Msc air

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UNIT - I
LESSON 1 – IMPORTANCE OF ATMOSPHERE
Contents
1.0 Aims and objectives
1.1. Introduction
1.1.1. Importance of atmosphere
1.2. Composition of atmosphere
1.2.1. Importance of atmospheric gases and other constituents
1.2.2. Role of carbon dioxide in atmosphere
1.2.3. Role of other constituents
1.3. Structure of atmosphere
1.3.1. Layers of atmosphere based on composition of constituents
1.3.2. Layers of atmosphere based on temperature variation
1.4. Let us sum up
1.5. Lesson-end Activities
1.6. Points for Discussion
1.7. Check your Progress
1.8. References
LESSON 1 – IMPORTANCE OF ATMOSPHERE
1. 0 AIMS AND OBJECTIVES
The aim of this lesson is to know the importance of atmosphere it, its properties and
structures:
· What is atmosphere?
· What is it made of?
· What is its composition?
· To what height atmosphere does extend?
· Is atmosphere uniform throughout? Or does it have different layers?
1.1 INTRODUCTION
The earth is the only known planet, on which life exists. The present condition and
properties of earth’s atmosphere are one of the main reasons for earth to support life. First
of all, we should know, what is atmosphere? Atmosphere is a gaseous layer surrounding
the earth. In other words, we can say that our earth is surrounded by a thin layer of gases,
called atmosphere. Yes, it is thin when compared to the size of the earth. Yet, this thin
layer has its own influences on various processes that take place on earth. It contains a
mixture of gases with some impurities.
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The atmosphere is special because it contains life-sustaining oxygen in large
quantities (about 21% by volume). In fact, it took millions of years to reach the present
condition by various processes. Along with its development, life came into existence and
evolved.
The aim of this material is that at the end, you will be amazed to know how
perfectly, all the natural processes on earth are functioning harmoniously. In fact, the
processes of the atmosphere do not take place in isolation. Atmosphere constantly
exchange energy and matter with other components of the earth – lithosphere, hydrosphere
and biosphere.
Hereafter, the term, atmosphere is used for earth’s atmosphere. Atmosphere
extends from a few meters below the earth’s surfaces or water’s surface to a height of about
60,000 km. However, about 90% of the atmosphere is within few km from the ground.
Most of the mass of the atmosphere is near planetary surface, as the gravity pulls them
towards the earth’s center.
1.1.1 IMPORTANCEOFATMOSPHERE
Atmosphere is very important to sustain life on earth. It contains life-supporting
oxygen in sufficiently large quantities. Constant concentration of oxygen is maintained
through oxygen cycle. The oxygen cycle does not take place in isolation but It takes place
along with other bio-geo chemical cycles. These cycles connect the atmosphere with
hydrosphere, lithosphere and biosphere. It is send that the atmosphere does not function by
itself, but it functions in conformity with other spheres on earth.
Atmosphere is the transparent layer through which, life-sustaining solar radiation
passes and reaches the earth’s surface or into the water. It is awonder, why solar radiation
is mentioned as “life-sustaining” one. It is mentioned so, because, solar radiation is the
only source to supply energy for photosynthesis to take place on earth, which thereby
supports all other life.
When solar radiation passes through the atmosphere, happen. Harmful ultra violet
radiation was “absorbed” by ozone layer, which is a part of our atmosphere. Ozone layer
prevents about 95% of harmful ultra-violet radiation from reaching the earth’s surface.
Solar radiation when it passes through the troposphere, the lowest layer of thea
atmosphere, part of the radiation, may be reflected back by the clouds, scattered by
atmospheric constituents, thereby reducing visibility, absorbed by atmospheric constituents
and the remainder reach the earth’s surface (land or water).
Heated earth emits the energy in the form of infra red radiation during nighttime
and this radiation is absorbed by carbon dioxide, water and few other gases. This process
results in “green house effect”. Thus, the atmosphere is kept warm during night, it will
become so cool and intolerable for living organisms.
Earth is not heated by solar radiation uniformly due to its inclination. As a result,
different weather patterns exist over the earth. In order to compensate these differences, air
sets in motion resulting in winds and circulation of air. These wind currents are of global

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scale as well as of local scale. They are responsible for disastrous storms like cyclones,
dust storms, tornadoes etc. These wind currents also influence water currents in the
oceans; they in turn affect the wind currents. Consequently, understanding the atmosphere
and its functions and behavior is quite complex. However, with available knowledge,
scientists try to comprehend to the extent possible.
Thus, understanding of the atmosphere has become very essential and important.
All the above processes make the atmosphere, a dynamic atmosphere. In forthcoming
sections, we shall learn more about this life-sustaining sphere, atmosphere. In fact, you
will be fascinated to know about the atmosphere.
Self-check Exercise 1
State two reasons why we should know about our atmosphere?
Note: Please do not proceed unless you write answers for the above two in the space given
below:
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1.2. COMPOSITION OF ATMOSPHERE
The comportion of the atmosphere are gases present in large amounts, water vapor
and solid particles in considerably less amounts. Gases that in the atmosphere are divided
into two kinds, based on their concentration, viz., constant gases and variable gases.
Constant gases are the ones, whose concentrations do not change over time, and
their concentrations almost remain same. But, variable gases are present in different
concentrations at different places and times.
Nitrogen and oxygen are the two major constant gases that make up 99 percent of
the air. Both are important to sustain life on earth. Nitrogen constitutes 78.09% and
oxygen 20.94 percent by volume thus, making the bulk of the atmosphere. The remaining
0.97% is constituted by nitrous oxide and inert gases such as, argon, helium, krypton,
xenon and also by variable gases – carbon dioxide, water vapor and ozone.
1. What are the two major constituents of the atmosphere? Name them with their
average proportions.
2. Distinguish between constant gases and variable gases.
below:
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1.2.1 IMPORTANCE OF ATMOSPHERIC GASES AND OTHER
CONSTITUENTS
As such, nitrogen in its gaseous form in the atmosphere is not that important. It is
non-poisonous. But, indirectly, it is very important, as it is converted into useful forms for
life by certain micro organisms. Nitrogen is very important in the formation of amino
acids, which are building blocks of proteins, and also in the formation of nucleotides,
which are part of the genetic materials, viz., DNA and RNA. This is one of the best
examples that the natural processes are in perfect harmony with each other.
It is needless to stress the importance of oxygen. Every living organism, including
humans, need oxygen for respiration. Respiration is the process through which the
chemical energy (food) is converted into usable form of energy by the living cells.
Inert gases, as such do not play any role in the environment. However, they are
used for commercial purposes such as in neon lights.
1.2.2 ROLE OF CARBON DIOXIDE IN ATMOSPHERE
Carbon dioxide is emitted by all living organisms as an end product of respiration.
This carbon dioxide, in turn is used by producer organisms (green plants and certain micro
organisms) for the synthesis of food. This process is called, photosynthesis. This is
another example how natural processes are inter-related and inter-dependant. As a large
amount of carbon dioxide is utilized by plants, on average, it comprises only 0.04 percent
of dry air. Nevertheless, it plays significant role in keeping the atmosphere at temperatures
that permit life.
The concentration of carbon dioxide, nowadays, is increasing due to human
activities. Today, man burns large quantities of fossil fuels for various purposes. As a
result, huge amounts of carbon dioxide are emitted into the atmosphere. Its ever increasing
concentration has already resulted in global warming and in some places, melting of ice.
Water vapor, though present in small quantities, plays a crucial role. Itis
responsible for cloud formation in the atmosphere and precipitation. Its concentration
varies over time at a given place and at different places. It is an important component of
the atmosphere in determining the weather of a place at a given time. It is responsible for
fog formation during early morning hours in winter, for feeling sultry near coastal regions,
during summer.
Water vapor also absorbs outgoing radiation from earth as CO2 does and it has a
crucial role in green house effect. Thus, both CO2 and H2O along with ozone (in
troposphere), methane and N2O are called, green house gases.

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1.2.3 ROLE OF OTHER CONSTITUENTS
In addition, water present in clouds, can reflect and absorb the part of the incoming
solar radiation. It also determines the weather of a place at a given time to a large extent.
Water vapor is also transported along with the wind and its circulation. As a result, it is
distributed to the parts away from sea. Nevertheless, the places that are far away from sea
will have less moisture content than the places near the sea.
Other variable gases are also present in the atmosphere but in minute quantities.
They are hydrogen, helium, sulfur dioxide, methane, carbon monoxide and oxides of
nitrogen. Some of them are air pollutants, emitted by human activities.
Solid particles are present in minute quantities in the atmosphere. They might have
originated either naturally or by human activities or both. In acceptable quantities, they are
beneficial as they initiate and take part in cloud formation. The solid particles absorb and
scatter the solar radiation. When their concentration goes high, they reduce the visibility of
the atmosphere and are deleterious to human health. They also affect other living
organisms in many ways.
Table 1.1 – Constituents of atmosphere
Gas
Average percentage
(by volume in dry air)
Nitrogen 78.09
Oxygen 20.94
Argon 0.9
Carbon dioxide 0.03
Neon 0.002
Helium 0.0005
Methane 0.0002
Krypton 0.0001
Hydrogen 0.00005
Nitrous oxide (N2O) 0.00005

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1.3 STRUCTUREOFATMOSPHERE
The earth is surrounded by a thin layer of air, called, atmosphere. The atmosphere,
from the surface of earth extends up to 60,000 km. You may wonder how such a thickness
would be called a thin layer. Where, most of the mass of the atmosphere is found near the
planetary surface. It is near the earths surface from surface to about 80 to 100 km. This is
due to the earth’s gravity, which pulls the atmospheric constituents, towards its center. As
a result, the most of the atmosphere are within this thickness.
1.3.1 LAYERS OF ATMOSPHERE BASED ON COMPOSITON OF
CONSTITUENTS
It is the thin layer, in which most of the atmospheric processes take place. According
to the concentration of the gases, atmosphere is divided into:
1. Homosphere: the lower region, extending from the surface of the earth to a height
of 80 to 100 km above the earth. In this layer, gases are more or less uniform in
their chemical composition.
2. Heterosphere: it starts from the upper portion of homosphere and extends to height
of 60,000 km above the earth. In this layer, chemical composition changes with
height. Concentration of gases keep decreasing as one goes up. Inter molecular
distance increases with height and hence, concentration decreases.
Distinguish between homosphere and Heterosphere.
below:
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1.3.2 LAYERS OF ATMOSPHERE BASED ON TEMPERATURE
VARIATION
Atmosphere again can be divided into four distinct layers according to their temperature
characteristics:
1. Troposphere: it is the bottom layer of the atmosphere. It contains 70% mass of the
atmosphere. It extends to an average height of 12 km. However, its thickness
varies with latitude: over the poles, only about 8 km; above the equator, it is about
16 km. A very important feature of this layer is that temperature decreases with
height in this layer. The rate of decrease of temperature with altitude is called,
lapse rate. Average lapse rate in troposphere is -6.4 °C / km. Troposphere ends at
tropopause. Tropopause is just like a lid over the troposphere, where temperature
stops decreasing with height.

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2. Stratosphere: it lies just above the tropopause. It extends to a height of 50 km
from earth’s surface. Ozonosphere, a very important layer is found within this
stratosphere. Ozone present in the ozonosphere prevents the harmful ultra-violet
rays from reaching the earth, thereby protecting the life. Thus, ozonosphere acts as
a protective umbrella.
Stratosphere is a calm layer consisting of relatively clean air. Water vapor in this
layer is almost absent, and hence clouds will not form in this layer. In this layer,
temperature increases with height; just opposite to that in troposphere. This
temperature increase with height prevents the vertical winds. Only horizontal
winds are seen. These horizontal winds flow almost always parallel to the earth’s
surface. Absence of vertical winds and the existence of horizontal winds parallel to
earth’s surface result in relatively calm atmosphere (absence of turbulence). This
ensures smooth travel for fights. Absence of clouds also provide with good
visibility for pilots. All the above features led to the operation of jet air planes in
this layer. The flying of jet air planes was partly responsible for the destruction of
ozone. The detail account on ozone layer depletion will be discussed in the
forthcoming sections. Above the stratosphere, temperature neither decreases nor
increases with height up to some level. This small layer is called, stratopause.
3. Mesosphere: it starts from the edge of the stratopause, just at an approximate
height of 52 km from earth’s surface. It extends to height of 80 km from the
ground. In this layer, temperature decreases with height as in troposphere. This
layer, as such does not have any impact on life. But, it gains importance as it plays
crucial role in radio communication. How does it exhibit its influence? Sunlight
passing through this layer, converts the individual molecules to individual charged
ions i.e. ionization. Ionized particles are concentrated as a zone called the D-layer.
This D-layer reflects radio waves sent from earth. But this D-layer blocks the
communications between earth and astronauts.
In this layer, during summer, at night times, a spectacular display of wispy clouds
can be seen sometimes over high latitudes. It is presumed that meteoric dust
particles coated with ice crystals reflect the sunlight resulting in wispy clouds. Just
above the mesosphere lies, mesopause, in which temperature neither decreases nor
increases.
4. Thermosphere: it is found approximately above 80 km from earth’s surface. It
extends to the edge of space at about 60,000 km from earth’s surface. Temperature
keeps rising with altitude in this layer. It is likely to reach 900 °C at an altitude of
350 km. However, these high temperatures are not felt as in lower layers. The air
molecules are so far apart in this layer. As a result, these temperatures really apply
to individual molecules only. In this layer too, ionization of molecules take place.
It results in individual charged ions. This process produces two ionized belts, viz.,
E- and F-layers. These layers also reflect radio waves and have influence over
radio communications. In the upper thermosphere, further concentration of ions are
seen that comprise the Van Allen radiation belts. This layer is sometimes called,
magnetosphere. It is thus called as earth’s magnetic field has more influence over
the movement of particles rather than earth’s gravitational field. Thermosphere as
such has no definable upper boundary and gradually blends with space.

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What is the significant of troposphere? Explain briefly.
below:
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Figure 1.1 – Vertical temperature variation of the atmosphere and its layers
1.4 LET US SUM UP
In this lesson, the following concepts are
· the importance of atmosphere
· what makes it important

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· why it is important
· the composition of atmosphere
· the structure of atmosphere – layers based on its composition and layers based
on temperature variation
By now, the importance of the atmosphere in sustaining life was understood
atmosphere plays many significant roles in maintaining the temperature of a particular
place. Atmosphere, as it exists, provide suitable conditions for life of a place. It
determines the life forms available at a particular place.
1.5 LESSON-END ACTIVITIES
· Take a sheet of paper and write down points that come to your mind about the
importance of our atmosphere
· Write what will happen if carbon dioxide constituted 21% of atmosphere
· Write down how the temperature varies with increasing altitude
1.6 POINTS FOR DISCUSSION
Now you are well aware that our atmosphere sustains the life. The composition of
atmosphere makes it possible to support life. Carbon dioxide plays a vital role in keeing the
earth warm. The temperature variation vertically, also we learned.
1.7 CHECK YOUR PROGRESS
· Can you state the major constituent of the atmosphere and its percentage?
· Can you divide the atmosphere into layers according to temperature variation with
approximate height of each layer?
· Will you be able to describe the properties of troposphere?
1.8REFERENCES
1. De Blij, H.J. and Muller, P.O. Physical geography of the global environment, John
Wiley & Sons, Inc., New York
2. Miller, Jr., G.T. Environmental Science, Thomson Brroks/Cole, CA, USA, 2004
3. De, A.K. Environmental Chemistry, New Age International Ltd, Publishers, New
Delhi, 1994
4. Rao, M.N. and Rao, H.V.N. Air pollution, Tata McGraw-Hill Publishing Co. Ltd.,
New Delhi
5. Stern, A.C. Air pollution, Academic Press, Inc., New York, 1976
6. Wark, K. and Warner, C.F. Air pollution – its origin and control, IEP A Dun-
Donnelly Publisher, New York, 1976
7. W.H.O. Glossary of air pollution, World Health Organization, Copenhagen, 1980
8. www.en.wikipedia.com
9. Critchfield, H.J. General climatology, Prentice Hall of India Pvt. Ltd., New Delhi,
1987

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LESSON 2 – ELEMENTAL PROPERTIES OF ATMOSPHERE
Contents
1.1. Elemental properties of atmosphere
1.2. Chemical and photochemical reactions
1.2.1. Photochemical reactions n ozone layer
1.3. Ozone layer depletion
1.3.1. Ozone layer depletion by CFCs
1.3.2. Ozone layer depletion by nitric oxide
1.4. Let us sum up
1.8. References
2.0 AIMS AND OBJECTIVES
In the previous lesson, we have learnt the atmosphere, its importance, its
composition and its structure. In this lesson, we will be learning about the properties of
atmosphere. In fact, the atmosphere is not a static gaseous medium; it is rather a dynamic
fluid in which many physical and chemical processes take place throughout.
· About chemical properties of the atmosphere
· Reactions taking place in the atmosphere.
· How ozone layer is depleted by some of the chemical reactions
2.1 ELEMENTALPROPERTIESOFATMOSPHERE
In the preceding section, importance of atmosphere, its temperature profile with
increasing altitude and its constituents. In this lesson, we will learn the chemical properties
of atmosphere and various chemical reactions taking place in it.
2.2. CHEMICALANDPHOTOCHEMICALREACTIONS
As it is mentioned earlier, atmosphere is dynamic. Many chemical and physical
processes are taking place in the atmosphere. Some of them occur naturally while others
occur due to man’s interventions. Atmosphere is a gaseous medium and is very large in
size when compared to any man-made reaction chamber for the reactions to take place.
Now let us see something about chemical and photochemical reactions that occur in
atmosphere.
First, we shall learn the reactions occurring in ozonosphere. These reactions help in
maintaining the concentration of ozone at this altitude. Then we shall move on to learn
some of the reactions occurring in troposphere and the chemical reactions taking place in
troposphere and stratosphere.

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2.2.1. PHOTOCHEMICAL REACTIONS IN OZONE LAYER
The existence of ozone layer was discovered by the French Physicists, Charles
Fabry and Henri Buisson in 1913. It is located in the lower part of the stratosphere
approximately between the altitudes of 15 and 35 km. Concentration of ozone at its
maximum is 10 ppm at an altitude of 25-30 km. Between the altitudes of 15 and 40 km, its
concentration ranges from 2 to 8 ppm. Concentration of ozone is maintained at their levels
in the above altitudes by “ozone-oxygen” cycle. Photochemical mechanisms in ozone-
oxygen cycle were studied by the British Physicist, Sidney Chapman in 1930.
When ultra-violet radiation strikes the oxygen molecule, O2, it splits the molecule
into two individual oxygen atoms (O and O). The reactions and wavelengths of the ultra
violet radiation in which the reactions take place are described below:
The atomic oxygen thus produced will combine with un-split oxygen molecule (O2)
to produce ozone molecule (O3) once again. In this reaction a third body, M plays a crucial
role in absorbing the excess energy liberated. This M may be a N2 or another O2.
Thus formed ozone will be split by striking ultra-violet radiation at the wavelength
around 308 nm into a molecule of O2 and an atom of oxygen.
By this ozone-oxygen cycle, the concentration of ozone is maintained. The actual
concentration of ozone is determined by the rate of its formation and its destruction.
Due to photochemical dissociation by ultra-violet radiation, other forms of oxygen
are also present in stratosphere: O+
, O•
and O•
2 – ionic form and excited form. These
reactions are as shown below:
They are also part of the ozone-oxygen cycle. This entire cycle is presented in figure 2.1.
The amount of ozone present in the stratosphere is measured by simple
spectrophotometer from the ground. This instrument was developed by the British
meteorologist, G.M.B. Dobson who explored the properties of ozone layer in detail. Total
amount of ozone in a column overhead is measured and expressed in “Dobson units”
named in honor of G.M.B. Dobson.
308 nm
– 260 nm
O3 + hν O•
+ O2
– 260 nm
O + hν O+
+ e– 260 nm
O2 + hν O2
+
+ e
– 260 nm
O2 + hν 242 nm
O + O
– 260 nm
O + O2 + M (N2 or O2) O3 + M
240 – 260 nm
O2 + hν O + O
O2 + hν O + O
135 – 176 nm

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1. In what layer does ozone layer exist?
2. Between what altitudes is the ozone layer present?
3. What is the significant role of ozone layer? State briefly.
below:
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2.3. OZONE LAYER DEPLETION
Today, due to human activities, this ozone layer is becoming thin. This thinning is
called, ozone depletion. At the zones, where thinning is too severe, they are termed as
“Ozone holes”. The ozone hole is defined as the area having less than 220 Dobson units
(DU) of ozone in the overhead column (i.e., between the ground and space). Ozone hole,
observed over Antarctic region is shown in figure 2.2.
Mechanism of ozone depletion is not well understood. Chloro-fluoro carbons
(CFC) used in refrigerators, air conditioners, propellants etc. and oxides of nitrogen emitted
by air crafts flying near stratosphere are found to be the causes for ozone layer depletion.
Ozone layer is depleted by free radical catalysts – nitric oxide (NO), hydroxyl (OH),
Figure 2.1 - Ozone-oxygen cycle
(Source:Wikipedia)

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atomic chlorine (Cl), and atomic bromine (Br). Halogens have the ability to catalyze ozone
breakdown.
A catalyst is a compound which can alter the rate of a reaction without permanently
being altered by that reaction and so can react over and over again. Chlorine atom or any
other halogen atom released from CFC or BFC by striking ultraviolet radiation is now
available for catalyzing ozone breakdown. Although these species occur naturally, large
amounts are released by human activities through the use of CFCs and bromofluoro
carbons.
2.3.1 OZONE LAYER DEPLETION BY CFCs
CFCs and BFCs are stable compounds and live long in the atmosphere. As the live
longer they are able to rise to the stratosphere. Cl and Br radicals are liberated from these
compounds by the action of ultraviolet radiation. These radicals initiate and catalyze
breaking the ozone molecules. One single radical is capable of breaking down over
100,000 ozone molecules. Ozone concentration is decreasing at the rate of 4% per decade
over northern hemisphere. CFCs have long life time – 50 to 100 years. As they remain for
such a long duration, they deplete ozone layer continuously. Moreover, this depletion rate
keep increasing as more and more CFCs are released.
Figure 2.2 – Ozone hole over Antarctic region
(Source: www.research.noaa.gov/climate/t_ozonelayer.html)

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Ozone depletion is by chlorine atom is illustrated in figure 2.3. The chemical
reactions that lead to destruction of O3 by CFCl3 are shown below. Similar reactions take
place with other CFCs and BFCs.
9/11/2007 C.Ravichandran 7
CFCl3 + hn CFCl2 + Cl•
Cl• + O3 ClO• + O2
ClO• + O Cl• + O2
In a cyclic reaction, each ClO• can initiate
a series of chemical reactions which
lead to destruction of about 100,000
molecules of ozone!!
200 nm
After realizing the seriousness of this problem, countries have come forward to ban
completely or phase out and the ban the use of CFCs. Sweden was the first nation to ban
CFC-containing aerosol sprays. The ban was brought to force on January 23, 1978 in
Sweden. Few other countries followed Sweden later that year. Ozone hole was discovered
in the year 1985. After this, countries came forward for an international treaty, the
Montreal Protocol for complete phase-out of CFCs by 1996. This effort has yielded
positive result. Scientists announced on August 2, 2003 that depletion of ozone layer had
slowed down due to the ban on CFCs.
Scientists have developed HCFC to replace CFC for the same purpose. These
HCFCs are short-lived in the atmosphere to reach the stratosphere and damage ozone layer.
Figure 2.3 – Chemistry of ozone depletion

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2.3.2 OZONE LAYER DEPLTION BY NITRIC OXIDE
Chemistry of ozone depletion by nitric oxide is shown below:
9/11/2007 C.Ravichandran 6
O3 + NO NO2 + O2
NO2 + O3 NO3 + O2
NO3 + NO2 N2O5
O + NO2 NO3
NO + NO3 2 NO2
O + NO NO2
M
M
When a nitric oxide (NO) molecule combines with O3, it is oxidized to nitrogen
dioxide (NO2). This NO2 now combines with another O3 molecule to become NO3 and O2.
Both NO2 and NO3 may combine to form N2O5. Even if atomic oxygen is available, it
readily combines with NO2 to yield NO3. Thus, O3 is completely utilized for the above
reactions and thereby depleted.
International community, after realizing its seriousness, has agreed to withdraw the
operation of jet airplanes that emit NO in stratosphere.
Write the chemical reactions involved in ozone layer depletion by
a) CFCs
b) NO
below:
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2.4 LET US SUM UP
In this lesson we have learnt the elemental properties of atmosphere. We studied, in
depth, the photochemical reactions taking place in ozone layer and learnt how these
reactions protect the earth from harmful ultraviolet radiation. We also learnt how these
reactions are interrupted by CFCs and other chemicals which lead to depletion of ozone
layer. We shall learn more in Unit 2 about this.

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· Take a sheet of paper, and write the sequence of photochemical reactions taking
place in the upper atmosphere
· Write the wavelength regions of ultra-violet radiation absorption by O3 and O2
You are now aware that the natural protection available in the stratosphere. It
protects us from harmful ultraviolet radiation. Man, by his activities releases CFCs into the
atmosphere and other gases that deplete the ozone layer.
· Can you write ozone-oxygen cycle on a sheet of paper?
· Can you write down the reactions of ozone depletion by CFCs and NO?
2.8REFERENCES
1. de Blij, H.J. and Muller, P.O. Physical geography of the global environment,
John Wiley & Sons, Inc., New York
2. Miller, Jr., G.T. Environmental Science, Thomson Brroks/Cole, CA, USA,
2004
3. De, A.K. Environmental Chemistry, New Age International Ltd, Publishers,
New Delhi, 1994
4. Rao, M.N. and Rao, H.V.N. Air pollution, Tata McGraw-Hill Publishing
Co. Ltd., New Delhi
6. Wark, K. and Warner, C.F. Air pollution – its origin and control, IEP A
Dun-Donnelly Publisher, New York, 1976
7. W.H.O. Glossary of air pollution, World Health Organization, Copenhagen,
1980
9. Critchfield, H.J. General climatology, Prentice Hall of India Pvt. Ltd., New
Delhi, 1987

17
LESSON 3 – METEOROLOGY
Contents
3.1. What is meteorology?
3.2.Incoming solar radiation – insolation
3.2.1. Radiation balance
3.2.2. Earth’s outgoing radiation
3.2.3. Green house effect
3.3.Weather
3.3.1. Temperature
3.3.2. Altitudinal temperature distribution
3.3.3. Horizontal temperature distribution
3.3.4. Atmospheric temperature measurements
3.3.5. Humidity
3.3.6. Measurement of humidity
3.3.7. Phases of water in atmosphere
3.3.8. Precipitation
3.3.9. Pressure
3.3.10. Measurement of atmospheric pressure
3.4.Let us sum up
3.8. References
In the preceding lessons we have learnt the importance of atmosphere, its
composition, its structure, its properties and reactions taking place in it. We also learnt
how the protective ozone layer is depleted by some chemicals.
In this lesson various physical processes and phenomena occurring in the
atmosphere. It will be interesting to note the influence of incoming solar radiation in
driving various processes in atmosphere. The study of temperature, its variation spatially
and temporally. Other parameters like pressure, humidity, wind, and precipitation will also
be discussed. By the end of the lesson, the learner will know all the above in details to a
greater extent.
3.1 WHAT IS METEOROLOGY?
In this lesson, you will be learning about physical processes taking place in the
atmosphere, especially in troposphere. These physical processes are responsible for

18
weather and climate of a place. All the processes are studied as a branch of science called,
meteorology. Meteorology is the science of atmosphere and deals with basic physical
principles as they apply to atmospheric phenomena. When it is said atmosphere, it may
represent atmosphere of not only earth but also of other planets. The study of “science of
earth’s atmosphere” is called metrology.
The atmosphere behaves like a mega heat engine, in which many interesting
processes like movement of air, storms, cyclones, formation of clouds and precipitation etc.
take place. Major driving force for this “engine” is solar radiation. Incoming solar
radiation (called, insolation) supply all the energy for all these processes. When radiation
strikes the earth, earth is heated up. This heat energy then transferred to the overlying air
through conduction and convection. Now air starts moving horizontally as well as
vertically. This causes all the weather and insolation is the only force for all these,
weathers.
3.2 INCOMINGSOLARRADIATION – INSOLATION
What is insolation? It is sun’s radiant energy that strikes the earth. Not clear?
Radiant energy from sun that strikes the earth is called insolation. It is short form of
“incoming solar radiation”. Radiation emanating from the sun consists of a range of wave
lengths, known as solar spectrum. Each wavelength contains its own energy. Solar
spectrum includes X-rays, gamma rays, ultraviolet rays, visible light, infrared rays,
microwaves, and radio waves. Of these, ultraviolet, visible and infrared portions constitute
more than 95% of the energy received by earth.
Most of the ultraviolet radiation is prevented by existing ozone layer in the upper
atmosphere. Part of the long waves is absorbed in the troposphere. The radiation which
ultimately reaches the earth mainly comprises visible light. This visible light is composed
of seven colors that make the rainbow after the rainfall at some times. As you know these
seven colors are: violet, indigo, blue, green, yellow, orange and red.
It is interesting to note that out of the total energy emitted by sun, only two-billionth
of it is intercepted by the earth. Only this small amount of energy is responsible for all the
physical processes taking place in the atmosphere and all the biological processes on earth.
Do you know how much energy reaches the earth? It is difficult to say how much
energy reaches the earth’s surface. But, scientists have estimated the amount of energy
reaching the edge of the atmosphere (top of the atmosphere). The average amount of
energy striking the outer atmosphere per unit area is known as the solar constant. It is
estimated that a mean value of 1.940 langleys per minute of energy strikes the outer
atmosphere. One langley equals to one gram-calorie per centimeter. In other words, 1.940
g cal per of energy reaches one centimeter area of outer atmosphere per minute (i.e. solar
constant = 1.940 g cal / cm / min).
In addition to the radiation, sun emits energy as “solar wind”. What is solar wind?
It is a stream of charged particles (i.e., plasma) ejected from upper atmosphere of the sun.
It also consists of magnetic fields. The effects of solar wind are not well understood.

19
However, it is known that it interferes with radio communication, knock out power grids on
earth, and disturbs earth’s magnetic field.
3.2.1 RADIATION BALANCE
Now let us turn to know more about solar radiation and its effects on earth and its
atmosphere. As far as earth is concerned, radiation means both incoming solar radiation
and outgoing radiation emitted by earth’s surface. Before radiation reaching the earth’s
surface, it travels through the atmosphere. When it passes the atmosphere, some of the
radiation energy is reflected by clouds, and some are scattered and absorbed by gases and
particles in the atmosphere. The remaining radiation finally reaches the earth; even some
of the radiation reaching the earth’s surface or water surface is reflected back.
The amount of radiation reflected depends upon the surface characteristics. The
reflected solar radiation is called, albedo. Albedo of a water body is different from that of
a land surface; albedo of a forest is different from that of a desert and so on. That is,
albedo values vary with the type of the surfaces on which the radiation strikes.
The scattered radiation by atmospheric constituents, may reach the earth and this is
called diffuse radiation. On a cloudy day, the light we receive is not direct radiation but
diffused radiation passing through the clouds.
Now let us turn our attention towards the amount of radiation received and the
amount of radiation emitted by the earth. Let us assume that total energy received by outer
atmosphere at a given location is 100 units (say 100%). Of these, some are reflected,
absorbed, and/or scattered. Some units finally reach the earth’s surface directly or through
diffuse radiation. The earth re-emits either all or part of the received radiation. All the
above details constitute radiation balance.
On average, out of all the energy received at the edge of the outer atmosphere, only
31% directly reaches the earth’s surface (or water surface; hereafter, the term earth’s
surface includes water surface too). This energy is called direct radiation. Almost 30% is
reflected by the atmosphere to the outer space. Clouds reflect approximately 25% and the
dust particles, about 5%. 17% of the energy is absorbed by clouds and otheratmospheric
constituents (dust particles and gaseous molecules). Some of the particles and gases scatter
the radiation in the atmosphere. Some of the scattered radiation eventually reaches the
earth’ surface. It is called diffuse radiation. Diffuse radiation reaching the earth’s surface
accounts for about 22%. Thus, earth receives on average, 53% of the solar energy both
directly and through diffuse radiation.
3.2.2 EARTH’SOUTGOINGRADIATION
Now, if all the energy received by the earth’s surface is retained, then earth will
become very hot and inhabitable place. Surface of the earth also reflect some amount of
energy. The earth’s surface is heated up by the energy received by it. Of course, the
remaining energy available after reflection heats up the surface. Some of this heat energy

20
is transmitted to overlying air through conduction and this in turn will initiate the
convection in the air above the earth’s surface.
As the earth’s surface is heated up, it will become hot and hold the thermal energy.
The earth at this point, earth starts emitting energy in the form of long wave radiation. This
is called outgoing radiation.
3.2.3 GREEN HOUSE EFFECT
Part of the outgoing radiation is absorbed by the atmosphere and retained as heat
energy. The remaining energy escapes into outer space. Carbon dioxide, water vapor, and
few other gases in the atmosphere are capable of absorbing the outgoing radiation. (called,
green house gases) the concentration of these gases, more the amount of long wave
radiation is absorbed. It is a natural phenomenon occurring in the atmosphere. It helps
preventing the earth from drastic cooling of the earth and its atmosphere. If all the heat
energy from the earth’s surface is lost to the space, then earth will become too cold. These
gases act like a blanket and provide the earth with “blanketing effect”. This process is
called Green house effect.
However, the intensity of this effect may vary from place to place depending upon
the concentration of these gases in the atmosphere. For example, over desert atmosphere,
the amount of water vapor is far less as it is a dry place. Carbon dioxide concentration is
also very less as life is sparsely distributed and less number of living organisms exist here.
Hence, more radiation will escape into the space and very little amount of radiation is
retained. Consequently, the desert atmosphere will be cool during night.
Define the following:
a) Albedo
b) Diffuse radiation
c) Direct radiation
below:
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3.3 WEATHER
Now let us turn our attention towards understanding the weather: what causes it
and how it is caused. First, we should know what weather is. Weather is the conditions of

21
the atmosphere experienced by humans at any given moment. Weather is used to refer to
the atmospheric conditions that exist for a short duration (hours or days). These conditions
fluctuate and vary temporally over a given place. Often people confuse this with the term,
climate. Climate refers to the average atmospheric conditions over a long period of time
(month or season) of a place.
What are those conditions? At times, we feel our atmosphere is very hot / cold,
sultry and humid / dry, rainy, stormy etc. All these experiences are due to atmospheric
conditions such as, temperature, humidity, wind speed and its direction, rainfall, etc. These
conditions vary both spatially and temporally.
3.3.1 TEMPERATURE
Temperature is an index of sensible heat. Bear in mind that it does not measure the
amount of heat energy. It indicates the degree of molecular activity or kinetic energy of
molecules. As it describes the kinetic energy, it specifies the speed of the movement of
molecules. In case of gas (air), molecules move and change their location; but in case of
solid, the molecules do not change their location but only vibrate it their place. The speed
of this vibration is described by temperature.
When a body has higher temperature, heat will flow from it to another body which
has lower temperature. That is heat flows from a body with high temperature to a body
with low temperature. Temperature is measured using thermometers. Temperature is
reported in any of the three scales, viz. Celsius scale, Fahrenheit scale, and Absolute
(Kelvin) scale. Of all the above, Celsius scale is used throughout the world to report the
atmospheric temperature. It is named after the Swedish Astronomer Andres Celsius.
0 °C is the triple point temperature at which the gaseous, liquid and sold states of water are
at equilibrium under standard pressure. The boiling point of water under standard
conditions is at 100 °C.
Kelvin scale is based on absolute zero. At absolute zero molecular activity ceases
theoretically. Absolute zero in Kelvin scale is equal to -273.16 °C. Each degree on Kelvin
scale is equal to that of Celsius scale. Accordingly, the value of 273 is added to Celsius
value for converting it into Kelvin.
Fahrenheit scale is an old scale used formerly. However, it is still used in United
States of America. It is also used in medical field to mention human body temperature. In
this scale, boiling point of water is 212 °F and freezing point of water is 32 °F. Fahrenheit
can be converted into Celsius by the following formula:
Air temperature is mostly measured using simple glass thermometers filled either
with mercury or alcohol. Other advanced thermometers are also available. Celsius scale is
widely used internationally to report air temperatures.
3.3.2 ALTITUDINALTEMPERATUREDISTRIBUTION
5
9
C = (F-32)

22
All the weather phenomena occur in lower atmosphere – troposphere. Yes, our
weather is seldom affected by conditions exist above the troposphere. Therefore, we limit
our discussion with troposphere when we learn about weather. Troposphere is the layer in
which we all live; movement of air occurs both vertically and horizontally.
As mentioned earlier, temperature decreases with height in troposphere. Rate of
decrease of temperature with height is called lapse rate. As already mentioned, average
lapse rate in troposphere is -6.4 °C / km. However, it may vary at different conditions. We
shall learn more about vertical temperature distribution when we study about atmospheric
dispersion of pollutants.
1. Distinguish between weather and climate
2. What is lapse rate?
3. What is the average lapse rate of the troposphere?
below:
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3.3.3 HORIZONTALTEMPERATUREDISTRIBUTION
Temperature differs horizontally at different locations. Temperature of a place
also varies at different times of a day and at different months and seasons of a year. These
variations are due to several factors. Incoming solar radiation is the major factor which
determines the temperature of a place. The angle of the sun’s rays and length of daylight
determine the amount of insolation received in a certain place. The angle of sun’s rays and
length of daylight are determined by latitudinal location of a place and time of the day.
As the earth revolves around the sun, one half of the globe is exposed to sun’s rays
and other half remains dark at any given time. The illuminated surface of the earth keeps
changing as the earth rotates itself on its own axis. This results in occurrence of day and
night time alternately over a given place. However, the entire half of the globe does not
receive equal amount of sun’s radiation. In addition, as the earth keeps rotating, the
amount of radiation received at a given location varies over time during daytime.
The sun’s rays strike almost perpendicular near the equator and at angles over high
latitudes. This means farther away from equator more will be the angle of striking of sun’s
rays over a place. Moreover, the earth’s axis is tilted at angle of 66½ degrees to the plane
of the ecliptic. This tilt is always in the same direction throughout its orbit around the sun.
The tilting of the earth in the same direction is called parallelism. This parallelism is the
key responsible factor for seasonal variation over the earth during a year.

23
Northern hemisphere is tilted to a maximum extent towards the sun around June 22.
Months closer to June are summer months in northern hemisphere. During this period,
northern hemisphere receives greater amount of sun’s energy than southern hemisphere
does. Around December 22, the southern hemisphere is tilted to the maximum extent
towards the sun. As a result, just opposite occurs during this period. That is southern
hemisphere will receive greater amounts of solar energy than the northern hemisphere does.
This variation determines the length of the daytime over a place. That is the length of the
daytime is a function of latitude and month of the year.
In general, the places near equator (lower latitudes) receive more amounts of sun’s
energy than the places over higher latitudes. In addition to this variation, morning hours
and evening hours receive less amounts of sun’s energy when compared to the sun’s energy
received during noon hours. During noon, the sun’s rays strike vertically overhead. But
during morning and evening hours, the sun’s rays strike at angles. These variations cause
the same amount of radiation fall over a small area during noon and over a relatively larger
area during morning and afternoon hours.
As the earth’s surface receives the energy from the sun, it gets heated up. But this
heating up also differs as the type of the surface differs. Different surfaces get heated up
differently. For example, rocky surfaces are heated up rapidly while water surfaces take
long time to get heated up. Similarly the rocky surfaces loose the energy rapidly while
water surfaces loose the energy slowly. Other surfaces too behave differently depending
on their thermal properties. Over a certain place many different types of surfaces may
exist. All the anomalies discussed above, determine the weather of a given place at a
given time.
Heat energy received by a surface is transferred both downward and upward. The
overlying air molecules are heated up due to conduction. Thus heated air become less
dense and they tend to move upward. As the heated air moves upward, the air above this
air (relatively cool air) tend to sink downward. In this way air starts moving up and down
and thereby, heat energy is transferred by air currents. This process is called, convection.
Through the processes of conduction and convection, air either is heated up or
cooled down. All these determine the temperature of overlying air - atmosphere. The
heating up of air is different as the underlying surfaces are different. For example, air over
water surfaces will be cooler than the air over land surfaces of a given place.
Consequently, air over the water will be denser than the air over the land surface. This will
result in movement of air from over water surface towards over the land surfaces. During
night time, opposite will occur – air from over the land move towards water. This
movement of air is called sea-land breeze.
As the different surfaces of the earth are heated up differently due to variations in
surface characteristics, the air above these places will have different temperatures
accordingly. This variation will tend to move horizontally. Horizontal movement of air is
termed as wind. Thus winds are generated. In the same way, the differences in air
temperature over different latitudes cause the movement of air in large scale. This large
scale movement of air is called global circulation.

24
Temperature of the atmosphere over a place changes during day and this is called
the diurnal cycle. It also changes during a year and this change is called annual cycle.
Temperature reaches at its maximum at around 3 p.m. of the day. Earth receives the sun’s
energy from dawn till dusk. Maximum energy is received at noon and as a result earth is
heated up to a maximum extent. Then this heat energy is transferred to overlying air.
There is a time gap between these two events: heating up of the earth’s surface and transfer
of the heat to the overlying air. Thus the maximum temperature is attained around 3 p.m.
After sunset, earth starts loosing its heat energy through outgoing infrared radiation.
As it looses gradually, the minimum temperature is attained around 4 a.m.
Briefly describe the reason for temperature variations
a) Diurnally
b) Seasonally
c) At different Latitudes
below:
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3.3.4 ATMOSPHERICTEMPERATUREMEASUREMENTS
Atmospheric temperature is measured by thermometers. Various kinds of
thermometers are available. Mercury-in-glass and alcohol-in-glass thermometers are very
simple and inexpensive thermometers available. Accuracy of the measurements depends
on the way they are manufactured and calibrated. Usually, for measurement of surface air
temperatures, the thermometers are mounted in a louvered instrument shelter.
Other types of thermometers are also available. Thermometers are made with
bimetallic elements so that temperature differences affect their shape and this difference in
shape is translated to record temperature. Electrical / electronic thermometers like
thermocouples and thermistors are also available to record temperatures automatically.
Specially made thermometers are available to record maximum and minimum
temperatures of a day. The simplest maximum thermometer is a mercury thermometer
with a constriction in the bore near the bulb. The minimum thermometer is made with
large bore and filled with colorless alcohol. A tiny, dark and dumbbell shaped index is
placed in the bore below the top of the alcohol column. Both these thermometers are
mounted horizontally and placed inside a specially designed shelter, Stevenson screen.

25
The constriction in the maximum thermometer allows the expanding mercury to
pass as the temperature rises. The column of mercury breaks at the constriction as the
atmosphere cools down. It leaves a part of the mercury in the bore at the same position
when maximum temperature was attained. Thus the maximum temperature is recorded.
Alcohol in the minimum thermometer contracts as the temperature decreases.
During its contraction, lower meniscus of the alcohol pulls the index along and leaves it at
the point where minimum temperature was attained.
After, noting down the readings of maximum and minimum temperatures, these
thermometers are reset for the next cycle of measurements.
For continuous recording of temperature, thermograph is used. It consists of an
element responsive to temperature change, a system of levers to translate these changes to a
pen arm and a cylindrical clock drum around which a calibrated chart is mounted.
However, the recordings are not as accurate as with the thermometers. Hence, frequent
checking and adjustments are made.
3.3.5 HUMIDITY
Humidity is the term used to represent the amount of water vapor in the air. Water
vapor present in the atmosphere plays important roles in determining weather of a place.
When high amount of water vapor is present in the atmosphere, we will feed discomfort.
This discomfort is called sultry. Atmosphere will be dry when water vapor is so less. This
also causes discomfort. The atmosphere of the places near the water bodies will have high
moisture content while that near the dry areas like deserts will have low or almost nil
moisture content. Water vapor enters the atmosphere from water bodies through
evaporation. Subsoil water enters the atmosphere both through evaporation and
evapotranspiration by plants.
Water vapor present in the atmosphere may form clouds as they are lifted to higher
altitudes. Clouds get saturated as they gain more and more water vapor. At certain point of
time after saturation, the water may fall from the atmosphere through the process called
precipitation. The falling of water either in liquid form or in solid form from the
atmosphere is called precipitation. Thus atmosphere becomes part of the hydrological
cycle.
At any given point of time, atmosphere holds water vapor. The amount of water
vapor held by atmosphere varies over time during a day and over a year. Air is a mixture
of gases present in the atmosphere. As such air does not hold the moisture; but the space
available among the gaseous molecules hold moisture. The water vapor holding capacity
of air varies according to the temperature and pressure of air. Therefore, the amount of
water vapor present in the atmosphere is expressed in various ways.
Specific humidity: it is the ratio of the mass of water vapor actually in the air to a unit
mass of air, including the water vapor. For example, specific humidity is 10 g per kg when
a kilogram of air contains 10 g of water vapor.

26
Mixing ratio: it represents the amount of water vapor in one unit mass of dry air (dry air is
the air without moisture content). When 10 g of water vapor is present in 1 kg of dry air
then mixing ratio is 10 g per kg. However, the total amount of air and water vapor would
be 1010 g (1.01 kg).
Vapor pressure: it is the partial pressure exerted by water vapor in the air. Vapor pressure
is expressed in the same units used for barometric pressure – mm of Hg or millibars.
Saturation vapor pressure: when air space contains the maximum possible amount of
water vapor at a given temperature and pressure, it is said to be saturated. The partial
pressure exerted by water vapor at this condition is called saturation vapor pressure. The
temperature at which air is saturated with water vapor is called dew point. Below dew
point, condensation of water vapor occurs.
Relative humidity: it is the ratio of the amount of water vapor present in a parcel of air to
the maximum amount of water vapor that air could hold at the same temperature. Thus if a
kg of air contains 9 g of water vapor when its maximum holding capacity is 12 g at a given
temperature under constant pressure, the relative humidity is 75%.
When temperature increases the saturation vapor pressure also increases resulting in
increase in holding capacity of water vapor. For example, at the increased temperature, let
us assume, the vapor-holding capacity of air is 15 g and only 9 g is present then the relative
humidity is 60%.
Relative humidity reaches its diurnal maximum in the early morning hours when
temperatures are low and then decreases to a minimum in the early afternoon. In the same
way, it is greater in winter than in summer over land. However, during monsoon months in
summer, moist air pass over the land and thereby high relative humidity values are attained.
Over oceans, relative humidity reaches maximum due to evaporation of sea water. Mid
latitude mountains also tend to have high relative humidity values during summer due to
stronger convective flow of moist air.
a) Humidity
b) Specific humidity
c) Relative humidity
d) Vapor pressure
e) Saturation vapor pressure
f) Precipitation
below:
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27
3.3.6 MEASUREMENTSOFHUMIDITY
Direct measurements of humidity and water vapor content are not possible with
ordinary instruments. Hence, they are measured indirectly using psychrometer. It consists
of two thermometers, both mounted on the same frame. One thermometer is wet-bulb
thermometer and other one is dry-bulb thermometer. Wet-bulb thermometer is mounted
little lower than the other and its bulb is covered with a cloth wick that can be wetted with
water during observation. The psychrometer is either swung in the air or aerated by a fan
to provide passage of air over the bulbs of the two thermometers. While air passes over the
wet-bulb thermometer, evaporative cooling lowers the wet-bulb reading. The difference
between the two readings is called wet-bulb depression. The dry-bulb temperature, wet-
bulb depression and atmospheric pressure are used to determine the humidity values from
psychrometric tables derived already from mathematical relations. If there is no difference
between wet-bulb reading and dry-bulb reading, then the wet-bulb depression is zero. The
zero value is attained when relative humidity is 100 percent.
Hair hygrometer is a device available for measurement of relative humidity
directly, though this instrument does not provide accurate values at low temperatures. It is
operated on the principle that human hair lengthens as relative humidity increases and
contracts with decreasing relative humidity. Its principle is utilized in the hygrograph,
which is a recording hygrometer with a clock drum and pen arrangement.
Yet another device, infrared hygrometer is available for measurement of humidity.
It employs a beam of infrared radiation projected through the air to a detector. Two
separate wavelengths are used; one is absorbed by water vapor and other passes through
undiminished. The ratio of the radiation transmitted by the different wavelengths gives the
indication of the amount of water vapor in the path of radiation.
Other devices like hygristor and dew point hygrometers are also available for
measurement of humidity. Hygristor is a device through which electrical current is passed
across a chemically coated strip of plastic. The passage of electricity is proportional to the
amount of moisture absorbed on its surface. This hygristor is commonly used in
radiosondes used for upper air observations and other instruments operated remotely to
measure the water vapor content.
3.3.7 PHASES OF WATER IN ATMOSPHERE
As already mentioned water vapor present in the atmosphere plays significant roles
in determining weather and climate. Water vapor in the atmosphere is also a part of water
cycle. In order to understand its influence on weather, we should know the properties of
water. Water is a chemical consisting of two hydrogen atoms and one oxygen atom. Its
chemical form is H2O. Water may be present in the atmosphere in all of its three physical
forms: solid, liquid and gas. It may change its form at any point of time in the atmosphere
by exchanging latent heat.
The change of state from liquid to vapor is called evaporation. It results when
molecules escape from any water surface. Evaporation takes place by utilizing vast

28
amounts of energy. In this way, a large amount of energy in the form of latent heat is
transferred from earth’s surface to the atmosphere.
The rate of evaporation is determined by three factors: vapor pressure, temperature
and air movement. Evaporation increases as the saturation vapor pressure at the water
surface becomes greater than the actual vapor pressure of the adjacent air. Hence,
evaporation takes place at faster rate into dry air than into air with high relative humidity.
When temperature of water increases with other factors being equal, the rate of evaporation
also increases. That means, whenever temperature of water is greater than that of air,
evaporation always takes place. Wind movement over the surface of water replaces moist
air by relatively dry air. This also increases evaporation.
Latent heat of vaporization: this is the heat energy utilized for the transformation liquid
into vapor and/or evolved during transformation of vapor into liquid without change in
temperature. The heat used for evaporation is retained by the water vapor without rising
temperature. The latent heat of vaporization ranges in value from nearly 600 calories
(2,510 joules) per gram of water at 0 °C to about 540 calories (2,259 joules) at 100 °C.
This variation is due to decrease in the difference between kinetic energy of the escaping
molecules and the average kinetic energy of all the water molecules as temperature
increases.
Evaportranspiration: loss of water vapor by terrestrial plants through their leaves and
stems is called transpiration. The combined losses of water vapor by evaporation and
transpiration from a given area are called evapotranspiration. Thus the atmosphere over
thick vegetation like forest will always have greater humidity values than that over non-
vegetated or less vegetated atmospheres.
The amount of solar energy received by a place is an important factor for
evapotranspiration. The intensity of solar radiation is determined by the latitude of a place.
Mean annual rates of evapotranspiration are greatest between 20° N and 20° S where the
highest amount of net radiation is received. Greater evapotrnspiration is accounted over
southern hemisphere than that over northern hemisphere as southern hemisphere consists of
larger proportion of oceanic surface.
When water changes its state from vapor to liquid, it is called condensation.
Condensation is a very important process for the formation of clouds. As the moist air
comes into contact with cool surfaces, it gets cooled to the dew point temperature and a
part of the vapor condenses into liquid form on that cool surface. Condensed water over
the surface is called dew.
During the condensation process, the heat is released as the heat is assimilated
during vaporization. This released heat is called latent heat of condensation. Liberation
of the latent heat of condensation may slow down the cooling process. This latent heat
released during condensation is one of the energy sources for atmospheric processes.
Condensation also occurs from cooling in the free air. However, it requires the
presence of very small particles called condensation nuclei. Some of these particles are
hygroscopic and hence have greater affinity towards water. As a result condensation will

29
occur on the surface of these hygroscopic particles even above the dew point. On the
surface of other particles, water vapor may condense upon cooling i.e. at dew point.
After sufficient amount of vapor is thus condensed they become visible as haze.
When more amount of vapor condenses, condensed water droplets increase in number and
become dense to form fog or cloud. Since the particles play vital role in the process of
condensation and the formation of clouds, they are called cloud condensation nuclei
(CCN). When water changes from liquid to solid, it is called freezing and when ice melts
into liquid, it is called melting. While melting takes place at 0 °C, freezing does not occur
at 0 °C but below 0 °C.
Water may contain foreign materials in dissolved form and/or in suspended form. It
may also be covered by foreign substances. Presence of foreign substances in water will
lower the freezing point below 0 °C. Amount of water involved and electrical charges also
lower the freezing point.
Water may bypass liquid state and become vapor from ice and vice versa. This
process is called sublimation. Sublimation is the process of becoming vapor from solid
phase or becoming solid from vapor phase. When dry air with a temperature well below
the freezing point comes in contact with ice, some molecules of water from ice will become
vapor. When water vapor comes in contact with cold surface well below the freezing point
vapor may immediately become ice.
1. Describe the role of particles in the atmosphere in formation of clouds.
2. Define the following:
i. Latent heat of vaporization
ii. Latent heat of condensation
iii. Sublimation
iv. Evapotranspiration
below:
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3.3.8 PRECIPITATION
Precipitation is water falling on earth in either liquid or solid form. Precipitation
occurs in the form of rain, drizzle, snow, hail and/or their modifications. Precipitation is
an important part of the hydrological cycle which brings water over the continental regions.
The precipitated water either runs as stream or infiltrate into the ground to become ground
water. Some part of the precipitated water is stored in natural and/or artificial reservoirs
such as dams, lakes, ponds and tanks.

30
Water vapor condensed over condensation nuclei form clouds in the atmosphere
and these clouds give out the water as precipitation. Of course, the formation of clouds and
the process of precipitation are quite complex.
Rain: it is precipitation with large liquid water droplets. Rain drops range from less than 1
mm to 5 mm in diameter.
Drizzle: it is precipitation consisting of uniformly minute droplets of water (less than 0.5
mm in diameter).
Snow: it is precipitation consisting of large ice crystals called snow-flakes. It is formed by
the ice-crystal process. These crystals do not have time to melt before they reach the
ground.
Hail: it is precipitation in the form of ice pellets called hail stones. The process of
formation of hailstone is complex one.
The process of precipitation (in any form) is quite complex. Details of precipitation
are not required for this course and hence not explained.
a) Rain
b) Drizzle
c) Snow
d) Hail
below:
………………………………………………………………………………………………
………………………………………………………………………………………………
………………………………………………………………………………………………
……………………………………………………………………………………………..
3.3.9 PRESSURE
You might have come across the news of cyclone and depressions in news bulletin.
What are they and how are they formed? To know the answers for these questions, we
should know about pressure. What is pressure? The definition for the word pressure is “the
force per unit area acting on a surface”. The SI unit for pressure is the pascal. One pascal
is equivalent to 1 N m-2
. The atmospheric pressure is expressed in millibars. 1 millibar is
equivalent to 100 N m-2
(=100 pascal).
What is atmospheric pressure? Atmosphere is held by earth by its gravitational
pull. The combined mass of all air molecules lying above the earth exert a force on earth.
So, atmospheric pressure is the force exerted by the column of air per unit area above the
earth’s surface. The amount of air in a column over a unit area will differ from place to
place as there is variation in its altitude. Yes, over top of a hill, the amount of air in a
column will be less than the amount of air in a column over a sea surface. Hence, the
atmospheric pressure will be less over the hill than over sea surface or a land surface at low
lying areas. As the height of a place increases from the mean sea level, the pressure
decreases. Of course, as you go deep under the earth, the pressure will be more than that

31
over land. Under the sea, the water column along with overlying air will exert pressure
which will be unbearable.
The atmospheric pressure over mean sea level under normal conditions is 1013 mb.
The atmospheric pressure over the top of the Mount Everest is 320 mb.
In addition, cold air will generally be denser than warm air. As a result a column of
warm air will exert less pressure than that by a column of cold air. The differential heating
of surfaces (due to differences in surface characteristics), the air lying over these surfaces
will be heated differently and hence will exert pressure differently. This will cause
anomalies of pressure values in horizontal atmospheres. Consequently, air from high
pressure region tends to move towards the low pressure region. Thus, winds are generated.
The strength of the winds is determined by the difference in pressure values. Greater the
difference in pressure values, stronger will be the speed of the wind. Let us discuss about
winds and their pattern in detail in the next lesson. We shall now turn our attention to the
measurement of atmospheric pressure.
What is atmospheric pressure?
below:
………………………………………………………………………………………………
………………………………………………………………………………………………
………………………………………………………………………………………………
………………………………………………………………………………………………
………………………………………………………………………………………………
……………………………………………………………….
3.3.10 MEASUREMENTOFATMOSPHERICPRESSURE
Atmospheric pressure measurements are essential in analyzing charts of winds,
storms, etc. A simple measuring device is mercurial barometer. It was first developed by
the Italian scientist Evangelista Torricelli in 1643. He filled a glass tube with mercury and
then placed the tube upside down in a dish of mercury. In his experiment he found that
instead of mercury running down into the dish, the atmospheric pressure pushing the
mercury in the dish down and thereby the mercury in the dish supported the mercury in the
tube from moving down. As a result of this experiment, Torricelli invented the world’s
first pressure measuring instrument, barometer.
At sea level, under standard conditions, mercury remained at the height of 76 cm
(=760 mm) in the column. This unit, mm of Hg became the expression for atmospheric
pressure. 760 mm of Hg is equivalent to 1 atmospheric pressure. However, the pressure
measured in units of height is not satisfactory. Pressure can be expressed in different units
and these units are inter-convertible. It is convention among meteorologists to express the
atmospheric pressure in millibars.

32
Aneroid barometer is an instrument used for measurement of atmospheric pressure.
It consists of a partially evacuated metal wafer called sylphon cell. This cell expands or
contracts in response to changing pressure. The fluctuation is linked mechanically to an
indicator on a calibrated dial. Barograph is the aneroid barometer designed to record the
pressure continuously on a chart. It consists of a pen arm attached to sylphon cells whose
expansion or contraction activates the pen to record the pressure on chart.
3.4 LET US SUM UP
In lesson 3, we have learnt about meteorology. We could distinguish between the
term weather and climate. We learnt that the insolation (incoming solar radiation is almost
responsible for all the atmospheric processes occur. Now, you have become familiar with:
· Incoming solar radiation
· Radiation balance
· Outgoing radiation and green house effect
· Temperature, its variation vertically and horizontally
· Humidity – what is humidity, and various expressions of humidity
· Role of water vapor in the atmosphere – formation of clouds and precipitation
· Atmospheric pressure and its measurements
Now, you must be able to explain all the above concepts clearly.
· Go through weather bulletin of news paper daily and record each of the
temperature, pressure and rainfall values on a graph sheet separately for a period of
three months.
· Do you infer any change in your records? If so write down.
· By now, you know that solar radiation strikes the earth’s atmosphere and pass
through it
· During its passage, some of the radiation is reflected, scattered, and absorbed
· Earth in turn emits its heat energy in the form of infrared radiation which is again
absorbed by certain gases of atmosphere causing green house effect.
· Water plays important role in the atmosphere.
· Can you distinguish between incoming solar radiation and outgoing radiation
· Can you explain the horizontal temperature variation?
· Can you describe the roles of water in atmosphere?
· Did you understand the concept of pressure?

33
3.8REFERENCES
1. De Blij, H.J. and Muller, P.O. Physical geography of the global
environment, John Wiley & Sons, Inc., New York
2. Miller, Jr., G.T. Environmental Science, Thomson Brroks/Cole, CA, USA,
2004
3. De, A.K. Environmental Chemistry, New Age International Ltd, Publishers,
New Delhi, 1994
4. Rao, M.N. and Rao, H.V.N. Air pollution, Tata McGraw-Hill Publishing
Co. Ltd., New Delhi
6. Wark, K. and Warner, C.F. Air pollution – its origin and control, IEP A
Dun-Donnelly Publisher, New York, 1976
7. W.H.O. Glossary of air pollution, World Health Organization, Copenhagen,
1980
9. Critchfield, H.J. General climatology, Prentice Hall of India Pvt. Ltd., New
Delhi, 1987

34
LESSON 4 - FORMATION OF WINDS AND GLOBAL
CIRCULATION
Contents
4.1. Winds and their formation
4.2. Geostrophic wind
4.3. Gradient wind
4.4. Turbulence
4.5. Local winds
4.5.1. Sea-Land breeze
4.5.2. Mountain-valley winds
4.5.3. Other local winds
4.6. Atmospheric circulation
4.6.1. Atmospheric circulation – hypothetical model
4.6.2. Atmospheric circulation – idealized model
4.6.3. Actual atmospheric circulation pattern
4.7. Secondary surface circulation and Indian Monsoon
4.7.1. South West Monsoon
4.7.2. North East monsoon
4.8. Let us sum up
4.12. References
In this lesson we are going to learn about the formation of winds and their patterns.
In fact, the study of winds is also a part of Meteorology. But for our convenience, we will
be studying about it in a separate lesson. Winds are very important as they play a vital role
in distributing the energy over a large area – even at global scale.
The earth is heated up differently at different latitudes. The places near equator
receive surplus energy while places near poles and high latitudes receive less energy;
places near poles and at high latitudes become energy-deficient. The surplus energy over
equator and low latitudes is converted into kinetic energy for the formation of winds. Thus
the surplus energy in transferred from equator and low latitudes to the high latitudes by
winds. In this lesson we will be learning all these in detail.
4.1 WINDS AND THEIR FORMATION
The movement of air is called wind. Where from do they originate and where do
they end at? Are there any pattern exist in the flow of winds? Yes winds follow same
pattern globally. This pattern is called global atmospheric circulation. In the past the
sailors took the advantage of the wind for the movement of ships and boats. The sea routes

35
were chosen according to the winds blown and their direction. Now let us learn how these
winds are formed and how they flow across.
Earth receives unequal amount of radiation and hence heated differently at different
latitudes. Earth also rotates on its axis. These two factors cause the formation of winds
and result in global circulation. Through this the surplus energy received over low latitudes
are transported as kinetic energy in the form of winds towards high latitudues.
Different types of surfaces of the earth are heated up differently as their thermal
properties differ. Even if places are close to each other in their latitudinal position,
differences in their surface characteristics make them have different thermal properties.
Consequently, the pressures over these different places also differ. This will create
horizontal pressure variation over an area of few meters to several kilometers. This
variation result in a gradient of pressure anomalies over the entire area. This horizontal
pressure gradient is responsible for the movement of air, wind. This pressure gradient
drives the air to move, and hence this force is called pressure gradient force.
Winds may blow gently or violently or with moderate speeds. Gentle winds bring
relief and comfort from heat of the day. Violent winds may sometimes bring devastating
effects. Storms, cyclones, tornado etc. are such violent winds that bring devastating effects.
In addition, winds carry moisture, heat, pollutants etc. from one place to other.
What is pressure gradient force? Explain briefly.
below:
………………………………………………………………………………………………
………………………………………………………………………………………………
………………………………………………………………………………………………
………………………………………………………………………………………………
………………………………………………………………………………………………
……………………………………………………………….
4.2 GEOSTROPHIC WIND
Due to pressure gradient force, air moves from high pressure region to low pressure
region. When a surface is heated up the overlying air is heated up and become light. The
light air tends to ascend. As the air rises, the pressure over that surface becomes low. To
compensate this low pressure air will start moving from a high pressure region towards this
low pressure region. When air moves from high pressure region, the overlying air at that
point will tend to sink. The air ascended will move towards the region where from air is
sinking. This completes the circulation of air between two different places vertically.
Consider a large area, and the pressure keeps varying over this area as you move
away from one place farther and farther. That means pressure keeps changing as you move
away from a place to another little by little. The line connecting areas with equal pressure
is called isobar. When pressure changes over a large area, that area may be occupied by

36
several isobars, each isobar denoting a particular pressure value. Sometimes, these isobars
may be parallel or circular over a large area of consideration.
Once the air start moving, other forces come into action: Coriolis force and
frictional force.
Coriolis force: First we shall learn what the Coriolis force is? It is a deflective force. Any
moving object, stream of water or air over the surface of spinning planet, will be deflected
due to the rotation of earth. This is called Coriolis force. Coriolis force will deflect
moving bodies towards their right in northern hemisphere and towards their left in southern
hemisphere. Due to this Coriolis force, the moving air over the earth’s surface will be
deflected accordingly. The existence of this force was discovered and demonstrated by
Gaspard de Coriolis in 1844.
However, the effect of Coriolis force is nil over equator and its effect increases toward the
poles. It acts at an angle of 90° to the horizontal direction of the wind and is directly
proportional to horizontal wind speed. When pressure gradient initiates the air to flow, the
Coriolis force deflects it to flow parallel to the isobars. Remember, the winds must flow
from an isobar with low pressure value towards the isobar with high pressure value.
Instead of this perpendicular flow, the wind flows parallel to the isobars as a result of
Coriolis force. The wind thus flowing is called geostrophic wind.
· What is Coriolis force? Explain briefly
· How is geostrophic wind formed?
below:
………………………………………………………………………………………………
………………………………………………………………………………………………
………………………………………………………………………………………………
………………………………………………………………………………………………
………………………………………………………………………………………………
……………………………………………………………….
4.3 GRADIENT WIND
If the air moves along curved isobars, a net centripetal acceleration tends to pull it
toward the center of curvature. This will result a rotating motion of air relative to earth’s
surface. Such wind is called gradient wind. If this gradient wind circulates in counter-
clock direction, then it is termed as cyclone in northern hemisphere while it is called
anticyclone in southern hemisphere. Similarly, the clockwise motion of air is called
cyclone in southern hemisphere while it is called anticyclone in northern hemisphere.
At the place under the low pressure region, air flowing toward this region tends to
converge and ascend and hence, this place is called area of convergence. Likewise, under
the high pressure region, air flows away from it as the air sinks from above and hence, this
region is called area of divergence.

37
In cyclonic circulation, surface air moves towards the centre, the low pressure cell,
and converges. Thus converged air ascends vertically in the center of the low pressure cell.
The reverse happens in the center of an anticyclone – as the surface air diverges from the
center of the anticyclone (high pressure cell), the air sinks from high in the center of the
anticyclone (high pressure cell). In these ways, the cyclones are associated with rising air
at their centers while, anticyclones are associated with descending air in their centers.
4.4 TURBULENCE
A third force may come into force when air starts moving due to friction exhibited
by surface. The earth’s surface may consist smooth surfaces like water, or highly rough
surfaces like rocky terrain, urban buildings etc. When air flows over the smooth water
surface, the friction encountered is almost zero. But when wind flows over a rough terrain
like a land with rocks or a land with urban buildings of various heights, it encounters
friction. The magnitude of friction differs as the surface features differ.
The friction lowers the speed as well changes the direction of the wind. It creates
turbulence in winds. Turbulence is associated with lulls, gusts and eddies. Turbulence
also increases with the increase of the wind speed. However, the surface relief does not
pose any friction at the height of about 500 m. At the height of about 1000 m, the winds
are no more under the influence of friction. As a result, the winds flowing at this height
will be approximately equal to the theoretical geostrophic wind and/or gradient wind.
4.5. LOCAL WINDS
Before we turn our attention towards the atmospheric circulation at global level, let
us see the local winds generated by differences in surface characteristics: sea-land breeze,
mountain-valley winds, and other local winds.
4.5.1 SEA-LAND BREEZE
Some land areas are very close to large water bodies. Lands near sea, large
reservoir, lakes and wide rivers are the best examples. During daytime the incoming solar
radiation heats both the land surface and water surface. However, the land surfaces are
heated up rapidly while water surfaces are not. The rapidly heated land surface in turn,
heats up the overlying air in relatively short duration. Heated air becomes light and
ascends high creating a low pressure region over the land surface. At the same time, the air
above the water remains relatively cooler and denser to create a high pressure region over
the water. This anomaly results in pressure gradient between the land and water surfaces.
Now this pressure gradient force will start driving the air lying above water to move
towards the land, this wind is called sea breeze.
During night, the earth looses it heat energy rapidly and gets cooled down. The
water does not cool down so fast but remain relatively warmer during night. Relatively
warmer water surface transfer its heat energy to the overlying air and thereby air becomes
warmer and light to ascend high. This will create a low pressure over the water. At the

38
same time, the overlying air at the land will be relatively cooler and denser creating a high
pressure region. This anomaly results in pressure gradient (just opposite of what exists
during daytime) between the land and water. Now the air from land will tend to move
towards the water and this wind is called land breeze.
4.5.2 MOUNTAIN-VALLEY WINDS
On mountain sides, under a clear night sky the lands situated at high radiate heat
and get cooled down. This in turn cools the air lying over those lands. Now the cool and
dense air tends to slide down the mountain slopes into valleys and low lands. This air flow
is called mountain breeze.
In the morning hours, temperature inversions may occur over the low lying areas.
As a result, the valley bottoms are colder than the hillsides. In general, inversions over a
land area prevent the vertical motion of air. But the mountain breeze reached the valley,
may gain considerable velocity and generate enough turbulent mixing to break the
inversion. On warm sunny days, the slopes of mountain will be heated up and thereby the
overlying air. This will generate an upslope flow of air towards the top of the mountain.
The flow of air from slope of the mountains to the top is called valley breeze. When warm
air moves up, it is replaced by cooler air from above the valley. This results in moderation
of surface temperature.
4.5.3 OTHER LOCAL WINDS
Under the influence of gravity, the cold and dense air moves downward along the
steep slopes. These winds are called gravity winds or katabatic winds. These winds are
prominent under calm, clear conditions where the edges of highlands plunge sharply
toward low lying terrain. These winds are fed by large quantities of extremely cold air that
collect over the high land.
Katabatic winds sometimes may reach destructive intensities as of a waterfall from
a high land. This kind of winds is experienced in the Rhône River valley of southern
France during winter. Icy, high velocity winds drain from the snowy mountains to the
valleys in this area. These winds are called mistral winds.
Another type of local wind is generated when the air is forced to pass across the
mountainous terrain. As the air move upward, the moisture present originally is almost
wrings out. After reaching the top, the winds cross over the mountain and slide down on
the other side of the hill along its slopes. As the air has already exhausted its moisture,
only dry and relatively warm air flows down with high speeds. Sometimes the speeds
reach to 120 km per hour. These winds are yet to be named by the scientists.

39
4.6 ATMOSPHERICCIRCULATION
Now let us turn our attention towards the atmospheric circulation at global level.
This circulation is called global atmospheric circulation. In real world, this global
circulation is a complex one. Due to differences in the presence of landmasses and water
bodies over the globe, the circulation becomes complex. However, we shall try to
understand from a hypothetical model and an idealized model. Later, we shall learn the
real existing atmospheric circulation.
4.6.1 ATMOSPHERICCIRCULATION - HYPOTHETICAL MODEL
Let us assume that earth is not rotating. As the places near the equator receive the
maximum insolation, the air above these places tend to rise and create a single cell of low
pressure around the equator. Polar Regions receive the minimum amount of radiation and
the air above these regions will be cold with maximum density creating a high pressure cell
at each pole. The air would subside at these poles. Due to the above processes, (ascension
of air over equator and subsidence of air over poles), the air will tend to move from poles
towards equator at surface level. The air reaching the equator converges, ascends high and
moves towards poles. This completes a full circulation. Thus two circulations of air are
created: one over northern hemisphere and another over southern hemisphere.
4.6.2 ATMOSPHERICCIRCULATION - IDEALIZED MODEL
Earth rotates on its axis. This rotation causes the deflection in the wind flow due to
Coriolis force. As the equatorial region is heated up throughout the year, a low-pressure
belt is formed over the tropical region. This belt is called inter tropical convergent zone
(ITCZ). ITCZ is also known as doldrums. But in real world, it fluctuates in its position
and intensity and is at times a weak, discontinuous belt. In this belt air rises as it is a belt of
low pressure region.
The risen air starts flowing towards the pole but for reasons not clearly understood,
the air descends at around latitudes of 30° N and 30 ° S. The descending air produce a belt
of high pressure each at the surfaces at these latitudes. These two high-pressure belts are
called Subtropical Highs.
The descended air diverges and part of the air flows towards ITCZ and another part
towards the poles. The descended air which flow towards equator is deflected to its right in
northern hemisphere and to its left in southern hemisphere. Thus northeast trade winds
and southeast trade winds are generated.
The surface air flowing toward poles from subtropical highs is also deflected by
Coriolis force and become Westerlies (west to east blowing winds). These winds form
broad mid-latitude belts between 30° and 60° in both hemispheres.
Over Polar Regions the air is dense and cool and hence exist high pressure cells
centering over these regions. They are called Polar Highs. The air from polar highs tends
to move towards the equator and as it flows is deflected by Coriolis force to become Polar
Easterlies (easterlies are winds that flow from east to west).

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