FYBA Geography Sem.- II ENG-munotes

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1Unit -1
COMPOSITION AND STRUCTURE OF THE
ATMOSPHERE AND DISTRIBUTION OF
TEMPERATURE
Unit Structure:
1.0 Objectives
1.1 Introduction
1.2 Composition and Structure of the Atmosphere
1.3 Insolation and temperature
1.4 Factors controlling distribution of temperature.
1.5 Distribution of temperature on the earth:
1.6 Significance of atmosphere and temperature
1.7 Conclusion
1.8 Questions
1.0. OBJECTIVES:
Atmosphere is one of the most important components of our earth.
To study the composition of earth’s atmosphere
To study the structure of earth’s atmosphere
To study the factors controlling the distribution of temperature.
To understand the variation in s patial distribution of
temperature on the surfa ce of the earth.
To know the significance of atmosphe re and temperature for
living kingdom.
1.1. INTRODUCTION
The earth’s atmosphere is composed of gaseous,
suspended particles and water molecules .I nt h ei n i t i a lp e r i o da f t e r
the formation of the earth the processes of chemical and biological
reactions on the earth formed the gaseous envelop encircling the
earth called as atmosphere. The differences in the cooling and
heating of the elements in the atmosphere due to incoming solar
energy and outgoing heat radiated from the earth helps to
differentiate betw een atmospheric layers.munotes.in

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21.2COMPOSITION AND STRUCTURE OF THE
ATMOSPHERE
1.2.1Composition of the Atmosphere :
The gaseous envelope which surrounds the Earth is termed
as the ‘atmosphere’. Most of the atmosphere is confined to a thin
shield around the earth, with the pressure and density of air
decreasing rapidly with altitude and gradually merging into the
emptiness of space. Fifty percent of the mass of the atmosphere is
within 5.5 kilometers of sea level; 90 percent is within about 16
kilometers of s ea level, and 99.9 percent is below 49 kilometers.
The atmosphere contains many gases. The atmosphere also
contains water vapour and aerosols (small microscopic particles).
Most of them are concentrated near the Earth’s surface. This is due
to the gravitat ional pull of the Earth and as t he density of air is
maximum .As we move up above the surface of the earth ,the
density of air decreases.
It is important to note that 50 percent of the atmosphere lies
below the height of 5.6 km. and about 90 per cent belo w1 6
kilometers . Only about 0.00003 percent of all the gases found in the
atmosphere are beyond 100 kilometers .
Table 1.1. Composition of air in the earth’s atmosphere
Component Symbol Volume of
dry air in %
Nitrogen N2 78.08 %
Oxygen O2 20.94%
Argon Ar 0.93
Carbon
dioxideCO2 0.03
Neon Ne 0.0018
Helium He 0.0005
Ozone O3 0.00006
Hydrogen H2 0.00005
Krypton Kr Trace element
Xenon X2 Trace element
Methane CH 4 Trace element
Nitrogen and oxygen together make up about 99 percent of
thevolume of atmospheric gases. Some of the minor gases such
as CO 2and O 3play an important role in the atmosphere. CO 2
absorbs heat radiated by the earth surface during night and so is
responsible for warming the globe .O3gases protect theearth by
absorb ing the incoming harmful ultraviolet (UV) rays ofthe sun.
This composition of the atmospheric gases remains the same up to
a height of about 80 km. Therefore, the lower parts of themunotes.in

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3atmosphere up to 80 km are termed as the Homosphere.
Composition of the atmosphere varies after 80 km of height. Hence,
it is termed as Heterosphere.
The lower parts of the atmosphere up to 10 -15 km contain
water vapour. It is derived from the evaporation and evapo -
transpiration from water bodies, soil cover and vegetation
respectively. The amount of vapour is determined by the
temperature conditions near the surface of the earth. As
temperatures are higher near the equator and decreases towards
the poles, the amount of vapour therefore is more near the equator
and become le ss as we move towards the poles. In terms of
vertical distribution of vapour content more than 90 %o f the total
atmospheric vapour is held up to an altitude of 5 kilometer.
Condensation of water vapor in the atmosphere is responsible for
different types of weather phenomena.
The atmosphere also c omposes of microscopic elements
such as dust and salt particles, etc. They are termed as aerosols.
The hygroscopic salt particles absorb water and act as
condensation nuclei in the formation of clouds. They also scatter
solar radiation. The sky appears blue due to the scattering of violet
rays of the incoming solar radiation.
Structure of the Atmosphere: On the basis of the characteristics
of temperature and air pressure the structure of the atmosphere is
divide d into four layers upwards from the surface of the earth
(Figure :1.1).These are:
i) Troposphere: Troposphere ’m e a n s‘ ar e g i o no fi n t e n s e
mixing of atmospheric components’ where weather conditions are
formed. Human activities are directly related to the weat her
conditions of this layer. It contains 75 percent of the atmospheric
gases, water vapour and aerosols. Hence this lowest layer of the
atmosphere is very useful to man. It is important to note that
temperature decreases with increasing height at the rate of6.50
Centigrade per 1000 meter (1 kilometer). This rate of decrease of
temperature is called as Normal Lapse rate (Figure 1.2).
However it should be noted that there is seasonal variation in the
height of the troposphere and changes from equator towar ds the
poles .The average height of the troposphere is about 16 kilometer
over the equator and 6 kilometer over the poles. The upper limit of
the troposphere is called tropopause .The boundary between
Troposphere and Stratosphere (the next layer) is known as
Tropopause . It is an inversion layer (i.e. temperature does not
decrease with increase in altitude in this layer). Tropopause acts as
a lid for Troposphere. Therefore, troposphere becomes a self -
contained layer of the atmosphere. Strong vertical convect ional air
currents are found near the Equator (Figure 1.3.).T h e r e f o r et h emunotes.in

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4height of the Tropopause is about 16 km at the Equator, but it is
only 8 km near the poles ( figure 1.4).
Exosphere
Figure :1.1. St ructure of the Atmosphere
Figure: 1.2.Normal Lapse ratemunotes.in

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Figure :1.3. Convectional Currents
Figure :1.4. Tropopause
ii) Stratosphere: Stratosphere extends from Tropopause up to
the height of 50 km. There is an absence of storm or changing
weather conditions and water vapour. Temperature remains
constant at the lower part of the stratosphere, but it increases with
increase in height. The lower part of the stratosphere is used by jet
aircrafts. Theozone layer is mainly found in the lower portion of the
stratosphere from approximately 20 to 30 kilometers above earth.
Its thickness varies seasonally and geographically. Ozone layer in
this region absorbs the harmful ultra -violet rays of the sun from
reaching the earth. The composition of the stratosphere also
changes with the latitude i.e.it begins with 9 km above the poles,
10 or 11 km in the middle latitudes, and 16 km at the equator, and
extends outward up to 32 km. Iti saz o n eo fd r y ,t h i na i r ,c old and
clear, with a horizontal temperature gradient. In Polar Regions the
temperature is-40°C to -46°C, but near the equator it ranges from
-62°C to below -74°C; in the middle latitudes it remains steady atmunotes.in

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6about -55°C (Figure :1.5).No weather occ urs in the stratosphere.
Air temperature slowly increases with height in the stratosphere.
Figure :1.5.Temperature changes in the stratosphere with
height
1.2.3. Role of the ozone layer: Concentration of ozone (O 3)is
found in this zone between 25 -50 km. Ozone is made up of three
atoms of oxygen (O 3)–ordinary oxygen molecule is made up of
only two atoms (O 2). Oxygen molecule is broken into two atoms by
UV radiation (Figure 1.6). Therefore the unstable atom combines
with the other molecule of oxygen (O+O 2=O 3). Thus, O 3or ozone
is formed. O 3molecule may also split into O 2molecule and the
unstable oxygen atom by the UV radiation. This constant
conversion of ozone to oxygen and oxygen to ozone utilizes much
of the UV radiation. Th us the amount of UV radiation reaching the
Earth’s surface is considerably reduced. (Excessive quantity of UV
radiation reaching Earth’s surface may create problems such as
blindness, skin cancer and can even damage vegetation)
Figure 1.7.
Figure :1.6.Effect of Ultra violet radiationmunotes.in

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Figure :1.7. Ultra Violet radiation and Ozone layer
Mesosphere: Mesosphere lies above stratosphere and b elow
thethermosphere . It extends from about 50 to 85 km from the
earth’s surface. Temperature in the mesosphere decreases with
height. The coldest temperatures in the Earth 's atmosphere
i.e.about -90°C is found near the top of the mesosphere.
Mesopause is a boundary that lies betwe en mesosphere and
thermosphere. Various types of waves and tides in the atmosphere
influence the mesosphere. These waves and tides carry ene rgy
from the troposphere and the stratosphere upward into the
mesosphere, driving most of its global circulation.
iii) Thermosphere: Above the mesosphere, in the
thermosphere and beyond, gas particles collide so infrequently that
the gases become somewhat sep arated based on the types of
chemical elements they contain .The thermosphere lies between
themesosphere and the exosphere . It extends from about 90 km to
between 500 and 1,000 km geographically above the earth surface.
Temperatures increase sharply in the lower thermosphere i.e. from
200 to 300 km height. The density of air is very low in
thermos phere. Solar activity strongly influences temperature in the
thermosphere. Theaurora (the Southern and Northern Lights)
primarily occurs in the thermosphere. Charged particles (electrons,
protons, and other ions) from space collide with atoms and
molecule s in the thermosphere at high latitudes, exciting them into
higher energy states. Those atoms and molecules shed this excess
energy by emitting photons of light, which we see as colorful
auroral displays. Thermo pause isab o u n d a r y that lies between
the t hermosphere and the exosphere. The Earth's thermosphere
also includes the region of the atmosphere called the ionosphere.
The ionosphere is a region of the atmosphere that is filled with
charged particles. The high temperatures in the thermosphere can
caus e molecules to ionize. The ionosphere represents less than
0.1% of the total mass of the Earth's atmosphere but is extremely
important because upper atmosphere is ionized by solar radiation
where the Sun's energy is strong at this level which breaks apartmunotes.in

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8molecules. Different regions of the ionosphere make long distance
radio communication possible by reflecting the radio waves back to
Earth (Figure 1.8and Figure 1.9.).
Figure :1.8. Broadcasting of Radio / T.V. Waves
Figure: 1.9.Gree nHouse effect munotes.in

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Figure: 1.10.Vertical Di stribution of Temper ature
iv) Exosphere :The exosphere is the outermost region of
Earth ’s atmosphere and gradually fades into the vacuum of space.
Air in the exosphere is extremel y thin or almost airless. The lower
boundary of the exosphere is called the exopause .A tt h i s altitude
barometric conditions are absent. Atmospheric temperature
becomes almost constant above this altitude. Exosphere lies at a
height of about 500 to 1,000 kilometers from the earth’s surface
depending on the solar activity. Inprinciple, the exosphere covers
distances where particles are stillgravitationally bound to Earth,i . e .
particles still have ballistic orbits that will take them back towards
Earth. The exosphere is a transitional zone between Earth ’s
atmosphere and space.
Check Your Progress:
Q1.Whatis atmosphere ?
Q2. What is the composition of atmosphere?
Q3. Give the structure of atmosphere.munotes.in

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101.3INSOLATION AND TEMPERATURE
The heat received from the sun travels in the form of short
and long wave’s radiation. The temperature of the atmosphere is
determined by the rate of insolation i.e. the amount of h eat received
from the sun by the earth which is released by the earth (long wave
radiation) that heats the atmosphere from below .The atmosphere
contains various green house gases such as c arbon dioxide, ozone
and water vapour thatabsorb out going heat r eleased by the earth
after sunset. This phenomenon is termed as the Greenhouse Effect
of the atmosphere. Greenhouse is a glasshouse constructed
normally in the temperate belt region for growing plant. As greenery
is inside house, it is termed as Greenhouse . Temperature inside
the greenhouse is more in comparison of outside the greenhouse.
This is possible due to dual properties of glass. Glass is transparent
and it is a bad conductor of heat. Hence, the solar radiation enters
into the glasshouse, i.e. green house -without any obstruction. There
it is converted into heat. This heat is trapped inside the glasshouse .
It is important to note that a tmospheric gases cannot prevent
insolation (short -wave radiation) from reaching the Earth and so
the earth ’s surface gets heated from above. However the heat
received by the earth is released in the form of long -wave radiation
that is absorbed by the atmospheric gases (Figure 1.11).A sa
result the atmospheric temperature is raised this is termed as
Greenhouse Effect of the atmosphere.
Figure :1.11. Electromagnetic waves and Short and Long wavemunotes.in

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111.4FACTORS CONTROLLING DISTRIBUTION OF
TEMPERATURE:
Temperature of different places on surface of the earth is not
same everywhere . This is because the amount of insolation
received at the surface of the Earth is controlled by the angle of
thesun, the state of the atmosphere, altitude, and geographic
location as explained below:
i) Latitude: Solar rays are vertical within the tropical belt (i.e.
from the Tropic of cance r to the Tropic of Capricorn) vertical rays
are concentrated over smaller surface area on the earth as
compared to the oblique solar rays (Figure 1 .12).S i m i l a r l yt h e
resistance offered by the aerosols and other elements in the
atmosphere is less in the ca se of vertical solar rays, than the
oblique solar rays. Intensity of solar radiation decreases due to the
resistance offered by the atmosphere i.e. due to reflection,
diffusion, scattering. Due to these two factors areas receiving
vertical solar radiation record higher temperature than the areas
receiving oblique solar radiation (Figure 1.13).
Figure :1.12.Temperature Belts of the Earth
Figure: 1.13: Vertical and oblique rays and heat receivedmunotes.in

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12Generally temperature decreases awa yf r o mt h ee q u a t o rt o w a r d s
pole (figure 1.14).
Figure :1.14.Temperature decreases from the Equator towards
Poles
ii) Altitude: Temperature decreases with increase in altitude
from the earth’s surface. Solar rays pass through the atmosphere,
in the form of short wave radiation. Earth’s surface is heated and
then it emits heat in the form of long wave radiation Earth radiation.
This long wave is absorbed by the water vapour enters into the
atmosphere by the process of evaporation fr om the earth’s surface.
Co is added to the atmosphere by the processes like burning,
combustion etc. As the concentration of water vapour and Co2 is
more near to the earth’s surface, the lower layers of the
atmosphere absorb more heat. As we move towards h igher altitude
the amount of water vapour and Co2 decreases, so the capacity of
the atmosphere to absorb heat. Hence the temperature decreases
with increase in the altitude (Figure 1.15.).
iii)
Figure: 1.15. Temperature decreases with increase in height .munotes.in

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13iv) Distance from the Sea: Temperature characteristics of land
and water are different. Land is heated more than water due to
solar radiation during day time as solar heat is absorbed and
released more slowly by water than by land. Water retains heat for
alonger time and hence land becomes cool while water remains
warm during night time. This difference in the heating properties of
land and water is the main cause of land and sea breezes in the
coastal areas. Difference in maximum and minimum temperature i s
less in the coastal areas but it is more in the interior parts of the
continents due to the absence of water Figure: 1.16..
Figure: 1.16.Range of temperature increases with distance
from the sea towards the land
v)Angle effect: (Effect of slope): Solar rays are slant in the
temperate region. South facing slopes of the mountains in the
temperate region of northern hemisphere receive ore sunshine
(insulation) than the north facing slopes. Hence vegetation and
settlements are found on the sunny slopes ( south facing) which
are termed as ‘Adret’. The north facing slopes of the mountains
in the temperate region, which remain in shade are known as
‘Ubac’ slope. This effect of slope on temperature is termed as
the Aspect effect Figure: 1.17..munotes.in

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Figure: 1.17.Aspect effect -Slope facing sun is warmer
.
v) Ocean Currents: Warm ocean currents are responsible for
increasing temperature of the surrounding region. e.g. Coastal
areas of west Europe remain warm due to the Gulf stream of the
Atlantic Oce an. Similarly cold ocean currents reduce temperature of
the surrounding areas , for example Labradaor cold current near
Greenland. .
vi)Local Winds: Warm local winds increase temperature of the
surrounding region and cool local winds reduce temperature of the
surrounding region.
vii)Nature of Cloud cover: Solar rays are reflected back due to
the cloud cover. Diffused solar radiation reaches earth’s surface,
due to the presence of cloud cover. Earth radiation is reflected back
to the earth due to the cloud cover. Difference in the maximum and
minimum temperature is less at the Equator due to constant cloud
cover, but d esert areas record extremes of temperature due to the
absence of cloud cover (Figure 1.18and1.19).
Figure 1.18.Effects of cloud cover on temperaturemunotes.in

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Figure: 1.19. Effects of cloud cover on temperature
viii)Duration / length of day: Duration of day is more during
summer than in winter hence more solar radiation is received
during summer than in winters. Hence tem perature is more in
summer and less in winter.
Check Your Progress:
Q. Name the factors controlling distribution of temperature.
Q. Explain inversion of temperature in the atmosphere.
Q. Write a note on Ozone Layer.
1.5DISTRIBUTION OF TEMPERATURE ON TH E
EARTH:
Distribution of temperature is explained as under:
a) Vertical distribution of temperature :The distribution of the
temperature varies vertically as well as horizontally on and above
the earth’s surface. Generally the temperature goes on decreasing
with increasing altitude from the sea level. It is caused due to two
reasons:
i) The atmosphere near the earth’s surface absorbs most of
thelong wave radiation. Thus the air gets heated from below that
makes air towarm -up and so it risesup. In this proces sof uplift of
warm air it gets expanded that releases the heat and so the
temperature goes on decreasing as we move upwards.
ii) Secondly the green house gases such as carbon -dioxide
and water vapour are more near the surface of the earth and
become thinner with increasing height. Hence lower atmosphere is
warmer while temperature becomes cooler with increasing height of
the atmosphere which is limited only to the lower region of the
atmosphere.munotes.in

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16However at times there is inversion of temperature in the
vertical temperature distribution. It occurs when there is absence of
winds blowing, the sky is clear, winters are long, air is cold and dry
and the land surface is covered with snow.
Inversion of Temperature: Temperature inversion is a condition in
which the temperature of the atmosphere increases with altitude in
contrast to the normal decrease with altitude. When temperature
inversion occurs, cold air underlies warmer air at higher altitudes .I t
may lead to different weather effects such as Pollutants hazard s.
Pollutants may get trapped below the inversion as the sky becomes
very hazy causing respiratory problems .It usually occurs in high
pressure zones, where the air is gradual sinking down in the lower
atmosphere.
b) Horizontal Distribution of temperature o nt h e earth: The
horizontal distribution of temperature across the globe is governed
by various factors. These are absolute location (latitudes and
longitudes); relative location (maritime / continental), time (season),
duration and intensity of sunlight r eceived (location –equatorial to
polar; day and night), soil type and vegetal cover, relief, water
bodies, winds, ocean currents, cloud cover. Th is horizontal
temperature distribution can be identified across the globe with the
help of isotherms for the m onths of January and July asbest
example. The solar rays are perpendicular on the lower latitudes
and become slanting as we move towards the higher latitudes. As a
result temperatures are higher in the tropical region and lower in
the pol ar region .T r o p i c of cancer (2 3½0North latitude) and
Tropic of Capricorn (2 3½0South latitude) are the upper latitudinal
limits of perpendicular solar insolation receiving maximum heat
from the sun. Isotherms are imaginary lines drawn with areas
having same temper ature. These isotherms generally run from
east-to west and are parallel to the latitudes. These isotherms are
irregular in the northern hemisphere due to more land surface with
its diversity whereas they are more uniform in the southern
hemisphere due to m ore water surface with less diversity. Variation
in temperature is rapid in the higher latitudes hence isotherms are
closely spaced whereas temperature variation is relatively slow in
the lower latitudes therefore isotherms are widely spaced . Isotherm
map for the months of July and January explains the given concept
of horizontal distribution of temperature.
1.6SIGNIFICANCE OF ATMOSPHERE AND
TEMPERATURE:
The existence of life on the earth is because of the gaseous
envelop of the atmosphere that provide sw a r m t h bytrapp ingheat
released by the earth and other necessary gaseous elementsmunotes.in

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17dissolving in rain water reaching the earth’s surface to enter into
ecosystem .
Advantages of the Atmosphere
Some of the advantages of the atmosphere are as follows:
i)It provides oxygen required for our survival as well as for other
forms of life.
ii)It provides carbon dioxide to plants for their survival.
iii)The atmosphere controls the temperature of the Earth, which is
essential for the existence of various for ms of life in the
environment. Without the atmosphere, the temperature of the Earth
would have increased to more than 2000C,which is certainly not
suitable for the environment found on the Earth.
iv)The atmosphere (ozone layer) protects us from the harm ful
effects of the ultra -violet radiation, which is a part of solar radiation.
v)‘Weather’ and ‘Climate’ are the terms used to indicate changes in
the atmospheric condition. Our daily and annual activities are
associated with weather and climate.
vi)Itprotects us from the harmful meteors, which are attracted
towards the Earth. Many of them are burnt down as they enter into
the Earth’s atmosphere.
1.7CONCLUSION
i)The vertical distribution of temperature, pressure, density, and
composition of the atm osphere constitutes atmospheric structure.
Spatial variation and distribution in temperature is determined by
the season, its geographical location, and the time (day time or
night time), and itsdistance and height from the sea.
ii)Amixture of a numbe r of gases is termed as air that constituents
abundantly molecular nitrogen (N2) , molecular oxygen (O2) ,a n d a
little amount of the inert gas argon (Ar). They together constitute
99.9 percent of the mass of dry air. This ratio of the number of each
molecu leis nearly stable up to a height of about 80 to90 kilometers
from the earth surface. Water vapor (H2O) and carbon dioxide
(CO2) are other importan tgases that absorb and emit long wave
radiation, and ozone (O3) gas that absorbs ultraviolet radiation fro m
the Sun as well as some long wave radiation from the Earth. The
distribution of these gases therefore affects the vertical temperature
distribution.
iii)Temperature thus cools with height throughout the troposphere,
but it then warms through the strato sphere, only to cool again
through the mesosphere .Finally it heats up in the thermosphere
and exosphere. This distribution comes about through changingmunotes.in

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18interactions among shortwave radiation from the Sun, long wave
radiation from the Earth, and various ga ses in the air.
1.8QUESTIONS
1.Explain the composition and structure of atmosphere. Draw
suitable diagrams.
2.Discuss the factors determining distribution of temperature.
Draw suitable diagram.
3.Describe the horizontal distribution of temperature. Draw
suita ble diagram.
4.Explain the vertical distribution of temperature. Draw suitable
diagram.
5.Examine the importance of atmosphere and temperature
giving suitable examples.
References:
1.Pednekar,Shinde,Thakur…..(2015): Physical Geography,
F.Y.B.A. Semester I and II, Sheth Publishers Private Limited,
Mumbai.
2.K. Siddhartha (2001) :Atmosphere, Weather and Climate,
Kausalaya publications Pvt. Ltd. New Delhi
3.Surender Singh (2005) :G e o g r a p h y ,T a t aM c G r a w -Hill
Publication New Delhi .
4.Savindra Singh (2011): ‘Climatology’, Prayag Pustak
Bhavan, Allahabad .
munotes.in

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Unit -2
THE ATMOSPHERIC PRESSURE BELTS
AND WINDS
Unit Structure:
2.0 Objectives
2.1 Introduction
2.2 Atmospheric Pressure and Wind belts
2.3 Distribution of Local winds, Monsoon and Planetary winds.
2.4 Variable Winds and Cyclones
2.5 Conclusions
2.6 Questions.
2.0 OBJECTIVES :
To understand the nature of atmosphere, its structure and
composition
To know the factors influencing pressure conditions and its
impact on temperature conditions
To study the distribution of local winds, monsoon winds an d
planetary winds
To study the variable character of winds in different areas of the
world.
2.1. INTRODUCTION :
Atmosphere contains air. Air is a mixture of gases. Most of
these gases have weight, therefore atmosphere has pressure. High
pressure and low pressure belts are formed on the Earth due to
insolation, rotation of the Earth, etc. Air always moves from the high
pressure area to low pressure area. Hence wind belts are formed in
between pressure belts. Similarly, various types of wind systems
are for med due to local variations in the atmospheric pressure.
2.2.ATMOSPHERIC PRESSURE AND WIND BELTS
Air Pressure :
Air is mixture of gases. Most of these gases have their own
weight. Therefore air exerts pressure on the Earth’s surface. The
weight of air on a unit area of the Earth is called Air Pressure.munotes.in

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Air pressure at sea level is equal to 1013.2 mb (millibar). Air
pressure decreases with increase in height/ altitude from the sea
level.
Figure :2.1Air pressure decreases with increase in altitude
In this, diagram the weight of air at sea level ‘(A)’ is maximum. The
weight of air at (B), (C) and (D) becomes less. Therefore the air
pressure at (B), (C) and (D) will be less. (D) records lowest air
pressure.
2.3DISTRIBUTION OF LOCAL WINDS, MONSOON
AND PLANETARY WINDS :
Pressure Belts
Two types of pressure belts are found on the Earth. They are :
a)High pressure belts and
b)Low pressure belts.
Before going into the details of these pressure belts, it is
necessary to understand how these belts a re formed. Temperature
and the atmospheric pressure are related to each other. There is an
inverse relationship between the temperature and the atmospheric
pressure. Areas of high temperature are generally associated with
low pressure, and the areas of low temperature are associated with
high pressure.
In this diagram, place (A) receives oblique solar rays.
Therefore it records low temperature. Place (B) receives vertical
solar rays. Therefore it records high temperature. (Figure :8.2)Due
to high tempera ture the earth’s surface at (B) will be heated more.
The air near to this surface will become warm and expand. So its
density will decrease. Air of low density is lighter than the air of high
density. Hence, the warm air being less dense will be lighter an d
will move in the upward direction. The upward moving air willmunotes.in

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displace upper air, due to the gravitational atmosphere. As a result,
the upper air will move towards nearby area, i.e. (A) in this
example. Now the accumulation of air is more at (A). Therefo re the
weight of air at (A) is more and, hence, (A) will be the area of high
pressure and (B) will be the area of low pressure.
Figure: 2.2. Solar radiation and Temperature amount.
As place (A) has high pressure and place (B) has low
pressure, air fr om place (A) will move towards place (B) in order to
maintain the balance in the pressure at (A) and (B) (Figure 2.3).
This air which moves from high pressure area to the low pressure
area is known as wind.
Figure :2.3. Low and High pressure Areas and wind
On a global scale, air near the equatorial area is more
heated due to more intense solar radiation. This air moves in the
upward direction in the form of convectional currents. Therefore low
pressure area is formed at the Equator which is termed as
‘Doldrums’. Air which is displaced by the equatorial low pressure
area descends down near 300Na n d3 00S latitudes. Hence high
pressure belts are formed at 300Na n d3 00S latitudes.munotes.in

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This pattern of alternate belts of low pressure and high
pressure are foun d on the entire Earth. The major pressure and
wind belts are given in the following diagram (Figure 2.4.).
i)Doldrums :Itis the low pressure belt situated between 50Nt o50S
latitude. Due to intense heating, low pressure area is developed. Air
rises up ward in the form of convectional air currents. The air is
generally calm in this belt. Therefore it is termed as ‘Doldrums’
which means ‘Belt of Calm’.
ii)Subtropical High Pressure Belt :This high pressure belt is
found at 300to 350north and south of the Equator. The upward
moving air at the equatorial low pressure belt descends in these
latitudes. Therefore these become areas of high pressure.
The descending air currents are found in this area of high
pressure. This creates calm air without much mov ement of air. In
olden times sailors found it difficult to steer their ships (due to lack
of wind) across these regions. In such situations horses that were
commonly carried as their cargo, were thrown into the sea in order
to conserve the food on the ship for survival of the sailors. Hence,
these latitudes are also termed as Horse latitudes. Air which moves
from this high pressure area to the equatorial low pressure area is
termed as Trade Winds.
iii)Sub-Polar Low Pressure Belt :These low pressure areas are
situated between 600to 650north and south latitudes in both the
hemispheres. Cold air at the poles is very heavy. Therefore high
pressure area is formed at the poles. Descending air at the
subtropical high pressure area is divided into two parts. On ep a r t
moves towards the Equator as Trade winds while the other part
moves towards the pole as westerlies. Due to the rotation of the
Earth, winds coming from poles i.e. polar easterlies and westerlies
–converge at 600to 650north and south latitudes for ming sub -polar
low pressure belt.
iv) Polar High Pressure Belt: The ascending air at the sub -polar
low pressure area descends down at the pole region. Thus forms
polar high pressure belt.
Shifting of Pressure Belts
The Earth’s axis is tilted by 23½0to the vertical plane.
Though the Sun is stationery but due to the tilting of Earth’s axis
and revolution of the Earth around the Sun, it appears that the
position of the Sun changes in different seasons or time of the year.
This is termed as the apparent m ovement of the Sun.
All pressure and wind belts shift to the north during summer
in the northern hemisphere and to the south during winter in themunotes.in

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northern hemisphere (or summer in the southern hemisphere).
Hence, the shifting of pressure and wind belts i s directly related to
the apparent movement of the Sun.
Figure: 2.4. Wind sand Pressure belts
Due to the shifting of pressure and wind belts, various
natural regions experience different types of climatic conditions in
different seasons. Rainfall is most impacted by them.
Distribution of Atmospheric Pressure
Distribution of atmospheric pressure is represented with the
help of isobars. Isobar is an imaginary line drawn on the map which
joins the places of equal atmospheric pressure.
Spacing between the isobars helps us to understand the
pressure gradient (Figure :2.5).
(a) Steep pressure gradient :When spacing between isobars is
less and Isobars are close together, hence, th e wind speed will be
higher. Generally, in the case of a cyclone, pressure gradient is
very steep and wind speed is more.
(b) Gentle pressure gradient :In this case the spacing between
isobars is less. Isobars are farther apart. Hence, wind speed will be
lower. Generally, in the case of anticyclones, pressure gradient is
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(c) Equal pressure gradient: There is no wind when the
atmospheric pressure is same at two places (Figure 2.6.).
Figure :8.5.Atmospheric pressure and wind speed
Figure :2.6.Equal atmospheric pressure –no wind
Check Your Progress:
1.Define air pressure.
2.What causes variation in atmospheric pressure ?
2.4.VARIABLE WINDS AND CYCLONES
Winds :
Distribution of the atmospheric pressur ei su n e v e ni nt h e
world. Winds are caused due to uneven distribution of pressure.
Winds move parallel to the Earth’s surface. (Vertical movement of
air is termed as air current.
Factors Affecting Direction and Speed of Wind :
Direction of wind means the direction from which wind blows.
For example, ‘easterly wind’ means wind coming from the east.
‘South -westerly’ wind means wind coming from the south -west
direction. Various forces are responsible for determining the
direction of winds. Some of them are a s follows:munotes.in

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Pressure Gradient Force :
The rate of change of air pressure between two places is
called Pressure Gradient. The direction of pressure gradient is at
right angles to the isobars. Air (wind) moves from high pressure
area to the low pressure are a. The velocity or speed of wind
depends on the pressure difference between the two places. Wind
speed is more when pressure gradient is more or steep. Wind
speed is less when pressure gradient is less or gentle.
Coriolis Force
The force which deflects wind and is formed due to the
rotation of the Earth is termed as the Coriolis force (after the name
of French engineer Coriolis).
Winds are displaced from the direction of wind gradient due
to the Coriolis force. Ferrel formulated a law to indicate the a ctual
direction of wind in the northern and southern hemispheres after
considering the impact of Coriolis force. This law is known as
Ferrel’s law (Figure 2.7).
Figure :2.7.D e f l e c t i o no fw i n d s
According to Ferrel’s law, if we stand facing the directi on in
which the wind is blowing, we will feel greater impact of the wind on
our right -hand side in the northern hemisphere and towards our left
hand in the southern hemisphere. The effect of rotation of Earth on
the direction of wind is minimum at the Equa tor and is maximum at
the poles.
Classification of winds :
Permanent Winds
These winds have influence all over the world. Hence they
are also termed as planetary winds. Due to their regular nature,
these winds are also known as prevailing winds.munotes.in

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Trade Winds
These winds blow from the subtropical high pressure belt
(300-350North and South latitudes) to the equatorial low pressure
area, i.e. Doldrums (50North and South of the Equator).
Characteristics of Trade Winds
These winds blow towards from t he subtropical high pressure
belts towards the equator.
Trade winds are deflected to the right of their path in the
northern hemisphere and to the left of their path in the southern
hemisphere.
They bring rain to the eastern margins of the continents but are
unable to provide rain to the western margins of the continents –
creating hot deserts on the western margins of the continent.
Hence, these deserts are also termed as Trade Wind Deserts.
Trade winds are known for their steady direction and wind
spee d.
Trade winds are replaced by the monsoon winds in the
monsoon region.
These winds are associated with cyclones and depressions.
Westerlies
These winds blow from the subtropical high pressure belts
(30-350N and S) to the sub -polar low pressure regio ns (600–650N
and S). These are also termed as anti -trade winds as their direction
is opposite to the direction of trade winds.
Characteristics of Westerlies
These are found in the temperature belt between 300-600Na n d
300–600Sl a t i t u d e s .
Their dir ection is south -west in northern hemisphere and north -
west in the southern hemisphere.
Since the expanse of sea is very large in the southern
hemisphere, the westerlies blow here uninterrupted. Hence, the
direction of westerlies is constant and the speed is very high in
the southern hemisphere. These are known by different names
in different latitudes as given below :
Latitude Westerlies (known as )
400S Roaring Forties
500S Furious Fifties
600S Stormy or Shrieking Sixtiesmunotes.in

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Westerlies are associate d with the changing nature and
accompanying changing weather. The velocity of the Westerlies is
more during winter as compared to their velocity during summer.
These winds cause rainfall almost throughout the year in
Western Europe, western Canada and so uth-west Chile in South
America.
Due to the shifting of pressure and wind belts, Westerlies
shift southward in the Northern hemisphere during winter and
provide rainfall to the Mediterranean region.
These winds play an important role in the balance of heat at
global level by carrying warm equatorial water and air to the
temperate region.
Polar Winds
These winds blow from Polar High Pressure areas towards
sub-polar low pressure areas, i.e. from 900N/S to 650N/S.
Characteristics of Polar Winds
These are very cold and dry winds as they come from the cold
Polar region.
Coriolis force is maximum near the poles. Therefore polar winds
are more affected by this force As a result their direction is
changed.
These winds are experienced between the Poles an dS u b -polar
low pressure belts.
Polar winds are better developed in the southern hemisphere.
DISTRIBUTION OF LOCAL WINDS, MONSOON AND
PLANETARY WINDS :
Monsoon (Seasonal winds) : The word ‘Monsoon’ is derived from
the Arabic word Mawsim meaning ‘season ’. These winds change
their direction according toe the season, i.e. the direction of the
winds in winter is opposite to that during summer season. Asia has
the greatest monsoon influence as it is the largest continent. Here
India, Myanmar, Indonesia, Chin a and Japan experience monsoon.
The other major regions of the world include parts of Australia,
North America, South America and the western coast of Africa.
Summer Monsoon winds : In the northern hemisphere, during
summer, the solar rays are vertical ov er the Tropic of Cancer.
Therefore the Asian land mass is heated more. As a result, low
pressure areas develop over at central Asia in China and Peshawar
in Pakistan. These low pressure areas are so strong that the airmunotes.in

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from southern subtropical high pressu re belt reaches the Equator
as south -east trade winds. After crossing the Equator these winds
change their direction due to rotation of the Earth and become
South -West Monsoon winds. As these winds come from the sea
they carry moisture. Hence they are abl et og i v er a i n f a l li nI n d i aa n d
other parts of Asia.
Winter Monsoon winds :The solar rays are vertical over the
Tropic of Capricorn in winter season. As a result, low pressure area
develops in Australia and high pressure area in Asia. Hence, the
monsoon w inds change their direction and become north -east
monsoon winds. As these winds mainly come from land, they are
not able to give rainfall to a large area The Coromandal coast in
Tamil Nadu gets some rainfall from the North -eastern Monsoon
winds ( Figure 2.8).
Figure 2.8.Monsoon winds in Summer and Winter
Sea breeze :During daytime land is heated faster than the water
along the coastal areas .Air near the land is also heated. As a
result it expands. The dens ity of warm air becomes less than the
surrounding cooler air. Therefore it moves in the upward direction.
Hence, low pressure area develops over the land. The air pressure
is comparatively higher over the sea, as water is relatively cooler.
Therefore the air from the sea moves towards the land as sea
breeze during daytime Figure: 2.9.
Land breeze : Water retains its heat for a longer period of time
than land. Hence the cooling process at sea is very slow. On the
other hand, land is cooled more quickly. Hence, high pr essure area
develops over the land and low pressure area develops over sea.
Thus the air from the land moves towards the sea as land breeze
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Figure: 2.9.Development stages of sea breeze
.
Land and sea breezes have moderating influenc eo nt h e
climate of the coastal areas. Hence in this region, the range of
temperature is less. Fishermen take advantage of these winds for
going into the sea for fishing and returning to the shore.
Figure: 2.10.Development stages of land breeze.
Local Winds :
Planetary or permanent winds have their influence on the
entire globe but local winds have their influence only on smaller
local areas over which they prevail. Therefore these winds are
termed as local winds. These are known by different names i n
different regions. Some of the local winds are as follows :
Mountain and Valley Winds :
These are formed in the hilly regions. During the night the air
at the mountain tops becomes quite cool. Its density increases and
it flows down the slopes into the valley below during nigh -time or
early in the morning before sunrise. As these winds come from
mountains these are termed as Mountain winds. These are also
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Figure 2.11. Mountain and Valley winds
During the day, air at the bot tom of the valley is heated due
to solar radiation. Its density becomes less. This light and warm air
moves in the upward direction along the hill slopes. As these winds
come from the valley, these are termed as Valley winds o r
Katabatic winds.
Chinook :
‘Chinook’ means ‘snow eater’. This local wind descends
down the eastern slope of the Rockies mountain range in the
western part of North America.
As Chinook descends down from the eastern slope of the
Rockies, it becomes warm (ascending air along hill s lope becomes
cool, while descending air becomes warm) Snow melts due to
Chinook in winter. Hence, it is termed as snow eater or Chinook. It
increases the temperature of the prairies and helps in farming
activities in the region.
Fohn :
The winds which cr oss the Alps and descend down in
Switzerland are called as Foehn (or Fohn). Other similar winds
which descend down the mountain slopes and increase the
temperature are as follows:
Norwester –In New Zealand
Santa Ana –in California
Samun –In Iran
Bergs–in South Afric a
Other types of local winds are –
Sirocco –(hot) –Sahara
Salano –(hot) –South Spain
Khamsin –(hot and dry) –Egypt
Simoon –(hot and dry) –Arabia
Mistral –(Cold) –France
Bora -(cold) –Adriatic coast of Yugo -slaviamunotes.in

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Check Your Progress:
1.Define winds. Give a classification of winds.
2.Explain land breeze and sea breeze .Draw diagram
3.What is Chinook?
These winds are associated with the pressure system, i.e.
low pressure or high pressure. These winds do n ot blow in any
particular direction regularly. Their origin and direction of movement
depends upon the formation of pressure system. The major types
of variable winds are Cyclones and Anticylones.
Cyclones :
Cyclones are associated with the low pressure system.
Cyclones which develop in tropical belt are known as Tropical
cyclones, while the cyclones which are developed in the temperate
belts are known as Temperate cyclones. Tropical cyclones are
more intense and more destructive than the temperature cycl ones.
Tropical Cyclones :
A tropical cyclone is represented by concentric circles of
isobars having low pressure area in the centre. The pressure
gradient is very steep and so the isobars are very close to each
other. Winds move towards the centre and t he direction of winds is
anticlockwise in the northern hemisphere and clockwise in the
southern hemisphere.
Major characteristics of tropical cyclones:
Tropical cyclones mostly develop over the seas (water -bodies).
Isobars are usually in the form of co ncentric circles.
Pressure gradient is very steep.
Wind speed is more than 100 km. per hour.
Tropical cyclones are small in size.
These are associated with heavy rainfall.
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The cyclones travel from east to west.
The centre of the cyclone is known as ‘eye of the cyclone’ .
There is no movement of air (‘calm’ condition) and no rainfall at
the centre of the cyclone.
Direction of wind is anticlockwise in the northern hemisphere
and clockwise in the southern hemisphere .
Tropical cyclones are known by different names in different parts
of the world, e.g. Typhoons in China, Tornadoes in USA, Cyclones
in India, Hurricanes in the Caribbean and Willy -Willies in Australia.
Tornadoes :These are more intense tropical cyclo nes, where the
wind speed is between 300 -500 kilometres per hour. These are
mainly found on the Mississippi plains of the USA. They cover a
very small area, i.e about 100 to 500 metres of diameter. Funnel -
shaped thick ground to sky. It appears dark as bri cks, sand and
other material on the Earth is lifted in the funnel by the strong wind.
Hence it causes maximum destruction.
2.5. CONCLUSIONS :
It is important to note that the differences in pressure caused
by temperature differences due to variable facto rs give rise to the
formation of different seasons. Hence throughout the year when it
is harvesting in northern hemisphere, the southern hemisphere
undertakes the sowing of seeds. This seasonal difference provides
us with food crop and other agricultural p roducts throughout the
year.
2.6.QUESTIONS:
1.Define air pressure. Explain factors determining atmospheric
pressure conditions on the earth.
2.Write a detailed note on distribution of pressure belts. Draw neat
diagram.
3.How do land and sea breeze i nfluence the pressure conditions?
4.Write a note on mountain winds.
5.How is a cyclone formed? Give different types of cyclone
6.Draw a neat diagram to represent Chinook wind.
7.Explain the following:
a) Doldrum b) Ferrel’s law c) Tornadoesmunotes.in

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References:
1.Pednekar,Shinde,Thakur (2015): Physical Geograph y, F.Y.B.A.
Semester I and II, Sheth Publishers Private Limited, Mumbai.
2.K.Siddhartha(2001):Atmosphere, Weather and Climate,
Kausalaya publications Pvt. Ltd. New Delhi
3.Surender Singh (2005): G eography, Tata McGraw -HillPublication
New Delhi.
4.SavindraSingh(2011): ‘Clim atology’, Prayag Pustak Bhavan,
Allahabad.
5.https://www.google.co.in/search?q=diagram+for+winds+and+pres
sure+of+world&biw=1366&bih=655&tbm


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Unit -3
HUMIDITY, CONDENSATION AND
PRECIPITATION
Unit Structure :
3.0 Objectives
3.1 Introduction
3.2 Humidity
3.3 Condensation
3.4 Precipitation and its types
3.5 Precipitation and its distribution
3.6 Conclusions
3.7 Questions
3.0.OBJECTIVES :
To understand humidity, its nature and its types.
To study the methods used for measuring humidity.
To know the process of condensation and its types.
To understand the formation of clouds and its types.
To study precipitation, its types and nature of d istribution .
3.1INTRODUCTION
The Earth is known as a Wet planet as 71% of it consists of
water, which is essential for all forms of life. Water is available on
the earth in various forms depending upon its location, position and
time.T h ef o l l o w i n gd i a gram represents three stages of water.
Figure: 3.1.Three forms of watermunotes.in

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Latent heat is required for the transformation of water from
one stage to another. This heat is not recorded on thermometers. It
is required for the change of state, e.g. when 80 c alories (cal) of
latent heat is given to ice, it melts and ice (solid) is converted into
water (liquid). Similarly, evaporation requires latent heat of 560
calories. This heat is released into the atmosphere at the time of
condensation which is termed as l atent heat of condensation.
3.2. HUMIDITY
Humidity is the amount of water vapour present in the air.
The proportion of water vapour in the air is not same everywhere. It
varies from less than one percent to about 4 percent by volume. It
is less in the arid regions and more in the humid regions.
Water vapour is invisible. It becomes visible when water vapour
is converted into water droplets of ice crystal by condensation
Types of Humidity :
When air contains large amount of water vapour it is termed a s
humid or wet air. When the amount of water vapour present in
the air is less it is termed as dry air. Humidity can be e xpressed
in three ways. They are :
i) Absolute Humidity: It is the actual amount of water vapour
present in the air. It is expressed i ng r a m so fw a t e rv a p o u rp e r
cubic meter of air. Absolute humidity varies with the expansion or
contraction of air, due to change in temperature. It is a theoretical
concept and is not much used in day -to-day measurements of
humidity.
ii) Specific Humidit y:It is the weight of water vapour per weight of
a given mass of air. Specific hu midity is usually expressed as
grams of water vapour per kilogram of air. It is not affected by
changes in temperature of air. It is maximum at the equator and
minimum at the poles. It is more on the oceans than on land.
iii) Relative Humidity: Relative humidity is a ratio between the
amount of water vapour actually present in the air -absolute
humidity (AH) to the amount of water vapour that air can hold at the
given tempera ture-saturation humidity (SH).
Relative humidity is expressed in percentage.100AHRHSHWhere:
RH : Relative humiditymunotes.in

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AH : Actual humidity, i.e. the amount of water vapour actually
present in the air.
SH : Saturation humidity, i.e. th ea m o u n to fw a t e rv a p o u rt h a tt h e
air can absorb/hold at the given temperature.
The capacity of air to hold moisture changes according to
the change in temperature. Air can absorb and hold more water
vapour when the temperature of air is more. Air is con tracted when
temperature is less. Therefore, less amount of water vapour is
absorbed in such air.
Relative Humidity Type of Air
RH=20% Dry air
RH=80% Humid air
RH=100% Saturated air
Wet and dry bulb thermometers (psych -rometer or
hygrometer) areused for the measurement of relative humidity.
Measurement of relative humidity :Wet and dry bulb
thermometers consist of two ordinary thermometers. The bulb
of the wet bulb thermometer is kept permanently moistened by a
piece of muslin (cloth) wrap ped around it and an attached length
of wick is dipped into a reservoir of water which is kept below
this thermometer.
Evaporation of water takes place from the wet muslin which
covers the wet bulb thermometers. Some amount of heat will be
extracted from the wet bulb thermometer. Therefore, the wet bulb
thermometer always records lower temperature than the dry bulb
thermometer.
The rate of evaporation is related to the amount of water
vapour present in the air. If the air is dry, there is a lot of space to
accommodate water vapour. Hence,, the rate of evaporation is
rapid. More heat is extracted from the wet bulb thermometer.
Therefore, it records much lower temperature than the dry bulb
thermometer.
Check Your Progress:
1.Define humidity.
2.Give different types of humidity .
3.How is humidity measured?munotes.in

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3.3.CONDENSATION
Condensation is the process through which water vapour
change into liquid state and become tiny droplets of water or ice
Figure: 9.2.This process is opposite to evaporat ion. In evaporation
water turns into vapour (gaseous) form. Thus any further cooling of
the saturated air starts the process of condensation. Temperature
of air decreases with increase in height from the Earth’s surface.
The rate of decrease in temperature with increase in height is
termed as Lapse Rate which is about 6.50C per kilometer. With
decrease in temperature, the capacity of the air to hold moisture
also decreases. As a result relative humidity increases. Air is said
to be saturated when the relati ve humidity is 100 percent. At this
stage the process of condensation, (conversion of water vapour
into water droplets) begins. The temperature at which relative
humidity becomes 100 percent and condensation begins is termed
as Dew Point Temperature.
Figure: 3.2.Condensation
Process of Condensation :
Condensation is a process in which water vapour is
converted into water droplets or snow (ice crystals).Water vapour,
which is in gaseous form in the air and is invisible, is converted into
liquid –visible water droplets, if the temperature of the air is above
00C (i.e. freezing point). If the temperature of the air is less than
00C, i.e. less than freezing point, then the condensation is in solid
form as ice crystal. Latent heat of about 560 calories is released
during the process of condensation.
Condensation depends upon two factors –
a)Relative humidity of the air
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a)Relative humidity of the air is less in desert region (where the
air is dry). Hence, greater degree of co oling is required to start the
process of condensation in these regions , where the air contains lot
of water vapour. Hence lesser degree of cooling issufficient to start
the process of condensation.
b)Condensation by degree of cooling is a result of f our distinct
processes.
i)Cooling by expansion –Adiabatic process :The Earth’s
surface is heated due to solar radiation during daytime. Air near the
Earth’s surface is also heated. After sunset and during the night,
the surface of the Earth cools down due to terrestrial radiation. As a
result, the air near the surface of the Earth is also cooled. Due to
the process of condensation, the water vapour present of
condensation, the water vapour present in the air near the surface
of the Earth is now convert ed (condensed) into dew (water droplets
on grass blades and other low level vegetation) or into frost (small
ice crystals).
ii)Cooling of air when it comes in contact wit h cold surface of
land or water :Air mass is a huge mass of air having uniform
characteristics, i.e., temperature and humidity. Condensation takes
place when warm and cold air masses come in contact with each
other (boundary between warm and cold air masses is known as
front.) Condensation and precipitation take place at the front.
iii)Cooling of air due to direct outgoin g radiation (Earth’s
radiation) :Atmospheric pressure decreases with increase in height
from sea level. In the diagram above at (A), the atmospheric
pressure is more as compare to that at (B) and at (C). Imagine an
air parcel moving in the upward direction from (A). This air parcel
will expand or increase in size as it rises to (B) and (C) positions.
Temperature of the air parcel will decrease due to the expansion of
air. This type of change is termed as adiabatic tem perature change.
Air is attracted towards the Earth’s surface due to the gravitational
force. The weight of the air at sea level is 1013 mb, i.e. average
atmospheric pressure.
A)Decrease in temperature due to increase in height:
In this case, the air parcel which is moving in the upward
direction expands due to reduction in air pressure at higher
elevation. Hence, the air parcel expands –molecules of air in air
parcel move further apart and temperature of the air parcel
decreases to 200C. This change in temperature due to reduction in
pressure caused by an increase in the altitude is known as
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B)Increase in temperature due to decrease in height:
The temperature of air parcel in this case is 200C. This air
parcel is m oving in the downward direction towards the Earth’s
surface. The temperature of this air parcel will increase from 200C
to 300C. This is mainly due to the increase in the outside pressure.
In the process of condensation water vapour needs some
kind of a surface on which it may condense, e.g. dew is formed on
vegetation at the ground level or on cooled metal objects.
At higher elevation such type of surface (and support) is
provided by very small, microscopic particles which are termed as
aerosols or con densation nuclei. These are added to the
atmosphere through various sources as ocean spray -salt particles,
dust, smoke, active volcano, fires, factories, etc.
Raindrops may grow up to 7mm (7000 microns) in diameter,
while drizzle droplets are less than 0 .5 mm in diameter. The
hygroscopic particles are much smaller (< 1 micron). The cloud
droplets vary in size from 2 -3 microns to 20 microns.
Relative sizes of raindrop, drizzle droplet, fog droplets, cloud
droplets and condensation nuclei (aerosol).These being very tiny,
are practically weightless. Therefore, clouds can remain suspe nded
in the air.
Forms of Condensation
Condensation can be in liquid form (water droplets), or in solid form
(ice crystals) depending on temperature of the air Figure 9.3.
i)Liquid forms of condensation : (temp >00C)–dew, fog, mist
and clouds.
ii)Solid forms of condensation : (temp <00C)–frost, rime and
clouds.
Forms of condensation can also be classified according to
theirplace of formation :
i)On the ground : D ew and frost.
ii)Near the ground : Mist, fog and rime.
iii)At higher elevation : Clouds.
i) On the Ground:
a)Dew:It is a liquid form of condensation formed on the ground,
on grass blades, etc. It is formed early in the morning before
sunrise when the temp erature of the Earth’s surface and the air
near the surface is the lowest. Dew is mostly found in the areasmunotes.in

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having temperature more than 00centigrade. The conditions
necessa ry for the formation of dew are :
Sky without cloud cover.
Calm weather.
Presen ce of sufficient amount of water vapour in the air.
Dew is very useful for plants. Desert areas have very less
humidity in the air, the plants depend on dew for water.
b)Frost :It is a type of condensation, at the ground level. It results
in the forma tion of solid ice crystals. Weather conditions required
for the formation of frost are same as those for dew, except that the
temperature is below freezing point. At the time of condensation, if
the temperature of air is below freezing point, water vapour is
directly converted from gaseous state (water vapour) to solid state
(ice crystals). Hence, frost should not be termed as frozen dew.
Valleys are the ideal locations for the formation of frost.
During right -time cold and heavy air accumulates in such l ocations.
(This air displaces warm air at the valley bottom and thus causes
the inversion of temperature). Hence, valley floors or bottoms are
affected by frost but the sides of valley remain unaffected.
Therefore, human settlements are located at these ar eas and not at
the valley bottom.
Frost is harmful to plants. Protection against frost is a great
challenge to horticulturists in some of the temperate belt countries
of the world. Various heat conservation methods are used for the
protection of plants, e.g. covering the plants with paper or cloth.
ii)Near the Ground
a) Mist and Fog: These are formed near the ground. They are the
forms of condensation in which minute droplets of water remain
suspended in the air. Mist and fog both reduce visibility. If the
horizontal visibility is up to 2 km, it is termed as mist. But if the
horizontal visibility is only up to 1km, it is termed as fog. Fog is
denser than mist. Therefore visibility is reduced in the case of fog.
Based on the visibility there are variou st y p e so ff o g .F o re x a m p l e ,
moderate fog, thick fog, dense fog, very dense fog, killing fog etc.munotes.in

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Figure 3.3.Types of Condensation
c)Smog : Combination of smoke and fog is termed as smog (smog
is also termed as pea -soup fog) which is more dense and
dangerous. Since smog includes pollutants, smoke, etc. people
suffering from chest problems such as asthama, bronchitis, etc. are
more affected by smog. Visibility is greatly reduced during dense
fog and smog. Therefore it is hazardous for traffic. In tem perate
belts, solar rays are slant and in winter days are shorter than the
nights. The presence of excessive smoke near the ground during
smog obstructs the solar radiation from reaching the Earth’s
surface. Therefore the Earth’s surface is not much heated .A sa
result a smog prevails for several days.
A combination of smoke and haze is termed as Smaze.
d)Haze is a mass of very small solid particles of dust, smoke, etc.,
due to which the visibility is reduced to below 2km, and up to a
minimum distance of 1 km.
Check Your Progress:
1.What is condensation?
2.What are the processes involved in condensation?
3.Give different forms of condensation.
iii)Clouds :
Clouds are a condensation at the upper level. Clouds are
composed of tiny water dr oplets or ice crystals. Clouds are
basically classified according to their shapes. Two main shapes of
clouds are (a) cumulus and (b) stratus (F i g u r e r 9.4).munotes.in

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Figurer: 3.4. Types of Clouds
World Meteorological Organisation (WMO) has classified
clouds i ntofollowing 10 types as per their occurrence at specific
heights. Figure:9.5.:
a)Upper level clouds (about 6 km.)
i)Cirrus clouds : These upper level clouds have fibrous
appearance and are white in colour.
ii)Cirrocumulus : These are in the form of thic k white patches,
made up of small ripples.
iii)Cirrostratus : Transparent, whitish clouds with hair -like smooth
appearance. They cover large portions of the sky and produce
halo effects.
b)Middle level clouds (approx. 4 km.)
i)Altocumulus : They are white o r grey rolls or rounded masses
of clouds.
ii)Altostratus : Greyish or bluish clouds in the form of sheets, or
layers. They cause precipitation.
c)Low level clouds (below 2 km.)
i)Cumulus : Dense with sharp outlines, developing vertically in
the form of ris ing mounds or domes. The bulging upper parts of
these colouds resemble cauliflower.
ii)Stratus : it is a low, grey cloud cover. This layer may produce
drizzle or snow.
iii)Stratocumulus : These are low, whitish or grey clouds with
dark patches.
iv)Cumulonimb us :Cloud with great vertical extent, in the form of
a huge tower. Upper part spreads in the shape of an anvil dome
(Figure 9.5). These clouds are associated with heavy rain,
thunder, lightning, hail and tornadoes.munotes.in

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v)Nimbostratus : These are low level clo uds associated with rain,
snow or sleet. Streaks of rain or snow falling from these clouds
but not reaching the ground are called ‘Virga’.
Figure:3.5. Types of Clouds at higher eleva tion
Figure:3.5.1.Cumulonimbus Clouds
Check Your Progress:
1.What are clouds?
2.Mention different types of clouds.munotes.in

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3.4.PRECIPITATION :
Process of Precipitation
If you are doing some chemical reaction in a test tube and
tiny particles are formed around it during the proces so ft h i s
reaction, these particles will slowly move in the downward direction
and settle at the bottom of the test tube. This deposition of tiny
particles in the test tube is known as precipitate.
The term precipitation is used for the deposition of wate ri ne i t h e r
liquid (e.g. drizzle, rain) or solid (e.g. snow, hail) form, which reach
the Earth from the atmosphere.
Forms of Precipitation
Precipitation can be in liquid or in solid form or a combination
of both. Major types of precipitation are as foll ows (Figure 9.6.):
i) Drizzle: It is a liquid form of precipitation. The size of the water
droplets in drizzle is very small (less than 500 micro -metres). It is
often associated with fog.
ii) Rainfall: It is a liquid form of precipi tation. The size of raindrops
is larger than the drizzle droplets. The size varies from 200 microns
to up to 7000 microns. Millions of cloud droplets are required to
form one single raindrop. For example, a raindrop of 3 millimetres
in diameter contains 27 million cloud dropl ets of 10 micron
diameter.
Figure :3.6. Drizzle and rainfall
iv)Sleet :Sleet is a mixture of rainfall and snowfall. If the
temperature of the upper air is less than freezing point (<00C) then
the condensation will be in the form of ice crystals. So me ice
crystals will be large and some will be small. If the temperature of
the air at the ground level is more than 00C then these ice crystals
will melt while coming down towards the Earth’s surface. Small ice
crystals will melt completely and will be co nverted into watermunotes.in

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droplets and large ice crystals will partly melt, but will remain as
smaller ice crystals. Hence, the precipitation will be a combination
of rainfall and snowfall which is termed as sleet.
v)Snowfall: In the case of snowfall temperatu re at the ground level
and upper level is less than freezing point (<00C).
vi) Hail: Hail or hailstones are small balls of ice having diameter
ranging from 5 to 40 mm. Hailstones consist of concentric layers of
ice alternating with layers of snow (like on ions). Hail is normally
associated with cumulonimbus clouds. Due to strong vertical air
currents the raindrops are taken at higher elevation where they
freeze and are converted into ice crystals. These ice crystals come
down due to the Earth’s gravitationa lf o r c e ,b u ta g a i nt h e ya r e
trapped in the strong vertical air currents and move in the upward
direction. More ice crystals and water droplets are added to original
ice crystals. As a result, the size of the original ice crystals
increases .U l t i m a t e l y ,i t falls on the Earth’s surface as hailstones.
Hailstones are very destructive. They can destroy crops. They are
also dangerous for aircraft.
Figure :3.7.Sleet, Snowfall and Hail
Types of Rainfall
The upward -rising air becomes cool. Its relative humidi ty
increases, and condensation begins, leading to cloud formation and
subsequent rainfall. Types of rainfall are based on the method by
which air is uplifted. Types of rainfall are as follows:
i)Convectional rainfall.
ii)Relief or Orographic rainfall.
iii)Cyclonic rainfall or Frontal rainfall.
i)Convectional Rainfall :This type of rainfall is experienced in the
equatorial region. Air near the equatorial region is heated more due
to high intensity solar radiation. It expands, becomes lighter and
moves in the upward direction in the form of convectional currents.munotes.in

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The rising air cools down. Relative humidity increases and
condensation begins. Clouds are formed and every day at about
4.00 p.m. the equatorial regions get convectional type of rainfall
Figur e:3.8. Hence, it is said that there are no seasons in the
equatorial region.
Figure :3.8.Convectional rainfall
ii) Relief or Orographic Rainfall :In this case, wind is obstructed
by mountain ranges and is forced to rise up. As the air rises
upward, i t cools, relative humidity reaches 100 percent (air is
saturated) and condensation begins. This cause the formation of
clouds. They provide rainfall to the windward side of the mountain
slope. Western side of the Western Ghat, i.e. Konkan coast,
receives t his type of rainfall. As in this rainfall air is lifted due to the
obstruction caused by the Western Ghats, it is termed as
Orographic (‘Oro’ means mountains) rainfall or Relief rainfall
Figure :3.9.
Figure :3.9.Orogr aphic/Relief Rainfall
When rain -bearing winds cross the mountain ranges, they
become warm and expand. Hence they are able to absorb more
water vapour. So the clouds coming towards this region are notmunotes.in

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able to give rainfall or very less a mount of rainfall is received in the
leeward side of the mountains. Therefore it is termed as the rain-
shadow zone.
Konkan coast, north -eastern part of India, Southern slopes
of Himalayas, East China, South -East United States and East
Brazil receive orogr aphic type of rainfall.
iii)Cyclonic Rainfall or Frontal Rainfall :Tropical cyclones
develop in the tropical region. Due to cyclonic activity, air is lifted up
with a great force. Condensation takes place due to cooling. As a
result clouds are formed. T he region gets heavy rainfall, but it lasts
only for a few hours Figure 3.10.
Figure: 3.10Tropical Cyclone
On the other hand, frontal rainfall is experienced in the
temperate regions due to the development of fronts. They are
formed due to the mergi ng of air masses having different
characteristics. Air mass is a huge quantity of air with respect to
temperature and humidity conditions. Thus, when two air masses
having different characteristics (cold and warm) move towards each
other, they do not mix i nstantly as their properties are different. The
warm air mass being lighter and less dense rides over the dense
cold air mass. Thus az o n eo f separation develops as a front. Warm
air uplifted along the front cools and clouds are formed along the
front. As ar e s u l t ,t h er e g i o ng e t sr a i n f a l l .T h i sr a i n f a l li si nt h ef o r m
of drizzle and covers larger areas. This type of rainfall may last for
many days. Figure: 3.11munotes.in

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Figure: 3.11.Temperate Cyclone
Thunderstorm
Thunderstor ms are different from cyclones. In the case of a
cyclone wind rotates around the centre but in a thunderstorm, the
wind does not rotate. Thunderstorms are associated with strong
upward moving air current, cumulonimbus clouds, lightning and
thunder with hea vy rainfall. They are sometimes termed as
cloudburst.
Due to intense heating of the Earth’s surface very strong and
hot convectional currents are generated. Cumulonimbus clouds are
formed having great vertical extent. Development of static electricity
(i.e. positive and negatively charged particles) causes flashes of
lightning and thunder is produced by the expansion of air due to the
tremendous heat of the lightning flashes. Thunderstorms are also
associated with hailstones.
3.5.DISTRIBUTION OF RAINFA LL
Distribution of rainfall on the Earth is not even all over. Some
regions receive rainfall throughout the year while some regions
receive it either in summer or in winter. Various factors influence
amount of rainfall received by a region. These are lo cation, nature
of relief, amount of vegetation etc. Distribution of rain amount is as
follows :
a)Rainfall throughout the year :Equatorial region and areas of
Western Europe, influenced by westerlies, get rainfall throughout
the year. Rainfall in the e quatorial region is heavy.
b)Summer Rainfall :The savanna region (continental interiors)
and tropical monsoon regions receive rainfall during summer.
Tropical monsoon regions in India receive heavy rainfall during
summer due to sough -west monsoon winds. Due to the apparentmunotes.in

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change in the direction of vertical sunrays, southern part of India
also receives rainfall from north -east monsoon winds during winter.
c)Winter Rainfall :The Medi terranean region is the region which
gets rainfall only in winter. Due to southward shifting of pressure
and wind belts, this region is under the influence of westerlies
during winter. Therefore, it gets rainfall in winter.
Distri bution of Rainfall in the World:
S.N. Nature of Rain Rainfall
amount in cmRegion
I Regions of Heavy
Rainfall>200 i)Equatorial
ii)Monsoon
iii)West Coast of Europe
and North America
II Regions of
Moderate Rainfall200-25 i)South -East Asia
ii)East China
iii)East Brazil
III Regions of
Scanty Rainfall<2 5 i)Tropical hot deserts,
e.g. Sahara.
ii)Mid-latitude deserts ,
e.g. Gobi, Australian
desert, etc.
iii)Polar region, i.e. cold
deserts such as
Antarctica
Check Your Progress:
1. Define precipitation.
2. What are the processes involved in precipitation?
3. Give different forms of precipitation.
4. Why rainfall is not e venly distributed in the world?
4. Explain the following:
i) Equatorial rain ii) Summer rain iii) Winter rainmunotes.in

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3.6. CONCLUSION:
The amount or precipitation in different forms that provides
fresh water supply to a region is governed by vario us processes of
humidity formation, its condensation and precipitation over the
period of time. This is determined by its absolute and relative
location, the nature of relief, amount of vegetal amount etc.
Interference in this natural mechanism invites nat ural disasters in
the form of floods or droughts, cyclones etc.
3.7. QUESTIONS:
1.Define humidity. How do we measure different types of
humidity?
2.Define Condensation. Explain the processes and forms of
condensation.
3.What are clouds? Give a class ification of clouds.
4.What is precipitation? Why do we get variation the type of
precipitation?

munotes.in

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51Unit -4
OCEAN RELIEF FEATURES AND
SALINITY OF SEA WATER
Unit Structure:
4.1Objectives
4.2. Introduction to ocean morphology.
4.3. Ocean morphology
4.4. Bottom relief features of the Pacific Ocean
4.5. Introduction to Salinity of Ocean
4.6. Worl d distribution of Salinity.
4.7. Conclusion
4.8. Questions
4.1OBJECTIVES
To study the morphology of ocean.
To study the distribution of relief features in Pacific Ocean.
To study the salinity of sea water.
To understand the factors responsible for va riation in salinity of
sea water.
To study the distribution of salinity of sea water in the world.
4.2.INTRODUCTION TO OCEAN MORPHOLOGY:
The configuration of the ocean basin with reference to their
nature and various dimensions is known as morphology of oceans.
Different marine provinces form the basis for identification of
various relief zones of ocean basins. Oceans cover approximately
65.7% (335 million square kilometers) of Earth's surface. Like the
land surface, the ocean bottoms represent various kinds of marine
features, i.e. plain, plateau, ridge, deep etc. The knowledge about
the relief of ocean floors was very limited in the past centuries but
the development of sound recording machine enabled the
oceanographers to get more knowledge about the dept and
topography of the ocean bottoms.munotes.in

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524.3. OCEAN MORPHOLOGY :
A hypsometric curve is essentially a graph that shows the
proportion of land area that exists at various elevations by plotting
relative area against relative height. The hypsograph ic curve is a
scientific way of describing the topography of the seafloor.
After the study of hypsographic curve, the ocean and sea
bottoms may be demarcated into various relief or depth zones as
given below:
i.Continental shelves
ii.Continental slopes
iii.Abyss al or deep sea plains
iv.Ocean deeps
Figure4.1: Hypsographic Curve: A Schematic Representation
of the Ocean Floor
i)Continental Shelves:
Continental shelves represent the shallow part, of the ocean
and seas. It extends between low tide level and 100 fatho ms depth
of the sea water. It is almost uniform zone of sea bed with a gentle
gradient. The width of the continental shelves varies from coast to
coast depending upon the geographical structure and
geomorphological nature of the coast. The average width of
continental shelves is about 70 km.
The continental shelves are formed by erosion of coastal
area by sea waves. Some continental shelves have been formed by
deposition of sediments in the sea brought by the rivers. According
to some views, the continen tal shelves were formed due to change
in the sea level or subsidence of coastal land by tectonicmunotes.in

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53movement. The formation of continental shelves is also thought due
to emergence of the ocean and sea bottom.
The continental shelves have been grouped in foll owing types on
the basis of their formation and location. a) Glaciated shelf, b) Coral
reef shelf, c) Riverine shelf, and d) mountainous shelf.
Importance of Continental Shelves:
a)Marine life is abundant due to availability of sunshine. Sunrays
penetrate t o the depth of the shelf and various kinds of grass,
seaweeds and planktons are available. They are used as food
for marine animals.
b)Various kinds of minerals like petroleum, natural gas are also
found in shelf area.
ii)Continental Slope:
Continental slope is the next depth zone of ocean and sea
floor to the continental shelf. It begins with outer edge of the shelf
where the gradient of slope becomes steeper abruptly. The
average slope of this zone is 4 degree but it varies from place to
place. This zone is free from the deposits due to steep slope and
about 60 per cent of sediments of continental slope are mud and
the rest is composed of sands, gravels and organic remains. It
covers smaller area of the oceans and seas. There are five types of
slopes :
a)Fairly steep slope dissected by canyons,
b)Gentle slopes with elongated hills and basins,
c)Faulted slopes,
d)Slopes with terraces, and
e)Slopes with seamounts.
The origin of continental slope is still not known, although
some may be as the r esult of large scale earth movement called
Plate tectonic. It does not always extend up to deep sea but
interrupted by wedge of deposited sediments. The deep waters of
thecontinental slopes are very ideal for catching deep -living
Rockfishes, Thorny heads ,Sable fishes and Dover Sole. Generally,
these species are caught as a group.
iii)Abyssal Plain or Deep Sea Plain
Beyond the continental slope a broad and featureless
deeper part of the ocean is found. This deep plain is known as
Abyssal Plain or Deep Sea Plain. It is found between 2000 to 6000
meters depth. Gradient of slope is very gentle and it appears as
uniform flat plain without any relief. Moreover, the deep sea plain is
formed by deposition of sediments of especially marine origin. The
abyssal plain exerts significant influence upon ocean carbon
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54concentrations over timescales of 100 –1000 years. The structure
and function of abyssal ecosystems are strongly inf luenced by the
rate of flux of food to the seafloor and the composition of the
material that settles.
iv)Ocean Deeps
On the ocean bottom al a r g en u m b e ro fn a r r o we l o n g a t e d
deep depressions have been recorded by the sound recorder
device. Their depth is rather greater than the abyssal plain and
some of them reach up to more than 9 kms below sea level. The
peculiarity of the distribution of ocean deeps is that they do not
occur amidst the ocean bottom but along the coast parallel to the
fold mountain. Most of the heat from global warming is absorbed by
the oceans as it covers 70% of the Earth's surface and therefore
has a capacity to absorb heata thousand times more than
theatmosphere. Oc eans are thus the main heat sinks of the earth .
Besides above discussed major relief of the ocean bottoms
submarines canyons, submarine ridges, abyssal hills, seamount,
guy outs and atolls are also are commo nly found in the oceans.
v)Submarine Canyons
A long narrow depression is commonly found on the
continental shelves and slopes. These depressions are called as
Submarine Canyons. They are usually perpendicular to the coast.
Their profiles resemble V -shaped valley with concave rock wall.
There are three groups of canyons on the basis of their location
and appearance: (1) submarine canyons which begin from
continental shelves and extend to the slopes, (2) canyons, which
are found at the mouth of the rivers an d extend up to continental
slopes, and (3) dendritic canyons which are cut down in many
branches.
Submarine canyons are diverse and complex in terms of
their origins, hydrography, geologic settings and biodiversity of
marine reserves in many locations be cause of their association with
higher biomass and biodiversity.
vi)Abyssal Hills, Seamount and Guyot
Some isolated hills on the bottom of the seas and oceans
are called Abyssal Hills. The abyssal hills attaining height of more
than 1000mt are called Seamou nts. Broad and flat topped
seamounts are called Guyots. Atolls, concentric coral reefs and
deep wide lagoon are common features in any ocean.
vii)Submarine Ridges
Like mountain chains on land, a long continuous chain of
mountains also spread out in the mid of the oceans. They form the
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554.4BOTTOM RELIEF FEATURES OF THE PACIFIC
OCEAN
Pacific Ocean is the largest and the deepest ocean of the
world. It extends from the Antarctic region in the south to the Arctic
in the north and lying between the continents of Asia and Australia
on the west and North and South America on the east.
Pacific Ocean, like the res t of the world's oceans, was
formed millions of years ago and has a unique topography. It also
plays a significant role in weather patterns around the globe and in
today's economy.
The Pacific Ocean has a highly varied topography. Oceanic
ridges are foun d in a few places in the Pacific Ocean and they are
the areas where new oceanic crust is being pushed up from below
the Earth's surface. The Explorer Ridge lies at the northern
extremity of the Pacific Ocean. The Jaun de Fuca lies west of
Vancouver.
Cont inental shelves cover rather less area while the deep
sea plain occupies largest area of ocean bottom. The depth of the
sea plains is also greater than that of sea plains in other oceans.
This ocean is associated with a large number of abyssal
hills, se a mounts, guyots, and plateaus like Albatross near western
coast of South America. An abyssal hill is a small hill that rises from
the floor of an abyssal plain . The greatest abun dance of abyssal hills
occurs on the floor of the Pacific Ocean . These Pacific Ocean hills
are typically 50 –300mi nh e i g h t ,w i t haw i d t ho f2 –5km and a
length of 10 –20km. They may be created along the flanks of the
East Pacific Rise ashorsts and graben features, and then become
stretched out with the passage of time. Nearly half of the world's
seamounts are found in the Pacific O cean ,a n dt h er e s ta r e
distributed mostly across the Atlantic and Indian oceans. Guyots are
flat-topped seamounts. Thousands of guyots, often in chains and
clusters, are spotted across the Western Pacific Ocean, especially
between the Hawaiian Islands and Japan.
Pacific Ocean is basically known for the island arcs and
ocean deeps or trenches. Out of 52 known ocean deeps, 32 are
found in this ocean. The Pacific is home to the deepest ocean point
in the world -the Challenger Deep in the Mariana Trench .T h i s
trench is located in the western Pacific to the east of the Mariana
Islands and it reaches a maximum depth of 35,840 feet. Kurile
Trench, Tonga Trench, Aleutian Trench, Japan Trench, Tuscarora
Deep, etc are the other important ocean deeps in this ocean. This
ocea n is exceptionally free from the submarine ridges. Themunotes.in

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56concentration of large number of deeps and island arcs from
Aleutian island to New Zealand through Japan is attributed to the
subduction of oceanic plate beneath the Asiatic continental plate
along the margin of convergence in Pacific Ocean. The northern
Pacific Ocean (and also the northern hemisphere) also has more
land in it than the South Pacific. There are many island chains and
small islands throughout the ocean. The largest island within the
Pacif ic is the island of New Guinea. Volcanic eruption and
earthquakes are very common phenomena in this ocean.
The characteristic features of this ocean are the feeble
development of continental shelves, absence of any continuous
middle dividing rise and ab undance of deeps and trenches.
Check your progress:
1.What do you understand by morphology of ocean?
2. With the help of hypsographic curve explain different relief
features of ocean basin.
3.Explain with examples various relief features found in th e Pacific
Ocean.
SALINITY OF SEA WATER
4.5INTRODUCTION TO SALINITY OF OCEAN:
Salinity is defined as “the ratio between the weight of the
dissolved materials and the weight of the sample sea water”.
Salinity is expressed as gram percentage (% ). Salinity can be
measured using a Handled Refract meter ,H y d r o m e t e ro r
Conductivity Meter. Salinity of the oceans and seas vary in open
seas. Usually where there is free mixing of fresh water, the
proportion of salinity remains constant, but where free m ixing is
absent, variation is seen. The average salinity of the sea water is
34.5%. The total amount of salt in sea water is gradually increasing
every year. This is because it is brought from the land every year.
Ditmar, during his Challenger Expedition i n1 8 4 4 ,r e p o r t e dt h e
existence of 47 types of salts in sea water.munotes.in

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Figure 4.2: Salinity
a)Sources of Salinity:
i)In areas where temperature is high evaporation will be greater
leading to lower dilution of salt. Togeth er with high temperature
if humidity is also low then salinity will be high.
ii)Secondly, if fresh water is continuously added in the form of
precipitation, rivers or icebergs, then salinity will not be high.
b)Factors Controlling Salinity in an Ocean
i)Rate o f Evaporation:
There is direct positive relationship between the rates of
evaporation and salinity. Greater the rate of evaporation higher is
the salinity. The sub -tropical region has highest salinity. Here the
sky is clear for more periods in a year, lan di sr e l a t i v e l ym o r ei nt h i s
latitudinal belt, and sources of fresh water supply are relatively low.
All this contributes towards higher rates of evaporation and so more
salinity in the sea water.
ii)Precipitation:
Higher the precipitation lower the salinit y, lower the
precipitation, higher the salinity. This is the reason why the
regions of high rainfall are associated with comparatively lower
salinity than the regions of low rainfall.
iii)Influence of river water:
Though the rivers bring salt from the land to the ocean, big
and voluminous river pours down immense volume of fresh waterinto the seas and so the salinity is reduced at their months.For example: -comparatively low salinity is found nea ro fm o u t ho f
the Ganga, the Congo, the Nile, the Amazon, St. Lawrence etc. The
influence of river water is more pronounced in the unclosed seas.
For example: -the Danube river reduces the salinity in Black Sea
[18%].munotes.in

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58iv)Atmospheric Pressure & Wind Direct ion:
Anticyclone conditions with stable air and high temperature
increases salinity of the surface water of the oceans. Sub -
tropical High Pressure Belt represents such conditions to cause
high salinity. Wind also helps in the redistribution of the salt in
the oceans and seas as winds drive away saline water to less
saline areas resulting into a decrease of salinity in the former
and increase in latter.
v)Circulation of Ocean Water:
Ocean current affects the spatial distribution of salinity by
mixing sea wa ter.
4.6WORLD DISTRIBUTION OF SALINITY
The latitudinal distribution shows that there is a low salinity
at the equator ranging from 34 to 35.5% because of high rainfall
and the large number of days with overcast sky. The region
between 20 -40 degrees N and 10 -30 degrees S are the region of
high salinity due to large number of cloudless days, which increase
heat and promotes evaporation. Sargasso Sea (37%) S. E. Brazil
(37%) Western Australia (36%) and near Peru -Chile (36.5%) are
the high salinity zones.
After obtaining maximum in the lower middle latitudes
salinity again decreases to 31% in the northern hemisphere and
33% in southern hemisphere that is (40 –60 degrees N and S).
Further pole ward salinity decreases due to the melting of ice. The
averag e salinity for northern hemisphere is 34% while in South it is
35%. This is attributed to the fact that in the south a comparative
less of mixing and less addition of fresh water takes place and
there is absence of land.
Salinity either decreases or incr eases with depth according
to that nature of water mass. Generally there is a decrease with
increase of depth. At the equatorial region of Indian Ocean salinity
increase with depth (34% to 35%) At the southern boundary of the
Atlantic surface salinity is 3 3% increasing to 34.5% and still deeper
it reaches up to 34.8%. Generally it can be said that in high latitude
salinity increases with depth due to denser water mass found at the
bottom.munotes.in

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Figure: 4.3. Distribution of Salinity in the World
Check your pr ogress:
1.Define salinity of sea water. Examine the factors influencing
salinity of sea water.
2.Account for the differences in the distribution of salinity sea
water in the world.
4.7CONCLUSIONS:
The study of ocean morphology has helped to unde rstand
the types of submarine relief features in the ocean basin .The
relative importance of different oceanic relief features provide with
different types of resources required either for bio -geo-chemical
cycle functioning or directly as resources in the form of marinemunotes.in

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60organisms, minerals and fossil fuels. The temperature balance of
the earth is largely maintained by the oceanic surface as it absorbs
huge amount of global heat.
Salinity of oceanic water is governed by different factors.
Differences in sal inity provides different of bio -reserves in oceanic
waters besides deriving salts from sea water.
4.8QUESTIONS:
1. What do you understand by morphology of ocean?
2. Explain the importance of ocean morphology.
3. Examine the distribution and location of bottom relief features of
Pacific Ocean.
4. ‘Salinity of sea -water varies in different areas’. Explain giving
reasons.
5. Account for the distribution of salinity of sea water in the world.
References:
1. K.Siddhartha, 2000: ‘Oceanography: A Brief Intr oduction’,
Kisalaya Publications, New Delhi.
2. Savindra Singh, 2011: ‘Oceanography’, Prayag Pustak Bhawan,
Allahabad.
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61Unit -5
TIDES AND OCEAN CURRENTS:
CAUSES, TYPES AND EFFECTS
Unit Structure:
5.0 Objectives
5.1 Introduction
5.2 Tides: Origin ,types and its significance
5.3 Ocean Currents: Factors of origin and modification of ocean
currents
5.4 Ocean Currents of the Atlantic Ocean
5.5 Effects of Ocean Currents
5.6 Conclusions
5.7 Questions
5.0. OBJECTIVES:
To study the origin and different types of tides.
To understand the significance of tides.
Understanding of origin of ocean currents.
To study the factors responsible for formation of ocean currents
To study the distribution of different types of ocean currents of
Atlantic Ocean.
To study the effects of ocean currents on the atmosphere and
human activities.
5.1. INTRODUCTION:
The sea water rises regularly twice a day at regular intervals.
This periodic phenomenon of alternate rise and fall in the level of
the se as is known as tides. Tides are produced as a gravitational
interaction of the earth, moon and the sun. The nature and
magnitude of tides vary from place to place figure 5.1.munotes.in

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62
Figure 5.1.H i g ha n dL o wT i d e s
5.2TIDES –ORIGIN, TYPES AND SIGNIFICANCE O F
TIDES:
a) Origin of tides :Origin of tides is due to the gravitational
force exerted by the moon, the sun and the earth .A st h e
position of the moon in the planetary system is close to the
earth than the sun, the gravitational pull exerted by the moon
istherefore twice as strong as that of the sun. Whereas the
sun though bigger in size in at greater distance from the
earth hence its influence is relatively less. The magnitude of
the tides therefore is determined by the position of the moon
in relation to theearth figure 5.2.
.
Figure: 5.2.High and low tid e with reference to position of
moon .
b)Following are different types of tides: These tides are
determined by the position of the moon, the sun and the earth
with reference to the rotation and revolu tion of the earth and the
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63Spring Tide: Highest tides known as ‘Spring T ide’occur on
two days in a month i.e. on full moon and new moon days ,i . e .
when the moon, the sun and the earth are almost in a line .
figure 5.3.
Figure :5.3.Spring Tide
Neap Tide: Lowest amplitude tides called as ‘Neap Tide’ occur
when the moon is at first and last quarter of full moon, and the
position of the sun and the moon are at right angles to the earth .
During this period the pull exerted by the sun and the moon tend to
balance each other resulting in the occurrence of lowest amplitude
of tides figure 5.4.
Figure :5.4.Neap Tide
ii)Aphelion and Perihelion Tides:
The proximity of the Moon in relation to Earth and Earth in
relation to the Sun also has an effect on tidal ranges (figure 5.5.)..
Earth moves around the Sun in an elliptic or bitthat takes a little
over 365 days to complete. Its gravitational force is greatest when
the Earth is at perihelion i.e. the position when it is closest to the
Sun in early Januar y; and gravitational force is least when the Earth
is at aphelion position i.e. farthest from the Sun in early July. The
Perihelion tides occur when the Earth, Moon and Sun are aligned atmunotes.in

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64perigee and perihelion , resulting in the largest tidal range seen over
the course of a year. So, tides are enhanced when the Earth is
closest to the Sun around 2ndJanuary of each year. Tides are
reduced during aphelion w hen i t is furthest from the Sun i.e. around
2ndJuly.
Figure: 5.5. Tidal range
iii)Daily and Semi -diurnal tides: is classified on the basis of time
interval between the tides Figure 5.6. These are:
Diurnal tides :The tides occurrin ga tt h ei n t e r v a lo f2 4h o u rs 52
minutes daily are called diurnal or daily tides.
Semidiurnal tides :Tides occurring at the interval of 12 hours
26 minutes are called as semi -diurnal tides
Figure 5.6. Position of earth and moon
c)Significance ofTides:
i) Inland Navigation in rivers: Tides generally help in making
some of the rivers navigable for ocean -going vessels. For example
river Rhine, Elbe, Danube in Europe, River Thames in London,
River Mississippi In U.S.A., River Hooghly in Kolkata.munotes.in

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65ii) Desilting of river mouths : Tides clear away the sediments
brought by the rivers and thus maintains the depth of water near
the coast that provides natural harbor for fishing and shipping.
iii) Tidal energy: The tidal force is also used as a source for
generating electricity. For example France and Japan have
installed power stations that convert tidal energy into electricity.
iv) Fishing activity: A large amount of fish are brought to the coast
during high tides that provides easy fish catch for fisher men to earn
their livelihood and a source of nutrient food, feed,a n df e r t i l i z e rt o
man, his animals and cultivable land respectively.
Check your progress :
1.Explain the origin of tides with the help of a diagram.
2.Discuss different types of tides with sui table diagrams.
3.Examine the importance of tides in human life.
11.3 ORIGIN OF OCEAN CURRENTS:
The general movement of a mass of oceanic water in a
definite direction is called ‘ocean currents’ Figure 5.7.O c e a n
currents originate in all parts o f the world and are most powerful of
all features of the ocean because they drive oceanic waters for
thousands of kilometers .
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66
Figure 5.7.1: Classification of Ocean Currents
The currents in the oceans a re originated due to combined
effects of several factors acting internally as well as externally
which are as follows:
1.Factors related to the Oceans : Local variations in the physical
properties of the ocean e.g. pressure gradient, temperature
differences, salinity differences; density variations etc. generate
ocean currents.
(i)Temperature Difference –Water moves from high temperature
belts to lower temperature belts. Thus, currents from the
equator move towards the pole. But, subsurface water also
moves fro m poles towards equator to compensate the loss of
water. Hence, temperature is an effective phenomenon.
(ii)Salinity Difference –Oceanic salinity affects the density
variation causes ocean currents. Salinity increases the density
of ocean water. If two areas having equal temperature are
characterised by varying salinity, the areas of high salinity will
have greater density than the areas of low salinity. Ocean
currents on the water surface are generated from the areas of
less salinity to the areas of greater salinity. Such system of
surface and sub -surface currents caused by salinity variation is
originated in open and enclosed seas.
(iii)Density Difference –Difference in the density of oceanic water
is the main cause for the movement of oceanic water as oceanic
currents. Water moves from the areas of lower density to areas
of higher density.
2.Factors related to atmospheric pressure :Ocean currents are
greatly influenced and controlled by atmospheric pressure and
its variation, wind direction, rainfall and evapora tion etc. These
are:
(i)Air pressure and Winds –Water moves from higher pressure
areas to lower pressure areas in the form of currents, due to
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67(ii)Rainfall and Evaporation –The sea water levels becomes
relatively higher in the area so fl o we v a p o r a t i o na n dh i g hr a i n f a l l
than those areas which record low rainfall but high evaporation.
Low evaporation coupled with high rainfall lowers the amount of
salinity and thus reduces water density. This mechanism results
in the rise of sea level .O nt h eo t h e rh a n dh i g he v a p o r a t i o na n d
low rainfall increases salinity and water density and thus lowers
the sea level. Thus, surface ocean currents are generated from
the area of low water level.
5.4OCEAN CURRENTS OF THE ATLANTIC OCEAN:
The type and movement of ocean currents is determined
the temperature conditions of the sea water in the ocean. Following
are the types of ocean currents found in the Atlantic Ocean.
1.North Equatorial Current (warm)
Normally, the north equatorial current is formed between the
equator and 100N latitude. The current is generated because of
upwelling of cold water near the west coast of Africa. This warm
current is also pushed westward by the cold canary current. On
an average, the north equatorial warm current flows from east to
west but this saline current is deflected northward when it
crosses the mid -Atlantic Ridge near 150Nl a t i t u d e .I ta g a i nt u r n s
southward after crossing over the ridge. This current, after being
obstructed by the land barrier of the east coast of Brazil, is
bifurcated into two branches e.g. (i) Antilles Current (ii)
Caribbean Current .
2.South Equatorial Current (warm)
South equatorial current flows from the western coast of Africa
to the eastern coast of America between the equator and
200latitud e. This current is more constant than the north
equatorial current. In fact, this current is the continuation of the
Benguela current. This warm current is bifurcated into two
branches due to obstruction of land barrier in the form of the
east coast of Bra zil. The northward branch after taking north
westerly course merges with the north equatorial centre near
Trinidad while the second branch turns southward and
continues as Brazil warm current parallel to the east coast of
America. This current is basically originated under the stress of
trade winds.
3.Counter -Equatorial Current (warm)
The counter equatorial current flows from west to east in
between the westward flowing strong north and south equatorial
currents. This current is less developed in the west du e to stress
of trade winds. In fact, the counter current mixes with the
equatorial currents in the west but it is more developed in themunotes.in

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68east where it is known as the Guinea Stream. The counter –
equatorial current carries relatively higher temperature and
lower density than two equatorial currents.
4.Gulf Stream (warm)
The Gulf Stream is a system of several currents moving in
north -easterly direction. This current system originates in the
Gulf of Mexico around 200N latitude and moves in north easterly
direct ion along the eastern coast of North America and reaches
the western coast of Europe near 700Nl a t i t u d e .
(i)Florida Current :This current is the northward extension of the
north equatorial current. These current flows through Yucatan
channel into the Gulf of Mexico, thereafter the current moves
forward through Florida Strait and reaches 300Nl a t i t u d e .
(ii)Gulf Stream: The Florida current after having the water of
Antilles current is known as Gulf Stream beyond Cape Hatteras.
This current is very wide and warm a nd is separated from the
Sargasso Sea to its right (in the east) and relatively cold water
near the coast to its left.
(iii)North Atlantic Current :The Gulf Stream is divided into many
branches at 450Nl a t i t u d ea n d4 50W longitude. All the branches
are collec tively called as North Atlantic Drift or current.
5.Canary Current (cold)
The Canary Current, a cold current, flows along the western
coast of North Africa between Madeira and Cape Verde. In fact,
this current is the continuation of north Atlantic Drif t which turns
southward nears the Spanish Coast and flows to the south
along the Coast of Canaries Island.
6.Labrador Current (cold)
The Labrador Current, an example of cold current, originates in
the Baffin Bay and Davis Strait and after flowing through th e
coastal waters of Newfoundland and Grand Bank merges with
the Gulf Stream around 500W latitudes.
7.Brazil Current (warm)
This current is generated because of the bifurcation of the south
equatorial current because of obstruction of the Brazilian coast
near Sun Rock. The northern branch flows northward and
merges with the north equatorial current while the southern
branch known as the Brazil current flows southward along the
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698.Falkland Current (cold)
The co ld waters of the Atlantic Ocean flows in the form of
Falkland cold current from south to north along the eastern
coast of South America up to Argentina. This current becomes
most extensive and developed near 300Sl a t i t u d e .
9.South Atlantic Drift (cold)
This current is originated because of deflection of the Brazil
warm current eastward at 400S latitude due to deflective force of
the rotation of the earth. The South Atlantic Drift, thus, flows
eastward under the influence of the westerly’s.
10.Benguela Current (cold)
The Benguela current, a cold current, flows from south to north
along the western coast of South Africa. In fact, the South
Atlantic Drifts turns northward due to obstruction caused by the
southern tips of Africa. Further northward this current merg es
with the South Equatorial Current.
5.5EFFECTS OFOCEAN CURRENTS:
The climate ofthecoastal region isalso influenced byocean
currents. Ocean currents therefore influence agricultural and other
economic activities ofcoastal regions. Following aretheeffects of
Ocean Currents:
i)Brings Precipitation : Warm ocean currents provide moisture to
the winds blowing from the sea to the land. It helps in reducing
the temperatures and so leads to precipitation (rainfall/snowfall /
drizzle etc). For example Gulf Stream near western coastal
Europe, North Atlantic drifts along the eastern coast of Mexico
and USA, Brazilian current along the eastern coast of Brazil.
ii)Weather and Climate: Warm Ocean currents of tropical move
towards Polar Regions to provide wat er. While cold ocean
current of polar region bring cool water to tropical water areas.
This exchange warm and cool water has a moderating climatic
effect on temperate coastal regions. High temperatures during
summers of coastal regions in temperate areas w ith cold and
dry climate are brought down by the presence of these warm
winds. For example north Atlantic drift (warm) current, which
flows along the western coast of Canada, makes the region
much warmer than other places on the same latitude.
iii)Scanty Rai n / no rain : Cold current are devoid of any moisture
and so the winds blowing from the sea are dry with no moisture.
These coastal regions therefore receive scanty rain or no rain.
For example Kalahari Desert and Benguela cold current of
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70iv)Confluence of ocean currents and fishing grounds : Places
where cold and warm currents meet are ideal for the growth of
Plankton that forms food for fish. These regions thus support a
large number of fish to develop as major fishing grounds of the
world . Newfoundland on the eastern coast of North America is
the meeting point of the Warm Gulf Stream current and the cold
Labrador Current.
On the other hand these places with confluence of warm and
cold current also give rise to the formation of thick fog that reduces
visibility and so are dangerous for shipping. It may cause accidents
of ship since the shipping traffic is heavy in this North Atlantic
Ocean.
v)Shipping and Navigation : Ships sailing with ocean current
gains natural speed, which helps to save fuel and time. Ships
moving against a current lose speed. Warm currents keep the
Arctic regions free from icebergs, which can be dangerous for
ships. Besides it keep ports free from freezing during winters
thus making shipping possible throughout the year .
5.6. CONCLUSIONS:
From the present study it is very clear that tides also play an
important role in influencing the weather and climate of the coastal
regions. They help in the functioning of aquatic ecosystem in the
coastal waters that facilitates fishing activity. Ocean currents largely
govern the temperature of waters in the sea. They influence the
weather conditions along the coast and play dominant role in the
temperate coastal areas. In addition Ocean currents help the
shipping and navigation a ctivities in the areas that are favourable.
5.7. QUESTIONS
1)How are tides formed? Draw suitable diagram
2)Explain different types of tides with neat diagrams.
3)What is the importance of tides in human life?
4)What are the causes and effects o f ocean currents?
5)What is the significance of ocean current?
6)Define current and describe the types and distribution of current
in Atlantic Ocean.
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71Unit -6
PRACTICAL PART B:
ATMOSPHERE AND WEATHER
Unit Structure:
6.0 Objectives
6.1 Introduction
6.2 Signs and Symbols used in Weather maps.
6.3. Weather instruments: Thermometer, Barometer, Wind Vane
and Anemometer, Rain Gauge (Diagrammatic
representation of weather instruments, its working and uses
of these instruments).
6.4 Conclusions.
6.5 Questions.
6.0.OBJECTIVES :
To study different weather phenomena that influence day to day
activity of mankind.
To understand the functioning of dif ferent weather instruments
and their recording of weather data.
To study different techniques used to know weather
phenomena.
To study weather maps with the help of conventional signs and
symbols used in weather maps.
6.1. INTRODUCTION :
Weather denote st h ea t m o s p h e r i cc o n d i t i o n so fw e a t h e r
elements at a particular place and time. The weather elements
include temperature, atmospheric pressure, wind, humidity and
cloudiness. These weather conditions are obtained from various
weather stations and recorde d by the meteorological department on
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72Meteorology is the study of the earth’s atmosphere and
weather. Scientists who study th ese conditions and forecast
weather conditions are called as meteorologists. Weather
instruments given in table 6.1 are used for recording weather
phenomenon at weather stations.
Table 6.1
INSTRUMENTS FOR MEASURING WEATHER ELEMENTS
S.N. Weather
ElementName of the
Weather
InstrumentUnit of Measurement
1 Temperature i) Maximum and
Minimum
thermometer,
ii) Thermographi) In British system :
Degree Celsius
(0C)
ii) In French system:
Degree Fahrenheit
(0F)
2 Atmospheric
Pressurei) Barometer
ii) BarographMillibar
(mb)
3 Humidity i) Wet and Dry
bulb
thermometerIn Percentage
(% )
4 Precipitation Rainguage Millimeter, centimeter
(mm), (cm)
5 Wind vane Wind direction North, South ,
East, West
6 Wind
VelocityAnemometer Kilometer per hour
The science of meteorology now with its advanced
technol ogy of Doppler is able to predict weather phenomenon more
accurately. Hence precautions are taken well in advance that saves
much of the loss of life and property. Weather forecasts are of great
importance in the field of aviation, shipping, agriculture et c.
Weather Instruments: Various instruments are used for measuring
different weather phenomena. Some of the common important
weather instruments given in table 6.1 are explained below:
i) Thermometer: is used to measure atmospheric temperature.
Most the rmometers are in the form of a narrow closed glass tube
with an expanded bulb at one end. The bulb and the lower part of
the tube are filled with liquid such as mercury or alcohol. Before the
other end is sealed off, the air in the tube is released by heat ing it.
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73cooled, as the case may be, as a result of which the mercury in the
bulb rises or falls. A scale is marked on the glass tube and
readings are recorded from the same.
The two mos t common scales used in the thermometer are
centigrade and Fahrenheit. On the centigrade thermometer, the
temperature of the melting point is marked 00Ca n dt h a to fb o i l i n g
water as 1000C, and the interval between the two is divided into
100 equal parts. On the Fahrenheit thermometer, the freezing and
the boiling points of water are graduated as 320Fa n d2 1 20F
respectively.
While the maximum and the minimum thermometer are used
to measure the atmospheric temperature, the dry bulb and the wet
bulb thermom eters are used to determine the humidity conditions in
the air. A set of these thermometers is kept in the Stevenson’s
Screen Figure 6.1
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74a)The maximum thermometer is designed to record the highest
temperature during a day . As the temperature increases, the
mercury moves up into the tube, however as the mercury cools, it
cannot moves downwards because of a constriction in the tube. It
must be reset again to bring it down. Figure 6.2
Maximum Thermometer Figure 6.2
b)Theminimum thermometer records the lowest temperature in
a day. In this thermometer, alcohol is used in place of mercury.
When the temperature decreases the metal pin in the tube goes
down and strikes at the minimum temperature. Figure 6.3
Minimum Thermo meter: Figure 6.3
c)The dry bulb and the wet bulb thermometers are used for
measuring the atmospheric humidity .T h ed r yb u l ba n dt h ew e t
bulb thermometers are two identical thermometers fixed to a
wooden frame. The bulb of dry thermometer is kept uncover ed and
is exposed to the air while the bulb of the wet bulb thermometer is
wrapped up with a piece of wet muslin, which is kept continuously
moist by dipping a strand of it into a small vessel of distilled water.
The evaporation from the wet bulb lowers it st e m p e r a t u r e .D r yb u l b
readings are not affected by the amount of water vapor present in
the air, but the wet bulb readings vary with it since the rate of
evaporation is dependent upon the amount of water vapor present
in the air. The greater the humidity in the air, the slower the rate of
evaporation and hence, the difference between the readings of dry
bulb and wet bulb will be small. On the other hand, when the air is
dry the evaporation from the surface of the wet bulb is rapid, which
would lower its t emperature and the difference between the two
readings would be larger. Hence, the difference of the readings ofmunotes.in

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75the dry bulb and wet bulb thermometers determines the state of the
atmosphere with regard to its humidity. The larger the difference in
the wet and dry bulb thermometer, the more arid is the air. Figure
6.4
Dry and Wet Bulb Thermometer: Figure 6.4.
2)Barometer and Barograph :These weather instruments are
used to m easur e and record air pressure at the weather stations
respectively .The unit of measurement used to measure air
pressure is millibars (mb).
There are two common types of barometer used to record air
pressure:
(a)Simple mercury barometer and (b) Aneroid barometer .
(a)Simple mercury barometer: This instrument has much greater
accur acy than aneroid barometer and is used in many weather
stations. A thick glass tube of meter length is filled with mercury
and its mouth is closed with the help of a finger. Another cup is
taken and is filled completely with mercury. This glass tube in the n
dipped into a cup filled with mercury. Care is taken while removingmunotes.in

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76finger from preventing air to enter the glass tube while inverting in
the cup . The mercury thus will flow out from the glass tube into the
cup. The level of mercury in the glass tube thu sg o e sd o w na n d
gets stable at certain level. This stable mercury level is measured in
millimeter. This height of the mercury level in the glass tube will
represent the air pressure. The vacuum portion in the glass tube is
called as ‘ Torricell i’s vacuum’. Figure 6.5
Simple mercury barometer: Figure 6.5
(b) Aneroid barometer: It is most convenient to use as it is light in
weight and easily portable. Aneroid barometer consists of a hollow
metal box with a spring placed inside the box that has flexible
sides.Any change in atmosphere makes the top of the box to bend
slightly. Thus when the air pressure is high the box bends in and
when the air pressure is low the spring pushes the box top
outwards. These movements of the box (bending in and out) are
magnifie d by a system of levers which control the movement of a
pointer over a dial. This aneroid barometer is attached to a rotating
drum on which a graph paper is fixed to get a chart. This instrument
is called as barograph. The pointer of the barometer is a pen
which continuously draws a line on the graph paper to given a chart
Figure 6.6and6.7munotes.in

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77
Aneroid Barometer: Figure 6.6
Figure 6.7barograph
3) Wind Vane andCup Anemometer :These weather instruments
are used to know w ind direction and wind velocity respectively.
AW i n dV a n e indicates wind direction. Al e v e rw i t ha r r o wa sa
pointer at one end and a small two metal plates attached at same
angle to the other end is fixed on a vertical rod. The arrow of the
wind vane alw ays points tothe direction from which the blows.
North, South, East and West directions are marked on the fixed
support of the wind vane Figure 6.8.To obtain accurate data amunotes.in

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78wind vane issited on the top of the tall building that is relatively
without a ny obstruction.
Wind Vane Figure 6.8
Cup Anemometer: It helps in measuring the velocity of the wind.
Four metallic cups are fixed on a frame that is balanced on a
vertical rod. As the wind blows the cup starts blowing in a horizontal
direction. wind in knots per hour or kilometers per hour Figure 6.9.
These rotations of the axel are transmitted through gears to a meter
which records the frequency of rotations that gives the speed of the
wind
Figure 6.9munotes.in

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794) Rain Gauge: It is a weather instrumen t used to measure rainfall
accompanied by a corresponding measuring jar in millimeter. It
consists of an outer metal casing (copper), inner container (glass or
copper) with a metallic funnel. The rainguage is fixed in an open
area without any obstructions in the form of trees, buildings, walls in
its vicinity. It is partly sunk in the ground having grass so that it is
not blown or knocked. The top of the funnel stands 30 cm from the
ground level so that rain from the nearby cannot enter into the rain
gauge .The rainfall gets collected in the inner container. The
overflow from the inner container is collected in the outer container.
The funnel has a tapering end so that evaporation of the collected
rainwater is minimized. The amount of rain is measured using a
measuring jar which is calibrated according to the diameter of the
gauge. The rainfall amount is measured daily or several times a
day depending upon the amount of rainwater received Figure 6.10.
The records are used to calculate the mean daily, mean mon thly
and mean annual rainfall received by a place.
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80Check your Progress:
1.Define weather. What are the elements of weather?
2.Who is a meteorologist?
6.2.SIGNS AND SYMBOLS USED IN WEATHER MAPS
Following chart gives various signs and symbols used in
weather maps for understanding the weather phenomena at a
particular place and time. Weather maps in India are produced by
India Meteorological Department referred to asIndia nd a i l y
Weather Report as given in Chart 1.
Signs and Symbol of Weather conditions : Chart -1
These are universally used signs and symbols that help to
have a comparative study with spatial analysis and spatial
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81SeaCondition: Chart 2.
SN Sea Condition Sign and Symbol
1 Calm Cm
2 Smooth Sm
3 Slight Sl
4 Moderate Mod.
5 Rough Ro.
6 Very Rough V.Ro.
7 High H
8 Very High V.H.
9 Phenomenal Ph.
10 Direction of Wave
Check Your Prog ress:
1.Draw all the signs and symbols used in weather maps.
2.How do these signs and symbols on a weather map help to
understand the weather of different places?
6.3CONCLUSIONS:
This module therefore explains the importance of signs and
symbol s and quantitative techniques in identifying different types of
weather phenomena to determine atmospheric conditions of a
particular place or a region on the surface of the earth.
Comparative analysis and spatial differentiation of weather and
atmospheric conditions is therefore possible with the help of such
geographical techniques used in practical geography.
6.4. QUESTIONS:
1.How is weather condition studied? Explain with correct
diagrams .
2.Explain the importance of studying weather conditions.
3.What are weather maps? How are they useful?munotes.in

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82References:
1.Gopal Singh (2001):‘Map Work and Practical Geography’, 4th
revised and enlarged edition, Vikas Publishing House Pvt. Ltd.
2.Linda lau (2000): Physical Geography, Union Book Co.P vt.Ltd.
andGreenwood Press ,Hong Kong .
3.https://www.google.co.in/webhp?sourceid=chrome -
instant&ion=1&espv=2&ie=UTF8#q=image+of+doppler+radar+i
n+India
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Question Paper Pattern for Semester End Assessment implemented
From 2020-2021
For Geography courses at F.Y.B.A

Duration of examination - 3 hours
Total Marks - 100 (per semester )

N. B. : (1) All Question s (Question No. 1 to 5) is compulsory.
(2) Draw neat sketches and diagrams wherever necessary.
(3) Attach map supplement to the main answer book.
(4) Figures to the right indicate full marks.
----------------------------------------------------------------------------------------------------------------------------- ------------

Q1. Attempt any One question (module 5) 20 marks

A)
B)

Q2. Attempt any Two questions ( module 1) 20 marks

A)
B)
C)

Q3. Attempt any Two questions ( module 2) 20 marks

A)
B)
C)

Q4. Attempt any Two questions ( module 3) 20 marks

A)
B)
C)

Q5. Attempt any Two questions ( module 4 ) 20 marks

A)
B)
C)



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