What is the importance of the atmosphere?
Air is essential to the survival of all organisms. Some organisms like
humans may survive for some time without food and water but can‘t survive even a few minutes
without breathing air. That shows the reason why we should understand the atmosphere in greater
detail.
Define The Atmosphere
The atmosphere is a mixture of different gases and it envelopes the earth all round. It contains lifegiving
gases like oxygen for humans and animals and carbon dioxide for plants.
What is the average height of the Atmosphere?
The air is an integral part of the earth‘s mass and 99 per cent of the total mass of the atmosphere is
confined to the height of 32 km from the earth‘s surface. The air is colourless and odourless and can
be felt only when it blows like the wind.
Can you imagine what will happen to us in the absence of ozone in the atmosphere?
In the absence of Ozone, life is not possible on the earth surface.
COMPOSITION OF THE ATMOSPHERE
The atmosphere is composed of gases, water vapour and dust particles. The proportion of gases
changes in the higher layers of the atmosphere in such a way that oxygen will be almost in negligible
quantity at the height of 120 km. Similarly, carbon dioxide and water vapour are found only up to
90 km from the surface of the earth.
COMPOSITION AND STRUCTURE OF ATMOSPHERE
Study the above table showing the composition of the
atmosphere and answer the following questions.
1. Which gas constitutes the highest % of atmosphere?
2. Name the gas which constitutes least % of
atmosphere
CHAPTER-8 COMPOSITION AND STRUCTURE OF ATMOSPHERE
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Gases
Carbon dioxide is meteorologically a very important gas as it is transparent to the
Incoming solar radiation but opaque to the outgoing terrestrial radiation. It absorbs a part of
terrestrial radiation and reflects back some part of it towards the earth‘s surface. It is largely
responsible for the greenhouse effect.
The volume of carbon dioxide has been rising in the past few decades mainly because of the burning
of fossil fuels. This has also increased the temperature of the air.
Ozone is another important component of the atmosphere found between 10 and 50 km above the
earth‘s surface and acts as a filter and absorbs the ultra-violet rays radiating from the sun and
prevents them from reaching the surface of the
earth.
Water Vapour
Water vapour is also a variable gas in the atmosphere, which decreases with altitude. In the warm
and wet tropics, it may account for four per cent of the air by volume, while in the dry and cold
areas of desert and polar regions, it may be less than one per cent of the air. Water vapour also
decreases from the equator towards the poles. It also absorbs parts of the insolation from the sun
and preserves the earth‘s radiated heat. It thus acts like a blanket allowing the earth neither to
become too cold nor too hot. Water vapour also contributes to the stability and instability in the air.
Dust Particles
The atmosphere has a sufficient capacity to keep small solid particles, which may originate from
different sources and include sea salts, fine soil, smoke-soot, ash, pollen, dust and disintegrated
particles of meteors. Dust particles are generally concentrated in the lower layers of the
atmosphere; yet, convectional air currents
may transport them to great heights. The higher concentration of dust particles is found in
subtropical and temperate regions due to dry winds in comparison to equatorial and polar regions.
Dust and salt particles act as hygroscopic nuclei around which water vapour condenses to produce
clouds.
STRUCTURE OF THE ATMOSPHERE
1.The atmosphere consists of different layers with varying density and temperature. 2. Density is
highest near the surface of the earth and decreases with increasing altitude.
3.The column of the atmosphere is divided into five different layers depending upon the temperature
condition.
Name the layers of the atmosphere
They are troposphere, stratosphere, mesosphere, thermosphere and exosphere.
The troposphere
1.It is the lowermost layer of the atmosphere.
2. Its average height is 13 km
3. extends roughly to a height of 8 km near the poles and about 18 km at the equator.
4. Thickness of the troposphere is greatest at the equator because heat is transported to great
heights by strong convectional currents.
5. This layer contains dust particles and water vapour.
6. All changes in climate and weather take place in this layer.
7. The temperature in this layer decreases at the rate of 1 °C for every 165 m of height.
8. This is the most important layer for all biological activity.
9. The zone separating the troposphere from the stratosphere is known as the tropopause.
The air temperature at the tropopause is about minus 800 °C over the equator and about minus 45°
C over the poles.
The temperature here is nearly constant, and hence, it is called the
tropopause.
The stratosphere
1. It is found above the tropopause and extends up to a height of 50 km.
2. One important feature of the stratosphere is that it contains the ozone layer.
3. This layer absorbs ultra-violet radiation and shields life on the earth from an intense, harmful form
of energy.
The mesosphere
1. It lies above the stratosphere,
2. which extends up to a height of 80 km.
3. In this layer, once again, the temperature starts decreasing with the increase in altitude and
Up to minus 100°C at a height of 80 km.
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4. The upper limit of the mesosphere is known as the mesopause.
The ionosphere
1 . It is located between 80 and 400 km above the mesopause.
2. It contains electrically charged particles known
as ions, and hence, it is known as the ionosphere.
3. Radio waves transmitted from the earth are reflected back to the earth by this layer.
4. Temperature here starts increasing with height.
5. The uppermost layer of the atmosphere above reaches up to minus 100
Exosphere
1. the thermosphere is known as the exosphere.
2. This is the highest layer but very little is known
about it.
3. Whatever contents are there, these are extremely rarefied in this layer, and it gradually merges
with the outer space.
Elements of Weather and Climate
The main elements of the atmosphere which are subject to change and which influence human life on
earth is
1. temperature,
2. pressure,
3. winds,
4. humidity,
5. clouds
6. precipitation.