The atmosphere is a protective covering around us
and broadly segmented into different zones. Each zone possesses specific
characteristics and composition. On the basis of temperature atmosphere is divided
into four major zones. The ascending order of these temperature zones of
atmosphere with respect to distance from earth is given as follows
1. Troposphere
2. Stratosphere
3. Mesosphere
4. Thermosphere
1. Troposphere:
This region of atmosphere lies nearest to the
surface of the earth and possesses about 70% of the total mass of atmosphere. This
the region of the atmosphere where life exists thus included in biosphere. The characteristics
of this zone are discussed one by one
a)
Distance
from earth’s surface
It starts from the surface of the
earth in form of gaseous phase and extends upto 11 km high from the surface of
the earth. To illustrate it further, it ranges from 0-11 km from the surface of
the earth.
b)
Composition
Fundamental chemical species in
this region are oxygen (O2), carbon dioxide (CO2),
nitrogen (N2) and water vapors (H2O). Water vapors mostly
evaporate from the surface thus usually present in the lower region of this
zone. It contains about 99% of the total water vapors of atmosphere. Constant flow
of air mix up these vapors with different gasses randomly.
c)
Lapse
rate
Laps rate can be defined as change
in temperature with respect to increase in altitude. The range of temperature
in troposphere is -56-15oC. Troposphere shows a negative lapse rate
as temperature shows a uniform decrease with increasing the altitude. At sea
level temperature of this zone is 15oC which decreases upto -56oC
at the height of 11 km above the sea level.
d)
Tropopause
Tropopause is a cold layer at the
boundary of this zone and stratosphere where temperature inversion occurs. This
is also called transition layer of troposphere above which temperature changes
dramatically as the next zone of troposphere starts.
e)
Residence
time of particles
Air is not uniform in this zone and
most of weather depends on conditions of troposphere. Non uniformity of air and
constant flow of air does not allow the molecules or pollutants to reside for a
long time in air.
f)
Density
Density of troposphere is directly proportional
to the altitude and it shows an exponential decrease with increasing the
altitude.
2. Stratosphere:
This is the second zone of atmosphere which lies immediately
above the troposphere. The main characteristics of this zone are given as
follows
a)
Distance
from earth’s surface
Stratosphere starts at the height
above 11 km from the surface of the earth and extends upto 50 km. It ranges
from 11 km 50 km above the earth’s surface.
b)
Composition
Stratosphere contains ozone layer (O3)
which serves as a protective shield from the hostile outer space.
c)
Lapse
rate
Stratosphere possesses a positive
lapse rate and is also named as quiescent layer as it shows an increasing trend
in temperature with increase in altitude. Temperature in this region increases
from -56oC at 11 km to -2oC at 50 km. The main
contributing factor of increasing temperature in the region is that ozone layer
absorbs ultraviolet radiations.
d)
Stratopause
This is the transition layer
between stratosphere and the next region of the atmosphere named mesosphere. At
stratosphere temperature inversion occurs among two zones.
e) Residence time of particles
The residence time of pollutants
and molecules is long in this region due to slow mixing mechanism in
stratosphere. The clouds do not mix up with air in this region thus the air is
usually dry. The hazardous effects of pollutants in stratosphere are more
drastic as compared to troposphere due to prolong residence time.
3. Mesosphere:
This is the third zone of the atmosphere lies
immediately above the stratosphere. Characteristic features of mesosphere are
discussed as follows
a)
Distance
from earth’s surface
This region starts from 50 km above
the earth’s surface and extends upto 85 km. this zone ranges from 50 to 80 km.
b)
Composition:
Chemical constituents of this
region are O2+ and NO+.
c)
Lapse
rate:
Mesosphere possesses a negative
lapse rate as the temperature in this zone decreases with increase in height. The
average temperature of the mesosphere decreases from -2oC at 50 km
height from surface of the earth to -96oC at a height of 85 km from
the earth’s surface. Minute amount of ozone and other ultraviolet absorbing
species are basically responsible for the temperature inversion in this region.
d)
Mesopause
This is the transition layer
between mesosphere and thermosphere where lapse rate changes from negative in
mesosphere to positive in thermosphere.
4. Thermosphere:
This is the fourth and last zone of atmosphere which
lies above the mesosphere and separated from mesosphere via mesopause. The characteristics
of this zone are discussed as follows
a)
Distance
from earth’s surface
This zone of atmosphere starts from
the height of 85 km above the surface of the earth and extends upto 500 km. the
range of thermosphere is from 85-500 km from earth’s surface.
b)
Composition
Positive ions including O2+,
O+ and NO+ are the main chemical species of thermosphere.
c)
Lapse
rate
Thermosphere possesses a positive lapse rate as temperature in this zone increases uniformly with increase in temperature. The temperature in thermosphere at the height of 85 km immediately above the mesopause is -92oC which rises upto 1200oC at the height of 500 km. Solar radiations in this region are being absorbed by oxygen and nitric oxide gasses which give rise to ionization responsible for the increase in temperature in this region. The lower region in this zone is specifically called as ionosphere due the presence of ionic species. The air in the upper region of this zone is quite less dense with respect to chemical species.
Figure 1 shows the various components of the atmosphere possessing different lapse rate.
0 Comments