·
The
air is a mixture of several gases and it encompasses the earth from all sides.
·
The
air surrounding the earth is called the atmosphere.
·
Nitrogen
and oxygen are the two main gases in the atmosphere and 99 percentage of the
atmosphere is made up of these two
gases.
Chemical Constituents of the
Atmosphere:
1.
Nitrogen (N2) - 78% (by volume).
2.
Oxygen (O2) - 21%.
3.
Argon (Ar) - 1%.
4.
Carbon Dioxide (CO2) - 0.038% (376 ppm) and rising.
Structure of the Atmosphere:
Troposphere:
·
It
is the lowermost layer.
·
Average
height is 13 km with 16-18 km at the equator and 6-8 km at the poles.
·
75-80%
of total mass.
·
Temperature
generally decreases with height; Average lapse rate is 6.5°/km.
·
Referred
to as the "weather
layer".
·
Climate
and weather changes occur here.
·
Zone
separating troposphere from stratosphere is called tropopause.
Stratosphere:
•
It
extends up to a height of 50 km.
•
Temperature
is constant with height initially, then it increases with height, known as an
inversion (temperature increases with height).
•
One
important feature of stratosphere is that it contains a layer of ozone gas.
•
This
layer is almost free from clouds and associated weather phenomenon,
making
conditions most ideal for flying aeroplanes.
·
So
aeroplanes fly in lower stratosphere, sometimes in upper troposphere where
weather is calm.
·
Sometimes,
cirrus clouds are present at lower levels in this layer.
Ozonosphere:
•
It
lies at an altitude between 30 km and 60 km from the earth’s surface.
•
This
layer reflects the harmful ultraviolet radiation.
•
The
ozonosphere is also called chemosphere.
Mesosphere (from the Greek meso
meaning middle):
•
It
extends up to a height of 80 km.
•
The
temperature gradually falls to -100°C at 80 km altitude.
•
Meteorites burn up in this layer on entering
from the space.
Thermosphere (from the Greek
therme meaning heat):
•
In
thermosphere temperature rises very rapidly with increasing height.
•
It
extends between 80-400 km.
•
Ionosphere is a part of this layer.
•
This
layer helps in radio
transmission.
•
The
International Space Station and satellites orbit in this layer.
•
Aurora’s
are observed in lower parts of this layer(Aurora Borealis and Aurora
Australis).
•
Topped
by exosphere.
Ionosphere:
•
This
layer is located between 80 km and 400 km and is an electrically charged layer.
•
This
layer is characterized by ionization of atoms.
•
Because
of the electric charge, radio waves transmitted from the earth are reflected
back to the earth by this layer.
•
Temperature
again starts increasing with height because of radiation from the sun.
Exosphere:
•
Light
gases like helium and hydrogen float into the space from here.
•
It
is the outermost layer.
•
This
layer coincides with space.
Vertical Structure of
Composition:
1.Homosphere:(homo means same):
•
Well
mixed, the chemical composition is constant.
•
This
layer extended up to 80 km from the surface.
•
This
part of the atmosphere continually circulates, so that the principal
atmospheric gases are well mixed.
2.Heterosphere:(hetero means
different):
•
Settling
of gases according to weight above 80 km.
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