Carbon dioxide (CO₂)	0.036
Neon (Ne)	0.002
Helium (He)	0.0005
Krypton (Kr)	0.0001
Xenon (Xe)	0.00009
Hydrogen (H₂)	0.00005

Nitrogen is the most abundant gas in the atmosphere, followed by oxygen.

Permanent Gases

Permanent gases remain in the atmosphere for long durations.

Their composition remains nearly constant.

Important permanent gases include:

• Nitrogen
• Oxygen
• Argon
• Neon
• Helium
• Krypton
• Xenon
• Hydrogen

Variable Gases

Variable gases remain in the atmosphere for a short time and their concentration changes from place to place and time to time.

They are also important because many of them contribute to the greenhouse effect.

Important Variable Gases

Constituent	Percentage
Water vapour (H₂O)	0–4%
Carbon dioxide (CO₂)	0.038%
Methane	0.00017%
Nitrous oxide	0.00003%
Ozone	0.000004%
Dust particles	0.000001%

Carbon Dioxide

Carbon dioxide is meteorologically very important.

It is transparent to incoming solar radiation but opaque to outgoing terrestrial radiation.

This means:

• Solar radiation can pass through it.
• Terrestrial radiation from the Earth is partly absorbed by it.
• Some heat is reflected back toward the Earth’s surface.

Because of this, carbon dioxide is largely responsible for the greenhouse effect.

The volume of most other gases remains nearly constant, but carbon dioxide concentration has increased in recent decades.

The main reason for this increase is the burning of fossil fuels.

Ozone

Ozone is an important component of the atmosphere.

It is mainly found between 15 Km and 35 Km above the Earth’s surface in the stratosphere.

Ozone acts as a filter.

It absorbs harmful ultraviolet radiation from the Sun and prevents it from reaching the Earth’s surface.

Water Vapour

Water vapour is a variable gas.

Its concentration:

• Decreases with altitude
• Decreases from the equator toward the poles

Water vapour acts like a blanket.

It prevents the Earth from becoming too hot or too cold.

It also plays an important role in:

• Atmospheric stability
• Atmospheric instability
• Cloud formation
• Precipitation

Dust Particles

Dust particles are also present in the atmosphere.

They originate from different sources such as:

• Sea salts
• Fine soil
• Smoke
• Soot
• Ash
• Pollen
• Dust
• Meteoric particles

Dust particles are generally concentrated in the lower layers of the atmosphere.

However, convective air currents may carry them to higher altitudes.

They are more concentrated in temperate and subtropical regions because of dry winds.

They are less concentrated in polar and equatorial regions.

Dust particles act as hygroscopic nuclei around which water vapour condenses to form clouds.

Structure Of The Atmosphere

The atmosphere has different layers.

These layers differ in:

• Density
• Temperature

Density is highest near the Earth’s surface and decreases with increasing altitude.

On the basis of temperature characteristics, the atmosphere is divided into five layers:

• Troposphere
• Stratosphere
• Mesosphere
• Thermosphere or Ionosphere
• Exosphere

Troposphere

The troposphere is the lowest layer of the atmosphere.

Its average height is about 13 Km.

It extends:

• About 8 Km near the poles
• About 18 Km near the equator

The troposphere is thicker at the equator because strong convection currents transport heat upward.

The word troposphere means “region of mixing or change.”

Most weather changes occur in this layer.

Features Of Troposphere

• It is the layer closest to the Earth’s surface.
• All weather phenomena occur here.
• Rainfall, fog, hailstorms and storms occur in this layer.
• It contains most of the water vapour and dust particles.
• Temperature decreases with height.

Normal Lapse Rate

In the troposphere, temperature decreases at an average rate of about 6.5°C per 1 Km.

This decrease in temperature with height is called the normal lapse rate.

The reason is that tropospheric gases absorb little solar radiation directly.

Instead, the Earth’s surface absorbs solar radiation and heats the air through conduction and convection.

Tropopause

The tropopause is the boundary between the troposphere and stratosphere.

Temperature remains nearly constant here.

It is about:

• -80°C over the equator
• -45°C over the poles

Stratosphere

The stratosphere is the second layer of the atmosphere.

It lies above the troposphere.

It extends from the tropopause to about 50 Km above the Earth’s surface.

Features Of Stratosphere

• It contains the ozone layer.
• It is almost free from clouds.
• Commercial airplanes fly in this layer.
• Temperature increases with height.

The temperature increases because ozone absorbs ultraviolet radiation and converts it into heat.

About 90% of atmospheric ozone is found in the stratosphere.

This is called good ozone.

The remaining 10% occurs in the troposphere and is often called bad ozone.

Stratopause

The boundary between the stratosphere and mesosphere is called the stratopause.

Mesosphere

The mesosphere lies above the stratopause.

It extends from about 50 Km to 85 Km.

Temperature decreases with altitude in this layer.

It may reach around -90°C, making it the coldest layer of the atmosphere.

Features Of Mesosphere

• It is the third layer of the atmosphere.
• It lies directly above the stratosphere.
• Temperature decreases with height.
• Most meteors burn up in this layer due to friction with atmospheric gases.

Mesopause

The boundary between the mesosphere and thermosphere is called the mesopause.

Thermosphere Or Ionosphere

The thermosphere extends from about 85 Km to 500-600 Km above the Earth.

Temperature may rise to 1500-2000°C or more.

This rise occurs because high-energy solar radiation such as X-rays and ultraviolet radiation is absorbed in this layer.

This energy causes ionization of gases.

The ionized region is called the ionosphere.

Features Of Thermosphere Or Ionosphere

• Gas molecules break into ions and electrons.
• Many satellites and the International Space Station orbit within this region.
• Auroras occur in this layer.
• The ionosphere reflects radio waves.

Charged particles from solar wind collide with oxygen and nitrogen atoms.

This produces colourful lights near the poles called:

• Aurora Borealis – Northern Lights
• Aurora Australis – Southern Lights

The ionosphere is important for long-distance communication and navigation because it reflects radio waves.

Thermopause

The upper boundary of the thermosphere is called the thermopause.

Exosphere

The exosphere is the outermost layer of the atmosphere.

It lies above the thermosphere.

This layer gradually merges with outer space.

Features Of Exosphere

• Air particles are extremely sparse.
• Earth’s gravitational pull is very weak here.
• Atoms and molecules can escape into space.

Magnetosphere

The outermost region of Earth’s magnetic influence surrounding the planet is called the magnetosphere.

It is associated with Earth’s magnetic field and helps interact with charged particles coming from space.

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