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Physical Geography

Luminaries and solar system

The boundless sky is all around our abode, the earth. This vast infinite sky is called
the firmament or the space. However, the scientists term it as a place where all the
particles and energy of the universe are concentrated or floating. The space has no
beginning or end. There are infinite number of luminaries in the firmament. If we look at this infinite space, we see the sun at day time and at night we see innumerable dots of light including the moon. These luminaries are revolving on their own axis with a particular speed in a very disciplined manner. The sun is a star and the moon is a satellite. These stars and their planets and satellites are called the luminaries.
Previously only the bright objects
found in the space were called the
luminaries but now the existence of
many objects that do not give light
have been discovered. Nowadays
luminaries include the moon, the sun,
planets, stars, meteors, comets and
besides, faded nebulae, pulser, Black
Dwarf, Black Holes are also included.
All these have formed the universe.
Stars : If we look at the blue and
bright night sky, we see many dots of
light twinkling. There are many such
luminaries in the firmament which are
called the stars . We can see
only a few thousands of stars with our
naked eyes. Some of this can be
seen from earth with the help of powerful telescope. The scientists, with the help of
powerful telescope have discovered more than 100 crores of such stars. The stars are
actually burning gaseous mass. Every star has its own light and heat.
On looking from the earth, the stars seem to exist in same plane. The distance of the
stars from the earth is measured by the velocity of the light. The unit of measurement
from the earth to the stars or from stars to stars is called light years. Since the sun is
the near most star, its distance from the earth is about 15 crore kilometres. A ray of
light from the sun takes 8 minutes 19 seconds to reach the earth.
The star next to the sun is Proxima Centauri. It is about 38,00,000 crore kilometres
from the earth. Scientific discoveries have enhanced our knowledge about the stars.
Some of the stars are round, some are spiral or lineal while some others are shapeless.
The density of Black Dwarf and Black Hole is very high and their gravitational force
is also very strong.
Constellation : Looking at the cloudless sky in a dark night, one can see a number of
stars clustered together. These are called constellations. The astronomers in the past,
connecting every individual constellation with imaginary lines and imagining different
figures, gave them strange names. Of them, the names of Great Bear, Orion,
Cassiopeia, Little Bear, Canis Major, Eridanus etc. are worth mentioning.
Galaxies : Billion of stars, dusts and huge pool of vapour have formed clusters of
luminaries which are called galaxy or the universe of stars. There are innumerable
galaxies in the space. Galaxies are far apart from each other. The milky way is a small
part of a galaxy.
The Milky Way : The cluster of stars that look like a brightly illuminated long
pathway extending from north to south in the dark sky is called the milky way. A
milky way is formed of millions of stars, some scientists take it to be a huge circular
region. Some other scientists consider it to be a coiled up hard nebulae. Looking at the
clear and cloudless night sky in winter, one would see a long line of translucent and
radiant lustre of light with a wide vicinity extended north to south. That is known as
the milky way. Our solar system including the sun is included in one of such a milky
way. The sun is positioned among billions of stars in this milky way.
Nebulae : The covering of the dimly lit stars in the space is called the nebulae. They
are visible with naked eyes or with powerful telescopes. Some of the nebulae are full of gaseous bodies. They have strong forms. These are called gaseous nebulae. The scientists think that many nebulae are far away from our milky way. One nebulae may contain billions of stars. Since the nebulae are billions and billions of light years away from the earth, the stars between them cannot be identified separately. The milky way is in the same plane with the nebulae.
Meteors : Sometimes in the cloudless night sky it looks as if a star is running about or
a star has just dropped off. This event is known as drop off of a star. These are not
actually stars. They are called meteors. Innumerable physical masses float in the
space. The gravitional force causes these physical masses rush towards the earth with a violent speed. When they come in touch with air, the friction with air makes them flare up. Majority of the meteors are small in size.
Comets : Sometimes a kind of luminaries appear in the space. They exist for a short
period and then disappear. They are called comets. Comets are amazing celestial body.
They revolve a long way around a star. When they come near the sun, they become
visible in the form of cloud. Gradually, the bright centre point looms like hair veiled
in mist. Then comes out as a long steamed tail just like a bright broom. The English
word comet has been derived from the Greek word ÔKomet’ that means dishevelled
hair. Comets exist in the solar system. The astronomer, Edmund Halley observed that
the comets follow the law of gravitation in the space. The comet he discovered is
known as Halley’s Comet which appears every 75 years. Halley’s comet appeared in
1759, 1835, 1910 and 1986.
Planets : Some celestial bodies revolve round the sun as a result of the gravitional
force. They have no light of their own. All of them rotate round the sun and get light
and heat from it. They do not twinkle in the sky. These luminaries are called planets.
The Earth, Mars, Venus, Saturn, Mercury, Jupiter, Uranus and Neptune are the eight
planets of our solar system.
Satellites : Some luminaries are caused by the gravitational force to rotate around a
planet. They are called satellites. They have no light or heat of their own but they get
light and heat from the sun or stars. The moon is the only satellite of the planet earth.
Every planet may not have satellite. Mercury and Venus have no satellite. The
satellites of the Saturn are the highest in number.
The Solar System
The huge universe beyond our vision is called the space. The space is so huge that it is
very difficult for us to conveive it. There are stars, comets, planets, satellites, meteors
and other heavenly bodies in the space. The universe is formed of these innumerable
celestial bodies. The sun is considered as a star of the universe. The solar system
includes the sun, its planets and satellites, billions of meteoroids and thousands of
comets.The sun is the centre around which all other members of the solar
system revolve. All the activities of the solar system centre around the sun. The solar
system is quite small compared to the vastness of the amazing universe. The earth is quite insignificant. The solar system is many billion times larger than the earth in size.
The solar system also includes various gaseous substances and dust particles. The
gravitional force of the sun make all the planets, satellites, comets and meteors to
rotate around the sun. The solar system is many times larger in area than the earth.
Sun : The sun is actually, a star. It is a yellow coloured star of moderate size among the billions of stars in the milky way. Its diameter is 13 lakh and 84 thousand kms. and mass is nearly 1.99 × 1013 kilograms. Of all the luminaries in the solar system, the sun has a very important place. Our relationship with the sun is very close. The sun is a heated star. The temperature at the centre of the sun is about 150,000,000° Celsius and at the surface it is about 6,000° Celsius. It is the source of light and heat of the earth but also of other planets and satellites. The earth would have been eternally dark without sun light.
There would be no pulsation of life and no animal or plant would live in this world.

The planets of the solar system are arranged according to their distance from the sun such as Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Of all the planets, Jupiter is the largest and the Mercury, the smallest. The description of these planets as arranged in the above order is given bellow :
Mercury : Mercury is the smallest planet of the solar system. It is nearest to the sun
from where the average distance is 58 crore kilometres and the diameter of this planet
is 4,850 kilometres. The planet takes 88 days to rotate once round the sun. So, for
Mercury 88 days make a year. It takes 58 days and 17 hours to revolve round its own
axis. So, one day of Mercury is equivalent to our 58 days and 17 hours. The
temperature of the planet is very high because of its closeness to the sun. The metals like zinc or lead dissolves into vapour in such temperature. There is no atmosphere in Mercury. There is no cloud, rain, wind or water and hence life is non-existent in this planet. This shows that the
surface of the Mercury is quite similar to that of the moon. It is rugged and full of
holes. There are innumerable hills and plain lands. Mercury has no satellite.
Venus : You must have heard about the morning star or the evening star and might
have seen it too. Venus is, in fact, not a star. It glows in the sky like a star which
prompts us to call it a star. Venus is covered under dense cloud. Naturally, the sun is
never visible from its surface. The sun is 108 crore kilometres from Venus. The Venus
is the nearest planet to the earth. There is little difference of light between day and
night. The dense cloud in Mercury is formed of carbon dioxide cloud. The rain that falls here is actually acid rain. The diameter of the Mercury is 12,104 kilometres. It takes 225 days for the Venus to revolve round the sun. Venus rotates on its axis very slowly. The sun rises twice and sets twice in the sky of Venus. Venus has no satellite.
Earth : Earth is the third closest planet to the sun. The average distance of the earth
from the sun is 15 crore kilometres. Its diameter is nearly 12,667 kilometres. The
earth takes 23 hours 56 minutes and 4 seconds to rotate on its own axis. The earth
takes 365 days 5 hours 48 minutes 47 seconds to revolve round the sun once. So, one year is equivalent to 365 days. The moon is the single satellite of the earth. The earth is the only planet in whose atmosphere there is existence of required oxygen, nitrogen and temperature that is necessary for the survival of plants and animals in the world.
Hence, the earth is considered as the most important planet of the solar system.
Mars : The orbit of the earth is next to earth in the solar system. Its diameter is about
6,787 kilometres which is half of the earth. The average distance of this planet is 22.8
crore kilometres from the sun and 7.7 crore kilometres from the earth. Mars has two
satellites. One of them is Phobos and the other is Deimos. The day and night in Mars
are nearly the same as that of the earth is respect of period of time. The earth takes
365 days to revolve round the sun. Whereas the Mars takes 687 days. The planet takes
24 hours and 37 minutes to rotate on its own axis. There are gorges and volcanoes in the surface of the Mars. The planet has little oxygen and water but the quantity of
carbon dioxide is so high that the existence of life is not possible. The planet looks
reddish since the rocks became rusty getting in contact with oxygen as the available reports show.
Asteroids : There is no planet within a distance of 56.31 crore kilometres from Mars.
There are many little luminaries in this vast space. All these luminaries having a
diameter ranging from 1.6 kilometres to 805 kilometres are called Asteroids. The
scientists have identified the orbits of thousands of Asteroids. As all these remain in a group, hence called Asteroids.
Jupiter : Jupiter is called the king of the planets because it is the largest planet. Its
diameter is 1,42,800 kilometres. It is 1,300 times larger than the earth in area. Its mass
is twice as much as any other planet in the solar system. This Planet is 77.8 crore
kilometres from the sun. The temperature of the surface of atmosphere is very low but
in the interior, it is quite high. The average temperature is 125° Celsius. Jupiter takes
nearly 12 years to revolve round the sun but it takes 9 hours 53 minutes to rotate on its own axis. So in one day as of our earth, the sun rises twice and sets twice in this
planet. The scientists have discovered, so far, 16 satellites. Of these, Lo, Uropa,
Ganimed and Callistro are the major satellites. The scientists think that heavy
atmosphere exists here.
Saturn : Saturn is the second largest planet of the solar system. The sun is 143 crore
kilometres away from Saturn. It is actually a huge gaseous globe and its diameter is
1,20,000 kilometres. Saturn is 760 times greater than that of the earth. Saturn takes 29 years and 5 months to revolve round the sun once but rotates on its own axis in 10
hours 40 minutes. There are thousands of rings surrounding the Saturn having
different colours. Saturn has 22 satellites outside its ring. Of these Titan, Hua, Dion,
Capitus and Tethris are remarkable. The surface of the Saturn remains covered by ice.
The atmosphere contains hydrogen, a mixture of helium, methane and ammonia gas.
Uranus : Uranus is the third largest planet. In order of distance from the sun its place
is seventh. It is located at a distance of 287 crore kilometres from the sun. The planet
takes 84 years to complete a single rotation round the sun but it takes only 10 hours 49
minutes to rotate on its own axis. Its diameter is nearly 49,000 kilometres. It is very light because it is formed of light substances. The atmosphere contains high
percentage of methane. Average temperature 170° Celsius. Recently, scientists have discovered some rings round this planet, but these rings are not bright. Uranus has 5 (five) satellites such as Mirinda, Ariel, Ambriel, Titania and Oberon.
Neptune : In order of distance from the sun, Neptune is placed in 8th position. It is
about 450 crore kilometres from the sun. This planet has got feeble light and heat.
Neptune in area is equivalent to 72 earths and 17 earths in mass. Its Diameter is
48,400 kilometres. The atmosphere which is formed of the mixture of gas mainly
methane, ammonia and other gases. This planet was first sighted by the scientists in
1846. It has two satellites such as Triton and Neroid. Moreover, two rings have been
recently discovered to exist round this planet.
Luminaries and Solar System
What we learnt from this article
Luminaries : The planets, stars, satellites, comets, meteors, pulsar, Black Dwarf,
Black Holes that exist in boundless space are called the Luminaries.
Stars : At night and clear sky, the luminaries that twinkle in the space are called stars.
Nebulae : The covering of the dimly lit stars in the space are called nebulae.
The Milky Way : The brightly, illuminated long pathway running from north to south
formed by cluster of stars is called the milky way.
Comets : Sometimes a kind of luminaries appear in the space. They exist for a short
period and then disappear. They are known as comets. Halley’s comet appear in the
sky every 75 years.
Planets : The celestial bodies having no light or heat of their own revolve round the
sun. They are called planets. The earth, mercury, saturn etc. are the planets.
Satellite : The celestial bodies that revolve round a planet are satellites. The moon is a
satellite.
The Solar System : The family of celestial bodies that include the sun and its planets,
satellites, asteroids, comets, and meteors is called the solar system. There are eight
planets in the solar system. They are Mercury, Venus, Earth, Mars, Saturn, Jupiter, Uranus and Neptune.

Latitude and Longitude

Some imazinary lines extending from east to west and from north to south are drawn
on a map of the world to determine the location of any place. These lines are known as the parallels of latitude and the meridians of longitude respectively. In geography, latitude and longitude are two important topics. One can determine the location of a place with the help of latitude and longitude. With the location of the longitude, one can determine the time of that place. Moreover, the northern and the southern position of any place can be determined through latitudes. Similarly, the western and the eastern position of any location from the Prime Meridian can be ascertained by longitude. Degree of latitude or a part of it is known as the latitude, while the degree of longitude of a part of it is termed as longtitude.
Latitude : Before we learn what is actually meant by the degree of latitude, we will
have to have an idea about the axis, the equator and parallels of latitudes. The
imazinary line which runs north to south through the centre of the earth is known as
axis. The northernmost point of this axis is known as the north pole and the
southernmost point is the south pole. Keeping both the poles in equal distance, an
imazinary line has been drawn which encircles the globe from west to east is known as the equator or the terrestrial equator. Due to the spherical shape of the earth, this line is also circular. So, this line is also known as the equatorial circle.
The equator has divided the earth into two equal halves. The part lying north to the
equator is known as the Northern Hemisphere and that to the south as the Southern Hemisphere. With the help of the
equator, one can determine the angular
distance of a place situated either in the
northern or in the southern hemisphere.
So, to find out the angular distance of a
place upon the surface, the only thing
is to be done is to connect with an
imazinary line, any point of that place
with the centre of the earth. Again, the
meridian which passes over that point
ultimately meets the equator on a
certain point, and that the meeting
point is connected by an imazinary line
with the centre of the earth.
The imazinary line is actually the radius of the earth sphere. The value of the angle
thus formed through those two lines is equivalent to the latitude of that particular place.
Therefore, the latitude is the angular
distance of a place north or south of
the equator. If a line is drawn from
any place on the earth to its centre
then that line will create an angle with
the equatorial plane. The value of that
angle is the latitude of that place. The latitude of a place situated to
the north of the equator is desigrated
as the north latitude and that of the
south as the south latitude. The total
value of the angle created by the
circle at the centre of the earth is
360°. These angles are being
divided into degrees (°), minutes (' ) and seconds ('' ). Thus the value of the equator is
0°. The latitude of the north pole is 90° North and that of the south pole is 90° South.
This is because that the angular distance of each of the poles is 90° from the equator.
These angles are being divided into degrees and minutes and on the basis of these
angles, the imazinary lines are drawn
parallel to the equator. These are
known as the parallels of latitudes.
The latitudes are parallel to one
another and every one of them is a
full circle. The circumference of the
latitudes decrease with the increase of
their values. Some of the parallels arc
quite important. The 23.5° latitude in
the northern and southern hemispheres
are known as theTropic of Cancer
and the Tropic of Capricorn respectively. Besides, the 66.5° north and south latitudes
are known as the Arctic and Antarctic circles respectively. The equator is
known as the great circle. The degree of latitude of all the places located in the same
parallel is the same.
The latitude of the region near the equator, in other words, the latitudes having the
value of 0° to 30° are called the low latitudes. Similarly, latitudes having the value of 30° to 60° are known as the mid latitude and those of 60° to 90° are the high latitudes.
There are different methods of determining the latitude of a place. Out of them we are
discussing about two methods.
(a) With the help of a pole star, and
(b) With the help of a sextant and the position of the sun in the horizon.
(a) Determining latitude with the help of a pole star : We know that the earth is
divided into two hemispheres, the northern hemisphere and the southern hemisphere.
Every night from any place of the northern hemisphere one can see the pole star in a fixed position in the northern sky. But the altitude of the pole star from the horizon does not remain the same. It differs with the difference of the place, that means, the altitude of the pole star in the horizon is different for different places. In the equator, the location of the pole star is in the horizon and its altitude is 0°. From the equator towards north pole for
every 1° of approach the altitude also increases by 1°. Ultimately in the pole, the altitude of the pole star rises to 90°. So, the degree of latitude for the equator is 0° and that of the north pole is 90°. In other words, the degree of latitude increases with any approach towards the north pole. Thus the latitude of any place in the northern hemisphere is equivalent to the degree of altitude of pole star. So, the latitude of any place can be derived from the altitude of the pole star. But it is not possible to determine the latitude of any place in the southern hemisphere by the pole star, nor possible to determine the latitude through this method in day time.
(b) Determination of latitude by sextant : Sextant is an instrument by which the
altitude of the sun can be measured. The latitude of any place can be determined after finding out the altitude of the noon sun with the help of a sextant. The following formula can be applied to find out the latitude of a place.
The formula is : 90° minus the altitude of the sun at noon ± the sun’s declination.
So, the latitude = 90°− altitude of the sun − declination
= 90 °− 50°−15° = 90°− 65° = 25° south.
If the place under consideration is in the northern hemisphere then it will require to
add the declination value of north and to subtract the declination value of south.
Similarly, if the place is in the southern hemisphere then the declination value of south is to added and that of the north to be subtracted.


Meridians of Longitude

The terrestrial equator can be graduated to degrees, minutes and seconds and through
these points imazinary lines can be drawn from north to south pole which are known
as the meridians of longitude. The
longitudes are also known as
meridians. The meridian which passes
through the Greenwich can be taken
as the Prime Meridian and the angular
distance of the meridians can be
measured from the prime meridian

Prime Meridian

The meridian which passes from north to
south pole through Greenwich in the
neighbourhood of London is termed as the
Prime Meridian. The value of this line has been determined as 0°.
Other meridians of longitudes can be drawn through the angular distance from the Prime Meridian. For example, the meridians that lie 45° East of the prime meridian is known as 45° East Longitude or east meridian and all the places lying on this longitude has the same longitudinal value of 45° East Longitude. So, we can say that the longitude of any given point on the globe is measured as an angular distance
eastward or westward from the prime meridian. We further know that the longitude of the prime meridian is 0°. The value of the angles created by the circumference at centre of the earth is 360°. So, if 360° meridians are drawn from pole to pole at equal interval of 1°, then they will lie at 1° of longitude apart. Hence, the total number of 360° meridians are equally divided into two parts of 180° each in the western and the eastern side of the prime meridian. As the earth is a sphere, so the 180° east and west longitude is virtually the same line. Like that of the latitudes, the longitudes can also be divided into minutes and seconds. One minute of longitude is 1/60th part of a degree of longitude. The value of both the latitude and longitude will be 0° when the prime meridian crosses the equator vertically. And this point has been recognised as a place located somewhere in the Gulf of Guinea.
Determination of longitude : The longitude of a place can be determined by two
methods. These are : (I) by the difference of local time, (2) by Greenwich mean time.
(1) By the difference of local time : We know that the earth is a sphere. It has been
moving continuously around its own axis from west to east. Consequently different
places of the earth have been facing the sun at different times. Whenever, the meridian of  any place comes in front of the sun i.e. when the sun appears just overhead, then it is considered as noon and the local time is taken as 12 noon. Ultimately other times of
the place are determined on the basis of local time. We know that there is a difference of 4 minutes of time for 1° of difference of longitude. In other way, we can say that for every 4 minutes, the longitude differs by 1°. For example, if in any place, the local time is 12 noon, then the local time for any place located 5° east of the previous place will be 12.00 noon + (5° × 4 minutes) = 12 hours 20 minutes. For the place lying 5° west of the former place, the local time would be 12.00 noon − (5° × 4 minutes) = 12.00 − 20.00 minutes = 11 hours 40 minutes.
(2) By Greenwich mean time : We all know that the longitude of Greenwich is 0°.
The actual time of Greenwich can be read from the Chronometer. To determine the
longitude of a place by sextant, one will have to record the highest altitude that the
sun attains there at noon and that position will have to be treated as 12.00 noon. The
longitude of the place mentioned above can now be determined from the difference of
time that occurs between the indicated place and Greenwich. If the place is located
east of Greenwich then the local time would be more than that of Greenwich and if it
is located west of the Greenwich, its local time would be less than that of Greenwich.

Differences between parallels and meridians :

1. All the parallels of latitude are parallel to each other but the meridians are not;
2. All the parallels are full circles and the meridians are half circles;
3. The lengths of each of the meridians are equal but that of parallels are not equal;
4. Parallels always run in a east-west direction, whereas the meridians in a north￾south direction;
5. The highest value of parallel is 90°, whereas that of meridian is 180°.

Local and Standard Time

Local Time : Everyday the earth moves round her axis from west to east. As a result, the sun appears earlier in the places located in the east. Due to rotation of the earth, the sun reaches the zenith of the sky, or in other words, the sun reaches its highest altitude on a certain place and that hour is treated as 12.00 noon. On the basis of this noon time, the other time for the day is determined. The time thus determined is the local time for that place. So, the local time of a place is determined on the basis of the highest altitude of the sun which can be observed with the help of a sextant.
The earth at the centre creates 360°. The earth requires 1,440 minutes (24 hours × 60
minutes) to cover this distance of 360° for one time. So, the earth needs 4 minutes
(1,440 ÷ 360) to rotate 1° of longitude. Therefore, for 1° difference of longitude, the difference of time will be 4 minutes.
Standard Time : If the sun’s noon position on the meridian is taken as 12 O’ clock
and if that is accepeted as the basis of determining the local time then this would
create confusion in maintaining, time among the different parts of the same country.
To avoid this confusion each country of the world has introduced a new concept of
maintaining time known as standard time. The standard time of the country is that
time which passes through the central part of the country.
There may be several standard times for a large country like the United States of
America and Canada. The United States of America has four different standard times,
whereas Canada has five. In those countries, more than one standard time has been introduced to run administration and other works more efficiently relating to postal, railway, wireless, telephone etc. The local time of Greenwich (0° meridian) has been accepted as the standard time for the world. Accordingly, the standard time in
Bangladesh is six hours ahead of Greenwich time. 90° East meridian has passed
through the middle part of the country and the standard time of Bangladesh is
calculated from the local time of that meridian.
Antipodes : Antipode means the relative position of two places which are situated on the earth’s surface just opposite to each other. To locate the antipode of a place an imazinary straight line is drawn through the earth from that place to the opposite position which passes through the centre. The point where this line touches the
opposite side of the globe is the
antipodal position to the former point. That means the later point is the antipodal to the former point.
If the latitude of a place is known then
the latitude of its antipode can be
ascertained. The latitude of a place
will be the same as that of its
antipode. The two places must be
situated on the different sides of the
equator, that means one is in the
northern hemisphere and the other
one in the southern hemisphere. So, if
the latitude of a place is 70° North, then the latitude of its antipode will be 70° South.
The total value of the meridian of a place and that of its antipode will always be 180°.
The meridian of a place if be east, the meridian of the antipode would be west. For example, the antipode of 40° East longitude will be 180°-- 40° = 140° West longitude.
The difference of time for these two places will be 12 hours. In fig. 7, the point B is
the antipode of A. The antipode of Dhaka is situated in the Pacific near Chile of South
America.

International Date Line

Travelling towards east or west from any place will require adjustment of local time
which ultimately creates problems even for adjustment of days of the week. This
problems stands acute if anybody crosses 180° longitude towards east or west from a
particular place. To avoid this problem or an imazinary line has been drawn absolutely over the waterbodies from north to south. This imazinary line is known as
International Date Line.
Necessity of the International Date Line :
We know that for the difference of 1° of
longitude, there is a difference of time of 4
minutes. So, for every 15° of longitude,
there will be a difference of an hour. Thus
travelling eastward from prime meridian
(Greenwich) up to 180°, one would be 12
hours ahead of the schedule and travelling
westward 12 hours behind the schedule. If it is 10 A.M. on Monday in Greenwich, then
the corresponding time would be 10 P.M. on
the same day at 180° East longitude,
whereas, the time at 180° West longitude
would be 10 P.M. on the previous day i.e.
Sunday. But 180° East or 180° West
longitude is virtually the same longitude.
So, it is observed that the local time differs
by 24 hours or one day in the same
longitude. There is a change of day and date
if one crosses this line which is known as
the International Date Line.
Any ship or aeroplane sailing towards
east from Greenwich will have to
subtract one day and sailing towards
west will have to add one day to adjust with the local time.
The International Date Line is virtually 180° east and west longitude. It crosses through the Aleutian on the north-eastern part of Siberia, Fiji and Chatham islands. To
avoid the problems of adjusting local time, the International Date Line has been so set
that it passes only over the waterbodies turning 12° towards east just near the Bering Strait, 7° towards west near the Aleutian islands and 11° towards east near Fiji and Chatham islands. Had it not been drawn this way, the people would have to count two different local times in two different parts of the same island.

What we learnt from ths aspect :
Equator : The imaginary line lying midway between the poles, which encircles the
earth from west to east.
The latitude : The angular distance of a point on the earthÕs surface north or south of the equator, as measured from the centre of the earth, is the latitude, of that place.
The lines of longitude : The imaginary lines which are drawn from north pole to
south pole through equator connecting each of the different points created by
graduating the equator into degrees, minutes and seconds.
Declination : The day when the sun gives its rays vertically over a latitude is the
declination of the sun for that day.
The Prime Meridian : The Prime Meridian is the line that passes from north to south
over Greenwich near London.
The Local Time : Due to rotation of the earth on its axis when the sun reaches its
highest position in the sky in some particular place, it is taken as noon-day and the time records as 12 O’ clock. This is accepted as the Local Time of the place and other time of the day is calculated from this local time.
Standard Time : The time which is referred to the mean time of a certain meridian.
Every country determines its time from the meantime of the meridian which passes
through the middle of that country and this is known as Standard Time of that country.
Antipode : A point on the earth surface is known as antipode of another point when it
is situated just on the opposite side of the globe corresponding to that point.
International Date Line : From any fixed place if any body crosses the 180°
longitude he will observe that it creates problems to determine the day of the week
along with the differences of local time. To avoid this anomaly depending on 180°
longitude, an imaginary line is drawn from north to south exclusively over the waters.
The imaginary line is known as the International Date Line.




The Motion of the Earth

Everyday the sun rises in the east and sets in the west. This process led many people
in the past to think that the sun is moving and the earth is fixed. The sun moves
around the earth. But with the advancement of science, this has been proved that the sun itself is moving and the earth has also motion. Later it has been revealed that the sun does not move around the earth, rather the earth moves around the sun. The earth not only moves around the sun, it also rotates on its own axis. The motion of the earth is of two types : (1) to rotate on its own axis, and (2) to revolve around the sun in its own orbit. The rotational motion of the earth is known as Diurnal motion and the revolutionary motion is called the annual motion.

Diurnal Motion

The earth has motion. Centering the sun, the earth moves from west to east on its axis.
This motion of the earth is known as rotation. The earth takes 23 hours 56 minutes and 4 seconds or nearly 24 hours
to rotate once on its axis. This
period of 24 hours is regarded as
one day. This diurnal movement
of the earth is the rotation. One
such full rotation of the earth is
known as solar day. The earth
takes a full day to complete
rotation once on her axis.
The speed of rotation is different
in different places. The speed of
rotation at the equator is highest.
Here the speed of rotation is more
than 1,610 kilometres per hour.
This speed decreases towards
north and south of the equator.
The speed of the rotation is nearly
zero at the poles. Though the
diurnal motion exists, but still we
don’t feel due to the following
reasons :
1. Compared to the size of the earth, we are very much insignificant, so we do not
feel the speed.
2. No fixed or movable object is available in front of the earth in the space which can
be taken as the basis to understand the rotational speed of the earth.
3. Keeping pace with the speed of the earth, the atmosphere also moves from west to
east. So, we, do not feel the rotational speed of the earth.
4. The rotational speed of each of the places of the earth is fixed.

Evidences of Diurnal Rotation

In the old days Ptolemy and his follower astronomers used to think that the earth was motionless. But subsequently astronomers like Copernicus, Kepler, Galileo and Newton gave their opinions in favour of the rotation of the earth.
1. Everyday the sun rises in the east and sets in the west. From this phenomenon we
can infer that either the sun of the earth is moving. This motion is accomplished
within one day or 24 hours. The sun is stationed about 15 crore kilometres away from the earth in the space. Being located at such a long distance, it is not possible for the sun to revolve round the earth within 24 hours. Moreover the tremendous speed the sun requires to move round the earth will be even more than the speed of the light, which is simply impossible. Because no object can move faster than light. So, it is not the sun but the earth which rotates on its own axis in 24 hours.
2. The shape of the earth : The earth is inflated in the middle and is compressed
towards the poles. If any pliable object or matter rotates around its axis, it acquires
such a condition. The present shape of the earth is the result of such rotational speed.
Scientist Newton had the opinion that due to the rotational motion, the earth acquired
the present shape.
3. Due to rotation of the earth, changes also occur in ocean currents and wind system.
According to Ferrel’s Law, it is due to the rotation of the earth that the air and the
ocean currents are deflected to the right in the northern hemisphere and to the left in
the southern hemisphere.
4. It is observed that if a stone is thrown from a high place, it does not reach the gound vertically, rather it moves slightly towards the east. This proves that the earth rotates from west to east or the earth has rotational motion
5. Primarily, tide occurs due to the attraction of the moon. The moon takes twenty seven days to complete one revolution around the earth. So, it is expected that the   would occur once every 27 days. But practically we observe that the high tide occurs every day which proves the rotational movement of the earth.
6. French scientist Foucault, with the
help of a pendulum proved the diurnal
motion of the earth. He attached a pin
at the bottom of the pendulum and
hang it on the top of St. Pantheon
church of Paris with a fixed motion,
so that it would oscillate from north to
south. Due to oscillation, the pin at
the bottom of the pendulum started
marking on the ground. Mr. Foucault
observed that every time the marks
have been deflected towards east. So,
it can be proved that due to diurnal
motion the earth rotates from west to
east.

Effects of Earth’s Rotation

1. Occurance of day and night : One of the effects of the earth’s rotation is the occurance
of day and night. We know that the
earth is round and it does not have
any light of its own. The earth
becomes illuminated by the light of
the sun. Due to diurnal motion, that
part of the earth which faces the sun
becomes illuminated. It is day in the
illuminated portion of the earth.
Sunlight does not reach the opposite
part of the illuminated portion. So,
it remains dark and it is night in the
dark portion of the earth (Fig. 11).
Due to rotation of the earth, alternately the illuminated portion becomes dark and the
dark portion becomes illuminated and that is why the day and night are changed.
When the dark portion is illuminated, it becomes day there. Similarly the illuminated portion turns dark, it becomes night there. Thus day and night have been occurring alternately. So, in some places it is 12 hours of day and 12 hours of night.
If the shape of the earth would have been flat instead of being round, the rotation of the earth would create only day or only night. In other words, either only day or only night would exist simultaneously over the globe. On a specific date, in some parts of the earth it is day whereas in other parts it is night. That means when a portion of the earth

Annual Motion
According to Newton’s Law of Gravitation, the planets move around the stars due to
their attraction. A law is also applicable to the solar system. In solar system each of
the planets moves round the sun. The earth being a planet of the solar system, also
moves on an elliptical orbit round the sun. The earth having a full rotation on her axis
for 24 hours, simultaneously moves around the sun on a set route i.e. orbit once in a
year. This movement of the earth is known as the annual motion. The earth takes 365
days 6 hours to revolve around the sun in her orbit. The period, the earth takes to
complete the revolution is known as the solar year.
The circumference of the earth’s orbit is slightly more than 93 crore 80 lakhs and 827kilometres. The average speed of revolution is 1,06,260 kilometres per hour. In other words, the earth moves around the sun at a speed of 30 kilometres per second.
Considering this speed, it requires 365 days 6 hours 48 minutes and 47 seconds to
move round the sun for one time. But the solar year is calculated to be of 365 days.
So, there remains a shortfall of about 24 hours or one day. To make up this deficit one extra day is added to the calendar in every fourth year. And that year is calculated as of 366 days. The month of February is calculated to be of 29 days instead of 28 days.
This year is regarded as Leap Year and the leap year is taken to be of 366 days.
Revolution of the earth is responsible to create fluctuation in the duration of day and night. It is also responsible to bring changes in the march of seasons.

Effects of Annual Motion

I. Fluctuation of day and night : The duration of the days and nights fluctuate in the same time in the different parts of the year due to the revolution of the earth. As a
result, it is observed that in the same latitude, the length of day and night fluctuate in different periods of the year.
In the process of revolutionary motion the earth, on 2lst June reaches in such a
position that the sun’s rays fall vertically on the northern most position of the earth,
that means, on 23.5° North latitude or on the Tropic of Cancer. At this time, the
northern hemisphere remains inclined more towards the sun and the southern
hemisphere remains at the furthest distance from the sun. That is why the length of the day is greater and the temperature is higher in the northern hemisphere. So, 21st June
is the longest day and shortest night in the northern hemisphere. The sun reaches its
northern most limit on 21st June and the date is known as Summer Solstice.
The sunshines vertically over the equator on 21st March and 23rd September. On these two dates, the sun remains in equal distance from the north and south poles. So, on these two dates, the duration of the day and night is equal (12 hours day and 12 hoursnight) over the entire world. The day when the day and night are equal over the globe, is known as the Equinoxes. 21st March is spring in northern hemisphere. So, 21st March is called the Vernal Equinox, and 23rd September is Autumn northern
hemisphere, so 23rd September is known as Autumnal Equinox.
The southern hemisphere shows maximum inclination towards the sun on 22nd
December. So, on that date it is the longest day and the shortest night in the southern
hemisphere. The sun reaches its southernmost position on this day and the date is known as the Winter Solstice. The sun gives its rays vertically over 23.5° South
latitude or Tropic of Capricorn.

Proofs of Annual Motion

1. The apparent motion of the stars : As we cannot feel the occurence of earth’s
rotation, similarly we cannot perceive the existence of earth’s revolution. But with the
changing locations of the stars from east to west at night sky, we can comprehend that
the earth has an annual motion or the revolution. By looking at the stars in a clear sky one can understand the gradual movement of the stars from east to west.
2. The changing locations of the sun in the sky : The sun is seen to be located in
different positions in different periods of the year. We can observe that on 21st March and 23rd September, the sun rises at a point due east on the horizon and sets at a point due west on the horizon. Again, we can see that on 21st June and 22nd December, the sun rises at north-eastern horizon and south-eastern horizon in the sky respectively.
From 21st June to 23rd December, the sun seems to have an apparent motion towards
south. This is known as southerly movement of the sun. Again from 22nd December to 21st June, it seems that the sun is apparently moving towards north and this movement of the sun is known as northerly movement.
3. The revolutionary motion of different planets : It has been observed through
telescope that all the planets are revolving around the sun. The earth as a planet also
has revolutionary speed or annual motion.
4. Direct observation : Recently the spaceman from spaceship has observed the
revolution of the earth.
5. The Law of Gravitation : In comparison to the sun, the earth is very small. The sun is 13 lakh times greater than the earth. So, it is naturally thought that due to gravitation the earth moves round the sun.
Change of Seasons
The whole year is being divided into 4 divisions on the basis of the variation of
temperature. Each of the divisions is known as a season. These are the Summer, the
Autumn, the Winter and the Spring. It is to be noted here that when summer prevails
in the northern hemisphere, winter sets in the southern hemisphere. Again, when there is winter in the northern hemisphere, it is summer in the southern hemisphere.
Similarly, when it is spring in the northern hemisphere, it is autumn in the southern
hemisphere and it is spring in the southern hemisphere when it is autumn in the
northern hemisphere.

Reasons for change of seasons : Due to the difference of temperature, the clange of
seasons does occur. The reasons for difference of temperature and as well as that of seasons are the following :
1. Difference in solar heat;
2. The earth is round. So, some places get direct sunshine, whereas in other places it
falls at a slanting position. As a result, it creates difference in temperature which in
turn causes to change the season.
3. The orbit of the earth is elliptical. So the distance of the earth from the sun
fluctuates. As a result, it creates difference in temperature which ultimately brings
changes in season.
4. The axis of the earth makes a fixed inclination of about 66.5° with the plane of the ecliptic. As a result, once in a year the north pole or the south pole shows inclination towards the sun. Temperature increases in the hemisphere when it remains inclined towards the sun and in opposite case the temperature decreases. As a result, the season changes.
Perihelion and Aphelion : The orbit of the earth is an ellipse. Its length is
93,80,51,827 kilometres. The sun is located in the centre or in the focus of the ellipse.
Perihelion : In the process of revolution around the sun, the earth on 1st to 3rd January reaches in such a position that the distance between the sun and the earth becomes the lowest. It is about 14 crore 65 lakh 83 thousand 5 hundred kilometres. This position of the earth on that date is known as Perihelion. In Perihelion position the axis of the earth lie aslant outside the orbit

Aphelion : As the orbit of the earth is ellipse, the distance between the sun and the earth does not remain fixed for all the time of the year. Sometimes the distance
increases and other times it decreases. In the first half of July, particularly on 4th July,
the distance between the sun and the earth becomes the highest being 15 crore 14 lakh
20 thousand and 500 kilometres. This position of the earth is known as Aphelion.

The process of changing of season : Normally, we know that there are four seasons in the earth viz., the summer, the autumn, the winter and the spring. Now we will try to understand how the changes of season do occur. The explanations for the change of seasons may be obtained from four situations of the earth during revolution around the sun.
1. Summer in the Northern Hemisphere and Winter in the Southern Hemisphere :
We know that the north pole comes nearer to the sun once in a year. During revolution
on her orbit, the north pole of the earth after 21st March, starts showing inclination
towards the sun. Consequently, a large portion of the northern hemisphere is being
illuminated by the rays of the sun. The area of illumination gradually increases. As a
result, the days become longer and the nights shorter in the northern hemisphere. As the days become longer the sun gives its rays for a longer time in the northern hemisphere.
The earth gets more time to receive heat and the heated earth in turn makes the
surrounding air to be hot. Nights are short and the amount of radiation from the soil is
less in comparison to the amount of heat that is stored during day time. Thus hot
weather prevails in the northern hemisphere and so it is summer there.
At this time opposite condition prevails in the southern hemisphere. Since the
southern hemisphere remains far from the sun and it gets sunshine for a shorter time.
So the nights are long and the days are short. The earth radiates the heat which she has stored at day time. As a result, the earth becomes cold. Cold weather prevails in the
southern hemisphere and it is winter there.
2. Autumn in the Northern Hemisphere and Spring in the Southern Hemisphere :
After 21st June, the length of the day starts decreasing and the length of night gets
increasing in the northern hemisphere. But until 23rd September in comparison to night, the length of the day still remains shorter. The north pole which is inclined towards the is moving away from the sun and from a distant position the southern hemisphere gradually inclined towards the sun i.e. the south polar region comes nearer to the sun.
Consequently temperature gradually decreases in the northern hemisphere and it increases in the southern hemisphere. This condition is known as Autumn in the northern hemisphere and Spring in the southern hemisphere. On 23rd September the day and night are equal throughout the world
•3. Winter in the Northern Hemisphere and Summer in the Southern Hemisphere :
We know that after 23rd September, the southern hemisphere starts showing
inclination towards the sun. One can perceive that everyday the sun moves southward.
This is known as southwardly movement of the sun. In comparison to night, the days
become longer in the southern hemisphere as the south pole shows inclination towards
the sun. As the sun’s rays fall directly on the Tropic of Capricorn, the southern
hemisphere gets sun rays for a longer period of time. Consequently, the temperature increases in the southern hemisphere. On 22nd December, the south pole approaches to the nearest position towards the sun and it is the longest day in the southern hemisphere. So, one and half months preceding 22nd December and the same period following that date the temperature remains high in the southern hemisphere.
Naturally, during this period, the temperature remains very low in the northern hemisphere. So, one and half months preceding 22nd December, it is summer in the southern hemisphere and it is winter in the northern hemisphere.
4. Spring in the Northern Hemisphere and Autumn in the Southern Hemisphere :
The sun starts moving towards the north after 22nd December. The length of the day
starts decreasing in the southern hemisphere and increasing in the northern hemisphere.
Consequently, the temperature increases gradually in the northern hemisphere and
decreases in the southern hemisphere. Again day and night become equal in duration on 21st March. On this date the sun,s position is vertical on the equator. This is known as autumnal equinox. This is spring in the northern hemisphere and autumn in the southern hemisphere




A presidential aspirant on the platform of the Peoples Democratic Party (PDP), Senator David Mark, on Friday evening met behind closed doors with former President, Chief Olusegun Obasanjo.
The meeting came less than 24 hours to the PDP National Convention where the party’s presidential candidate will emerge.
Mark speaking with journalists said he was in Abeokuta to consult with Obasanjo over his presidential ambition.
According to him, “Nigerians must get it right in the choice of leaders at all levels in order to move the nation to an enviable height.
“2019 general elections will be the first major polls that the Peoples Democratic Party would be contesting as an opposition party hence it is important.”
The presidential aspirant said he considers 2019 polls as a “test of the PDP’s strength, popularity and how well it had fared since the last general polls in 2015” which the party lost.
“Those who are afraid of contest have no business in the race.
”I came into the race well prepared after due consultation with various groups.
“The positive outcome of my consultations across the country and the overwhelming endorsements encouraged me to vie,” Mark added.


Physical Geography
Introduction
The landforms created by the different geographic processes under different
environments on the earth’s surface are included in the physical geography. The
subject matter of physical geography can be divided into three major divisions viz.,
lithosphere, atmosphere, and hydrosphere. Though each of these divisions possesses
individual qualities, yet they are interrelated. Besides, they collectively maintain the
biosphere. The lithosphere, atmosphere and hydrosphere are interlinked with each
other and the biosphere has been developed based on the elements taken from them. Let us cite an example. The people is using lithosphere for agriculture
depending on the river water or reserviors (part of the hydrosphere). Again, solar heat
is needed (part of the atmosphere) to increase the agricultural production. The
lithosphere, atmosphere, and hydrosphere are discussed respectively

Lithosphere
Earth’s crust and the interior of the earth : The earth is almost like a sphere. Its
radius is about 6,400 kilometres. There is no scope to observe the interior of the earth
by piercing the hard external covering. The geologists by observing the characteristics
of the seismic waves, gravitation and the characteristics of the magnetic field have
developed some idea about the elements and structure of the earth’s interior.
Depending on the worldwide collected information, the earth from the surface to the
centre has been divided into three strata. These are (a) the earth’s crust, (b) barysphere
and (c) centrosphere.
(a) The earth’s crust : The hard stratum of the rock covering the outer shell of the whole
earth is known as the earth’s crust (Fig. 17). The thickness of the earth’s crust is the lowest in
comparison to the other interior layers of the earth. On an average, it is about 20 kilometres.
The thickness of the earth’s crust on
an average is about 35 kilometres
beneath the continents and only 5
kilometres beneath the ocean.
Generally, the continental earth’s
crust is known as SIAL which is
composed of Silicon (Si) and of
Aluminium (Al). On the other hand,
the earth’s crust beneath the ocean is
composed of basalt which is heavier
than that of Sial layer and its major
elements are Silicon (Si) and
Magnesium (Mg) and is generally
known as SIMA. It is inferred that
probably this basalt stratum has been
existing throughout the world as a coating beneath the Sial and in the bottom of the
seas and oceans. The temperature beneath the earth’s crust increased by 30° Celsius
for every kilometre of depth.
(b) Barysphere : The stratum beneath the earth’s crust is the barysphere. But, there is
a thin layer between the earth’s crust and the barysphere. The Yogoslavian
Seismographer Mohorovicic in 1909 first discovered this layer of seperation. So, the
layer has been known as Mohorovicic Discontinuity. The layer of barysphere is 2,885
kilometres thick which is extended from Mohorovicic upto the outskirts  the centrosphere . The rocks of the upper part of the centrosphere are hard and
brittle and this layer is about 100 kilometres deep. The earth’s crust and the upper part
of the barysphere is 100 kilometres thick and this collectively known as rocksphere or
lithospere.
The rocks beneath the hard layer of barysphere being close to the melting point are
partially soft and this layer extends downwards for about 300 kilometres. This layer is
known as Asthenosphere. This hard layer of rock can move slowly over the
Asthenosphere. The upper part of the barysphere is made of silicate which is primarily
composed of iron and magnesium. It is assumed that the lower part of the barysphere
(700 to 2,885 kilometres) is constituted of the minerals composed of complete mixture
of iron oxide (FeO), Magnesium Oxide (MgO) and the Silicon dioxide (SiO2). The
temperature of the barysphere at 100 kilometres depth ranges from about 1,100°
Celsius to 1,200° Celsius which increases to 1,900° Celsius at a depth of 700
kilometres and to 3,000°Celsius at the outskirts of the centrosphere.
(c) Centrosphere : The centrosphere is a layer which is extended from the lower part
of the barysphere upto the centre of the earth. The thickness of this layer is about 3,486
kilometres. The centrosphere occupies about 16 per cent of the total area of
the earth and its weight is about one-third of the total weight of the earth. The pressure
of this layer is several lakh times more than the air pressure and the temperature ranges
from 3,000° Celsius to 5,000° Celsius. It is learnt from the siesmic waves that the
centrosphere has a liquid cover which is about 2,770 kilometres thick and has also ahard inner cover which is about 1,216 kilometres thick. The major two elements of the
centrosphere is Ferror (Fe) and Nickel (Ni) which is combinedly known as NiFe.
Lithosphere
What we learnt from this chapter :
Earth’s crust and the interior of the earth : The earth’s crust and the interior of the
earth can be divided into three divisions according to their mode of formation. These
are (a) Earth’s crust on Lithosphere, (b) Barysphere and (c) Centrosphere.
Earth’s crust : The thickness of the earth’s crust beneath the continent, on an average
is 35 kilometres and beneath the ocean it is 5 kilometres. The upper layer of the
earth’s crust is S1AL. The continents are primarily made of Sial rock.
Barysphere and Lithosphere : The bottom layer of the earth’s crust is barysphere.
This layer is about 2,885 kilometres thick. The earth’s crust and the upper part of the
barysphere upto 100 kilometres is collectively known as rocksphere or lithosphere.
Centrosphere : The layer extending from the lower part of the barysphere upto the
centre of the earth is known as Centrosphere. This layer is about 3,480 kilometres
thick.


Rocks and Minerals
The outer cover of the earth’s surface is known as the earth’s crust. The earth’s crust is
basically formed of the rocks composed of different minerals. These earth forming
minerals serve out different purposes. As for example, mineral oil and coal are used as
fuel. Moreover, the economic use of gold, copper, zinc, aluminium, nickel, iron etc. is
also very much important.
Different land forming processes like volcanism, depositional and erosional activities
are highly influenced by the structure of the rocks and minerals. It is necessary to
acquire some knowledge about minerals and rocks being the elements responsible for
formimg the earth’s crust.
First let us see what is meant by minerals? What is the difference between rocks and
minerals?
Minerals are formed by the composition of two or more basic elements. However,
some minerals may be formed of one basic element. Diamond, gold, copper, silver,
mercury and sulphur may be cited as examples. Of all these, the hardness of diamond
is highest and of talc is lowest.
On the other hand, the rocks are formed by the mixture of one or more minerals. Here,
we mention the term mixture because in nature the minerals are found in mixed
condition. Each of the rock forming minerals maintains its characteristic. Though
much of the rocks are formed of more than one mineral, but in such case, the minerals
and rocks are the same. As for example, calcite is a mineral while it is known as
limestone as a rock. The characteristics of minerals are controlled by nature of the
basic elements forming them. On the other hand, the property of rocks are regulated
by the nature of the minerals constituting them.
The rock family : The rocks are of three types according to their mode of origin.
These are igneous, sedimentary and metamorphic rocks. Igneous and metamorphic
rocks are formed by the processes accomplished in the interior of the earth. On the
other hand, the sedimentary rocks are formed by the processes acting on the surface of
the earth. Though the mode of formation of these rocks are different, yet they are
interrelated.
Igneous rocks : Igneous rocks can be formed under two environmental conditions
viz., intrusive and extrusive. In the deep interior of the earth due to cooling, magma
may be crystallized to form intrusive rock. If the magma finds its way to the surface of
the earth through the vents, then it is termed as lava. This lava after cooling becomes
extrusive rock

Intrusive rocks are sub-divided into plutonic and hypabyssal. Extrusive rocks are
again sub-divided into explosive and quiet type. In intrusive type of rock, the magma
solidifies in the deep interior of the earth. Granite, gabro, syenite, diorite etc. are the
examples of intrusive rocks. In hypabyssal type, the magma solidifies in the shallow
interior as Porphyry.
Magma when thrown by violent explosions into the air after being solidified gives
birth to the explosive Igneous rocks. Tuff and breccia may be cited as examples. In the
quite type of igneous rock, the molten magma flows out quitely in all directions and
solidifies. Rhyolite, basalt, andesite etc. are best examples of this type of rock.

Sedimentary rocks : The rocks which are formed by the deposition of sediments are
known as sedimentary rocks. In this type of rock, the sediments are deposited in
layers or strata. Sedimentary rocks cover an area of 5 per cent of the total area of the
earth’s crust. However, about 75 per cent of the total exposed part of the continental
landmass is formed of sedimentary rocks.
Sources of sediments and their characteristics : Different types of mechanical and
chemical weathering processes are acting on the earth’s crust. Due to the effect of
these processes, the outer crust of the earth gradually erodes out. These erodedmaterials are known as sediments. The sediments which are formed by detrital created
by the mechanical weathering is known as clastic sedimentary rocks. Clay, sandstone
and pebbles are the examples of clastic sedimentary rocks. Sedimentary rocks which
originate from the sediment accumulated by the chemical weathering is known as
organic sedimentary rock. In this process, the materials which remain in soluble
condition in the water gradually settle down at the bottom by different inorganic
processes and ultimately become consolidated. For example, if evaporation occurs in
any confined sea water, then the salts remain as residue. In this case, evaporation is an
inorganic process and the salt that remains as residue is considered to be sediments. In
some cases, the dead bodies of different acquatic animals are accumulated as organic
sediments and ultimately these are consolidated and transformed into organic
sedimentary rocks. Coal, limestone are the proper examples of such rock. Besides,
dolomite, chert and gypsum are the examples of chemical sedimentary rocks.
Classification of sedimentary rock : Primarily, the sedimentary rock is of three types
viz., mechanical, chemical and organic.


Characteristics of sedimentary rocks : Bedding, ripple marks, clay, cracks etc. are
found in the sedimentary rocks. The presence of fossils is one
of the major characteristics of the
sedimentary rock.
Metamorphic rocks : The rocks which
through temperature, compression and
chemical actions have been changed
either in form or in composition is
known as metamorphic rocks.
Fig. 20 : Bedding of sedimentary rocks
The agents of metamorphism : Heat, compression and chemically active fluids are
the main agents who work singly or collectively to transform rocks. These
metamorphic media increase the internal density of the rocks and the dimension of the
crystals and create clear foliation. Due to the application of temperature and pressure,
the minerals of the rock become compressed like the leaves of trees and are arranged
in parallel layers. This characteristics of layered structure of rocks is called foliation
Examples of the important metamorphic rocks :
Rocks with foliation
Slate : Slate is produced from shale. This type of rock contains very fine crystals
which are mainly formed by small plates created from mica. Slate is used to produce
writing slates, blackboards etc.
Non-foliated rocks
Marble : It is a coarse grained crystalized rock which has been formed from
limestone or dolomite.
Quartzite : It is a very hard metamorphic rock which is mainly created from sandstone.
Rocks
What we learnt from this aspect:
Rocks are divided into three types according to their mode of origin, viz. (1) Igneous rocks, (2) Sedimentary rocks, and (3) Metamorphic rocks.
The hot viscous type of matters in the interior of the earth is known as magma and if it finds its way to the surface of the earth, it is called lava.
Igneous rocks : Igneous rocks are formed by cooling of magma. Landforms created
by igneous rocks have diversified characteristics. Dikes, sills, laccoliths and batholiths may be cited as examples.
Sedimentary rocks : Rocks which are formed by the accumulation of sediments is
known as sedimentary rocks. In this type of rock, the sediments are accumulated in
layers. Limestone, coral and gypsum are the examples of sedimentary rocks.
Metamorphic rocks : Due to temperature, compression and chemical actions, the
elements and structure of the minerals of a rock are changed and transformed to a new
type of rock which is known as metamorphic rock. Igneous and sedimentary through changes are transformed to metamorphic rocks.
Slate : Slate is formed from shale. It is used to produce writing slates and
blackboards.
Marble : It is a coarser grained crystalline rock which is created from limestone and
dolomite.
Quartzite : It is a very hard metamorphic rock. Primarily, it is formed of sandstone.


Changes of the Earth’s Crust
Diastrophism

The earth’s crust is ever changing. Different landforming processes are responsible to
bring changes. Landforming processes are the activities by which the changes are
accomplished naturally in the formation of the land. For example, a river is building a
flood plain through deposition. So, the depositional activities of a river is a
landforming process. The landforming processes are accomplished with the help of
different physical agencies or forces such as the gravitational forces, interior
temperature and the solar energy. The different landforming processes with the help of the aforesaid agents bring changes on the earth’s crust either slowly or rapidly.
Generally, the landforming processes are associated with external forces. The changes brought about on the earth’s surface takes a longer period of time, so this type, of changes is known as slow changes. The slow changes are accomplished in two
processes.
Denudation : It means to make the things exposed. The processes by which the rocks
on the earth’s surface are broken into pieces through the application of external
physical forces and the debris are transported elsewhere is known as denudation. This denudation work is performed through three processes such as weathering, erosion and transportation.
Weathering : The weathering is a process by which the rocks on the surface of the
earth is broken mechanically into pieces due to snow or frost, the variation of
temperature and pressure or due to chemical (dissolution) action on the materials.
Even the rocks are dislodged by the animals. But the rocks weathered this way, are not transported elsewhere.
Erosion : Erosion and transportation are accomplished together. The process by
which the rocks of the earth’s crust are eroded by the river, wind, glacier, ocean
currents etc. and the eroded materials are transported elsewhere is known as erosion.
On the other hand, the rapid changes are brought about on the earth’s surface by the
landforming processes associated with the internal forces. The changes brought about
on the earth’s crust by the landforming processes are shown in a figure below :
According to this figure, the earthquake and the volcanism are mentionable of the
rapid and sudden forces responsible for the changes on the earth’s surface.
1. Earthquake : An earthquake is a vibration or oscillation of the surface of the earth caused by sudden release of enormous pressure. If a stone is thrown in the quite water of a pond, it will create ripples in the water which will scatter all around the pond. Sudden release of enormous pressure from the interior of the earth, creates waves in the surrounding rock like the waves in water which also scatter all around. The point where the energy is released is known as the centre. The point just vertically above the point of origin is known as Epicentre . Seismography is used to measure the intensity of the earthquake waves.
Types of earthquakes : Normally, an earthquake originates between 5 to 1,126
kilometres deep into the interior of the earth. The earthquake centre has been
classified into three classes according to their depth. These are :
Shallow : The earthquake originating at a depth of 60 kilometres is termed as shallow
earthquake.
Intermediate : The earthquake originating at a depth of 60 to 300 kilometres is
known as Intermediate.
Deep : The earthquake originating at a depth of 300 kilometres and above is known as deep earthquake.
About 90 per cent of the earthquakes that occur throughout the year over the world
originate within less than 100 kilometres of depth. The terrible earthquakes which
occured so far over the world are mostly of shallow type.
Causes of earthquake : The geologists opine that sudden tectonic actions in the
faultline or along the cracks are responsible to create earthquake. Besides, sudden
ejection of lava with great force can also produce earthquakes. Radiation of heat from
the earth’s surface causes shrinkage which in turn to maintain equilibrium, develops
cracks along the faultline or folds and the earthquake is caused. The rocks if dislodged
due to earth movement or development of faults will also cause the earthquake.
Strength of earthquake : The energy of earthquake means the amount of strength
which is scattered from the earthquake centre in all directions. Charles F. R. Richter, a seismologist first introduced an arithmetic scale to measure the intensity of seismicwaves. The scale is known as the ÔRichter scale’.
Earthquake Prone Areas or Belts
,it can be realised that the severity of earthquake is not equal throughout the world. Earthquakes mostly occur in the comparatively smaller areas which are long and narrow in size. Among them, the most noteworthy areas are the circular islands (Philippines and Japan), young fold mountains and ocean ridges.The earthquake prone areas can be divided into three principal zones :
(a) The Pacific Zone : Maximum number of earthquakes occur along the outer border
of the Pacific Ocean. Japan, the Philippines, Chile, Aleutian islands and Alaska have
been identified as the most earthquake prone areas.
(b) The Mediterranean and the Himalayan Zone : This region extends from the
Alps to New Zealand covering Caucasus, Iran, the Himalaya, Indo-China and East
Indies following the northern coast of the Mediterranean Sea.
(c) Ridges of the Mid-Atlantic and the Indian Ocean : The north-south ridges of
the Mid-Atlantic Ocean meeting with the ridges of the Indian Ocean extends upto the
Mediterranean zone through the Red Sea.
Besides these three specified belts, there are some areas located seperately within the
continents and in the deeps of the oceans where the occurrence of the earthquake is felt.
Effects of earthquakes : Earthquake is accepted as a natural calamity. Most of the
times, it causes great loss of lives and wealth in some parts of the earth.
Generally, it may be mentioned that the devastative nature of earthquakes brings the
following damages and changes in the earth’s crust.
(a) Faults, cracks or landslides occur in the earth’s crust due to the earthquake. The
course of the rivers is also changed. For example, the bed of the Brahmaputra river
was raised and the river changed its original course and started flowing through the
channel of the Jamuna due to the earthquake of Assam in 1787.
(b) Earthquake, if it occurs in a thickly populated area, then causes great devastation
to houses, roads and railways, water and gas supply line and other properties of the
area. Moreover, if the area is a hilly one, then the landslides and mud flow create
devastation to the settlements located at the foot of the mountains.
(c) Earthquake that originates in the ocean floor create tremendous waves on the
surface of water. Waves thus created by earthquakes are known as Tsunamis.
Tsunamis bring sudden flood in towns and other localities in coastal areas.
2. Eruption : Eruption is one of the major forces or processes bringing changes on
the surface of the earth. Eruption of the volcanoes or the magma is the main sources of igneous rocks on the surface of the earth. Volcanoes, by dint of their internal energy produce different landforms which are known as initial configuration of the land.
Different weathering agents start erosion on these initial landforms with various
external forces and such areas are gradually transformed into plain lands. Thus it can be noted that there are continuous attempts to maintain equilibrium between the internal and the external forces.

Volcanoes

Extreme temperature released from
the radioactive minerals in the interior
of the earth cause the rocks of the
lower part of lithosphere to melt. This
melted rock finds its way to the earth’s
crust through the cracks or weak
points. This erupted materials is
known as Lava, gradually accumulated
surrounding the vent and creates a
high landform which is called Volcano.
On the crest of the volcano, there is a
forepart with vertical slopes which is
known as crater. This crater through a tunnel is linked with the magma in the interior of the earth. Sometimes the molten
materials instead of ejecting out may be solidified inside. After ejection, the crater
collapses creating a hole which is known as caldera. Crater lake of Oregon State of the
United States of America is a Caldera. The volcanoes which eject lava from time to
time are active volcanoes. At present, there are about 850 active volcanoes in the
world. The volcanoes where volcanism remains suspended for a certain period of time are known as dormant and where the volcanic eruption has stopped permanently are known as extinct volcanoes.
Erupted materials : Large number of rock fragments, volcanic dust and fine volcanic
ashes are ejected with the lava. Besides, large amount of gaseous materials is also
ejected. In the ejection process, firstly comes the basaltic lava followed by the
andesite lava and lastly the rock fragments.
Types of volcanoes : Though the nature of each volcanic eruption is quiet different,
but the specialists on the basis of shape, size and eruptive nature have divided the
volcanoes in three different types as Shield Volcano, Cinder Cone and the Mixed
Cone.
Shield volcanoes : This type of volcanoes is mainly constituted of basaltic lava.
Mauna Loa and Kilauea of the Hawaii Islands are the best examples of this type of
volcano. Mauna Loa is 5,000 metres wide beneath the ocean and it rises to 4,170
metres above the ocean.
Cinder Cone : This type of volcano is created by rock fragments and lava. Parcutin and Mount Helena of Mexico are the proper examples of this type of Volcano.
Mixed Cone : This type of volcano is created mainly by andesite lava which is fluid
type with more viscous components. Hard rock fragments, volcanic dust, volcanic
ashes and the heated gas are thrown with violent explosions and are accumulated near the crater and the viscous lava accumulates above them. Mount Maon of the Philippines and the Fujiama of Japan are the examples of this type of volcanoes.
 Almost all the volcanoes are located along the border of the continents and the
oceans. The main belt stretches from New Zealand in the Pacific to the southern tip of
South America covering the Philippines, Japan, Alaska and the western coast of North America and this belt is known as Fiery Ring. Out of 850 active volcanoes of the world, this belt simply accounts for 75 per cent of the total. Moreover, traces of
volcanic activities are also manifested in the submarine ridges of the oceans. For
example, the landforming activities in the submarine ridges of Mid-Atlantic are still
active.

Effects of volcanism : Volcanic eruptions and heavy lava flow are a kind of natural
calamity which causes heavy damages to the living creatures and different human
activities. In the past, many towns and settlements were destroyed by lava flows and thousands of people died instantaneously due to the release of poisonous gas from the volcanoes. Many of the volcanoes are situated in high mountainous regions and their
crests always remain under heavy ice cover. But when eruption starts, the ice
coverings melted and creates mud flow and creates heavy damages to the localities
situated at the foot of the mountains. Moreover, occasionally the rock fragments and ashes ejected with explosive forces are being thrown into the air.
The landforms that are created from the volcanic eruptions and lava remain unutilised for a longer period and brings noteworthy changes in the landscape. However, the landforms thus formed by volcanoes are very fertile and are also very rich in mineral resources. Moreover, the volcanic landforms are very important for tourists.

Landforms

The earth is formed of diversified physical structures which is known as landform.
These physical structures include the high mountains, vast traces of plain lands and
also features of intermediate heights. The mountainous regions cover about 18 per
cent, plateaus and hills about 24 per cent and the plain lands about 58 per cent of the
total area of the earth.

Mountains

A mass of rock considerably very high occupying a vast area is known as a mountain.
Higher altitude and steep slopes are its main characteristics. This altitude may reach to several thousand metres from the sea level. Some of the mountains may be located in isolation viz, the Kilimanjaro of East Africa. Again some mountains with
several peaks may cover a large area
(several thousand kilometres). This
type of mountains are folded like the
waves) viz., the Himalayan
Mountain which extends from Pamir
Knot in the east. The Rocky and the
Appalachian Mountains of North
America, the Andes of South America,
the Alps, the Ural, the Caucasus of
Europe and the Fujiyama of Asia are
noteworthy among other mountains.
Several mountain building processes known as Orogenesis work together in the
formation of the mountain. Greek word ÔOros’ means mountain and ÔGenesis’ means formation or creation. Tremendous weathering particularly the wind, water and glacier bring substantial changes in the landforms.
Types of mountains : The mountains can be classified into four groups according to
their major characteristics viz., (a) Fold Mountain, (b) Volcanic Mountain, (c) Block
Mountain, and (d) Laccolith Mountain.
(a) Fold Mountain : This type of mountains are generally constituted with the union of
several vast and high mountain ranges. It can be said easily that when a tectonic plate
enters into another adjacent plate then huge quantity of sediments are accumulated along the joint line of the two
plates and due to lateral pressure,
these sediments are folded giving
rise to anticlines and synclines.
The fold mountains are formed
consisting of the synclines and
anticlines covering a vast area.
The Himalayan Mountains, the
Alps, the Ural and the Rocky
Mountains are the appropriate
examples of the fold mountain.
(b) Volcanic Mountain : Generally, the volcanic mountains are formed by the
accumulation and solidification of the molten materials ejected through the volcanoes.
This type of mountains are
generally conical is shape.
Depending on the nature of lava
flow ejected through volcanoes,
this type of mountain in some
cases, may cover small area with
steep slope as Mount St.
Helena of the United States of
America, and in other cases, may
cover a large area with gentle
slope like the Fujiyama (Japan) and the Mouna Loa (Hawaii island).
(c) Block Mountain : The block mountain has got at least one side to be very steep
with natural fault. Movement of the earth creates fractures in the earth’s crust. So, the
rock strata on both the sides
cannot match together and
displacement starts among the
strata. So, the earth’s crust is
displaced along the crack
which is known as fauly. So in
some part, this displacement
may be upward and in another
part it may be downward. The
uplifted part along the fault is known as Block Mountain. The Black Forest of Germany and the Salt Mountain of Pakistan may be cited as examples. The valley which is formed due to the downward movement of the land between two faults is known as the Rift Valley.
(d) Laccolith Mountain : The molten magma tries to come out of the interior of the
earth being dispalced by the pressure of the gas, but these molten materials, instead of
coming out to the surface of the earth
solidifies beneath the surface of the earth’s
crust. Due to upward thrust, the rock layer
takes the shape of a tomb. The mountain
thus formed is known as Laccolith or relict
mountain. The Henry Mountain
of the United States of America is an
example of this type of mountain.
Plateau : The vast plain area lower than the mountain but higher in elevation than the
plain land is known as Plateau. The formation of plateau is mainly linked with the mountain building processes
The volcanic lava, in some cases, has
formed a vast plain land as in the case of
the Deccan Plateau of South India.
Besides, the old mountainous regions, by
the process of weathering and
denudation has been converted into a
plateau. According to the mode of
location, the plateau may be classified into three types viz., (a) Intermontane Plateau,
(b) The Piedmont Plateau, and (c) The Continental Plateau.
(a) The Intermontane Plateau :
The elevation of the Intermontane
Plateaus range from 3000 to 5000
metres. These plateaus are generally
remain enclosed by mountains and
are formed by the different mountain building processes. The Tibetan Plateau is of this type. This plateau is surrounded on the north by Kunlun, on the south by
Himalaya and on the east and west
by other mountain. The average elevation of this plateau is more than 4000 metres and the area is also more than 52 lakh square kilometres. Besides, the Bolivian Plateau of South America, the Mexican Plateau of Central America, the Mongolian and the Tarim Plateau of Asia are examples of this type of plateau
(b) The Piedmont Plateau : This type of plateau is formed at the piedmont of high
mountains. Due to extensive weathering in the mountainous slope, the eroded
materials are deposited in the lower slopes of the mountains creating an extensive
plain land which is known as Piedmont Plateau. However, the lower part of this type of plateau joins with the adjacent plain lands. Colorado located in the south western tip of the Rocky Mountain of the United States of America and Patagonia of South America are the Piedmont plateaus.
(c) The Continental Plateau : The vast high plain land enclosed by seas or lowlands
are known as Continental Plateau. This kind of plateaus do not have any relation with
the mountains. Arabian Peninsula, Spain, Australia, Greenland are the best examples
of this type of plateau.
The population is very thin due to rugged relief and dry clilnate. However, these
plateaus are rich in different minerals like tin, copper etc. Moreover, hydroelectric
plants have also been established in the rapid flowing rivers of the plateaus.
Plains
An extensive high land above the sea level with gentle slope is known as Plains. Here,
the plain means the land which is less rugged. This type of plains have been formed by the erosional and depositional work of different landforming activities like river, glacier, and wind. Gently undulating land with less rugged relief is most suitable for agriculture, settlement and road construction. So, dense settlements have been developed on the plain lands.
On the basis of the origin of the plain
land, the plains have been classified into
two as erosional and depositional.
Plains formed by erosion : This type of
plains are formed due to continuous
erosion accomplished by river, wind and
glacier. The upper rocks are eroded
gradually and the lands with steep slope
are transformed to plains The
plains at the foot of the Appalachian and
the Siberian plains are the glaring
examples of this type of plains. Madhupur
and Barind region are the two examples
of such plains formed by erosion in Bangladesh. During the Pleistocene era that means about 8 to 10 lakh years ago,
these areas were uplifted from the ocean by diastrophic action. In the later eras, these
uplifted areas had been transformed to plains due to erosional activities by river.
Plains formed by deposition : The plains are also formed by deposition. Formation
of this kind of plains can be found anywhere starting from the mountainous regions to the sea coast. Rivers during their course can form valley such as the valley of Nepal.
Alluvial fan can be formed at foot of the mountain or hill by deposition. In the lower
course of a river, when its transportation power becomes sluggish, it overflows the
banks creating flood and the sediments are deposited gradually on both sides of the
river forming a plain land which is known as Flood Plain. Flood plains of the
Dhaleswari and the Jamuna can be mentioned as examples. A type of plain land is formed at the mouth of the river through deposition which is known as Delta (Fig.
36). The south western part of Bangladesh includes such a delta which is known as
the Ganges Delta. Besides, the plains are also formed in the coastal areas due to the
influence of ebb and tide are known as coastal plain (Fig. 36). The coastal plains of
Chittagong stretching from the mouth of the Feni river upto Teknaf is an example of
this type of plains. The plains are also formed by the deposition of glacial moraines in
the cold areas. The Prairie of Canada is an example of such plains.
Sometimes, the plains can also be formed by the depositional work of the wind. The
Loess soil of North China and the Pampa of Argentina are the examples of such
plains. The Loess soil is very fertile but very much erosion prone.




Atmosphere
The deep blanket of gas surrounding the earth is known as Atmosphere. The scientists opine that the atmosphere is about 35 years old. Its thickness is about 10,000 kilometres. However, about 97 per cent of the total atmosphere remains confined within 30 kilometres upward from the earth’s crust. The atmosphere remains in contact with the earthÕs crust due to the gravitational attraction. Again due to the
pressure of the layers of the atmosphere, density of air is highest at the sea level and
decreases with height.
Importance of the atmosphere : The atmosphere is of immense importance for the existence of the living organisms of the earth. The atmosphere safeguards the lives of living beings against the effect of harmful rays of the sun. Its gaseous elements like carbon dioxide rears the vegetation and oxygen safeguards the living creatures. The human activities on the surface of the earth helps to bring changes in the structure of the atmosphere, particularly large scale deforestation, industrial smoke, and burning
of coal, mineral oil and the natural gas polluting the atmosphere. For the interest of the living beings of the earth, we should refrain ourselves from contaminating the
atmosphere. So, we should have an idea about the structure of the atmosphere and
how it works.
Structure of the atmosphere : Atmosphere is constituted by the mixture of several
gases. The composition of the atmosphere remains fairly constant roughly upto an
altitude of 80 kilometres. Nitrogen and oxygen are the two important constituents of pure dry air. So far the volume is concerned, these two gases jointly constitute 98.73 per cent and the rest of 1.27 per cent being occupied by other gases. A list of different constituent elements are presented below :
Nitrogen (N2) 78.02 %
Oxygen (O2) 20.71 %
Argon (Ar) 0.80 %
Carbon dioxide (CO2) 0.03 %
Ozone (O3) 0.0001 %
Other 0.4399 %
Total 100.00%
Other gases include helium, krypton, zenon, hydrogen, methane, and nitrous oxide.
But the water vapour and dust particles are also found in different scale in the atmosphere. The amount of carbon dioxide in the atmosphere is only 0.03 per cent.
This meagre amount of carbon dioxide in the atmosphere is much important for the
atmospheric processes. Because, this gas helps the short waves of the sun’s rays to
reach the earth. The short waves rays after coming in contact with the earth’s surface
transform themselves to long waves. Carbon dioxide absorbs these long waves in the lower atmosphere. As a result, the atmosphere becomes hot.
Atmospheric layers : The atmosphere can be divided vertically into four divisions according to the characteristics (temperature, pressure, density) of its different elements. These are
Troposphere, Stratosphere, Mesosphere, and Thermosphere.
Troposphere : This is the lowest layer of the
atmosphere and it remains in contact with
the earthÕs crust. This layer is very much
important for weather and climate. The upper limit of the troposphere instead of terminating suddenly, its characteristics gradually fade away and mix up with the next stratosphere layer. The last part of the atmosphere is known as Tropopause. The thickness of the troposphere is 8 kilometres in the polar region and 16 to 19 kilometres in the equatorial region. The characteristics of the troposphere are the following :
(a) The density and the temperature decrease with the increase of altitude. The
decrease of temperature with increase of altitude is known as the normal lapse rate.
(b) The velocity of the wind increases with the increases of altitude.
(c) The lo wer atmosphere contains water vapour.
(d) The wind moves upward and downward.
(e) All sorts of weather and climatic processes are accomplished in this layer.
Stratosphere : The next layer of the atmosphere is stratosphere which extends upto 50 kilometres. Maximum amount of ozone gas is available in this layer. This
ozone layer absorbs maximum ultra violet rays of the sun. So, this earth has become
suitable abode of the living creatures. Both density and pressure are comparatively
less in this layer and the temperature also does not show a change in the lower
atmosphere. However, from the 20th kilometre upwards, the temperature records a gradual rise and continues upto 50 kilometres in the high stratosphere. There is no water vapour in stratosphere. Stratopause exists above this layer.
Mesosphere : The temperature decreases rapidly from the stratosphere upwards upto
80 kilometres. This vast area is known as mesosphere. But the temperature again
increases after the 80th kilometre. This layer is known as mesopause. Air pressure is very feeble in mesosphere.
Thermosphere : Thermosphere extends upward from the mesopause. The lower part of the thermosphere is known as ionosphere. Above the ionosphere, there are other two layers such as exosphere and magnetosphere.
Weather and climate : The daily average condition of temperature, pressure, wind,
humidity and precipitation for any place is the weather. Generally, climate is the
average condition of the weather for 30 to 40 years.
Elements of weather and climate : The elements of weather and climate are : (1)
Wind temperature, (2) Wind pressure, (3) Wind movement, (4) Humidity, and (5)
Precipitation.
Controls of weather and climate : The elements of weather and climate are
controlled by the following factors. These are latitude, altitude, distance from the sea,
location of the mountains, relief of the land, wind movement, ocean currents, slope of
the land, soil and the forests.
1. Latitude : This is one of the most important controls of climate as the incidence of sun’s rays varies with the latitude. The sun gives its rays vertically over the equator and as a result, the temperature is high in this region. On the other hand, the higher latitudes receive inclined or slanting rays and hence the temperature is low. Generally,
the atmospheric temperature decreases from the equator towards the poles.
2. Altitude : The atmospheric temperature decreases with the increases of altitude.
There is a decrease of temperature of 6° Celsius for every 1000 metres of increase of
altitude. Though two places being situated on the same latitude, but their climate will
differ due to the difference in altitude. As for example, Dinajpur and Shillong though
located on the same latitude but the climate differs due to the variation in altitude. The
temperature in Shillong is much lesser than that of Dinajpur.
3. Distance from the ocean : The climate of a place becomes mild if it is located
nearer to the sea or ocean. As the climate of Cox’s Bazar, Chittagong and Patuakhali
is milder than that of Bogra being situated nearer to the sea. No remarkable variation
of temperature between winter and summer and between days and nights is found in the coastal regions. This type of climate is known as equable climate. But the regions away from the coastal area experiences extreme climate during winter and summer, because the landmass gets warmer as well as colder more quickly than that of the waterbodies. For this reason, the interior parts of the continent during summer become very hot and equally these become very cold during winter. This type of climate is known as the continental climate or extreme climate.
4. Wind movement : The wind movement plays an important role on the climate of a
place. If any wind blows saturated with water vapour, then this may cause abundant rainfall there. During the rainy season, the monsoon full of water vapour brings heavy shower in Bangladesh. Again during the winter season, the continental air brings n in Bangladesh and the temperature decreases.
5. Ocean currents : The air of the coastal area becomes either cold or warm due to
the influence of cold or warm ocean currents. As for example, it can be mentioned that the eastern coast of the United States of America records an increase of temperature due to the influence of the Warm Gulf Stream. Again, the cold Labrador Current keeps
the eastern coast of North America cold.
6. Location of the mountains : Wind movement, when obstructed by the high
mountains, can create influence on the climate. The monsoon air being obstructed by the athwart situation of the Himalayan mountain in the north, brings heavy shower in India and Nepal. On the other hand, during winter the cold airmass of
Central Asia cannot cross the Himalayas and so the climate of the sub-continent never gets cold like Europe.
7. Slope of the land : The air and the land become heated where the sun’s rays fall
directly on the slopes of the high lands. But to the opposite side of the slope, the air
remains cold due to the inclined or feeble sun’s rays.
8. Soil : The structure of soil or its texture play a vital role in the preservation of
temperature. The sandy soil of the uncovered areas becomes hot as well as cold quickly. But in comparison to sandy soil, the alluvial and the clayey soil take much time to become either warm or cold due to heat preservation capacity.
9. Location of the forest : The vegetation by its evapo-transpiration activity helps the
air to be saturated with water vapour, and this saturated air being condensed brings
rainfall. Besides, the intensity of storms, and cyclones are reduced being obstructed by the forest. The air of the dense forest remains comparatively cold as the sun rays
cannot penetrate the dense to heat the soil.
Insolation and Air Temperature
Variation in insolation : The atmosphere receives 99.97 per cent of its total energy
from the sun. The energy thus received from the sun is transformed as heat energy or energy of motion in the atmosphere. The amount of energy received from the sun by
the earth through radiation in the form of small waves is the insolation. During
transmission, a considerable amount of this energy is absorbed, scattered and reflected
by the atmosphere. In the clear sky, about 80 per cent of this energy can reach the
earth through the atmosphere and the rest of 20 per cent is lost in the space. Even the
total energy that the earth receives from the sun is not distributed equally throughout the world. The amount of energy received by the equatorial region, in comparison to this, loses less amount of energy. On the other hand, the condition is just the opposite in the polar regions, in other words, the polar regions lose more energy than what they receive. The solar energy tries to bring equilibrium of this unequal distribution of insolation through the wind movement and the ocean currents.
The controlling factors of insolation : The amount of insolation that any portion
of the earth’s surface gets in a day will depend on two factors viz., (a) The intensity of insolation depends on the verticality or the inclination of the sum’s rays, (b) Duration of the day in comparison to night and the solar radiation. With the changing positions of the sun, the above two factors act seperately from the equatorial region to the poles in different latitudes and in different seasons, it is observed
that the intensity of the
vertical rays is greater as
it passes through a
comparatively thinner layer
of atmosphere and it
concentrates in a smaller
area. The different latitudes
lying between the Tropic of
Cancer and the Tropic of
Capricorn get vertical rays
at noon. On the other hand,
the oblique solar rays pass
through a thicker layer and
it covers a larger area. Due to these two reasons, the intensity of solar rays during
summer is more than that of the winter. Again the intensity of morning and afternoon
rays is less than that of the noon’s rays.
The earth’s crust gets sufficient time, if the day is long, to receive more solar energy
and consequently both the earth’s crust and the atmosphere become warm. On the
other hand, if any place, the nights are longer, then that place remains cold as it
receives less amount of insolation due to short duration of day time.
Air temperature : The warmth and the coldness situation of the atmosphere is the air temperature. The sun’s rays or the solar energy is the major reason for such warmth or coldness of the atmosphere.




Transformation of temperature between the earth’s crust and the atmosphere :
Temperature is transferred between the earth’s crust and the atmosphere by three
processes. These are radiation, conduction and convection.
Radiation : Electromagnetic waves can transfer energy (both temperature and light) into two objects without any medium. The same process also occurs in case of solar energy.
Sun rays of a fixed wave length reaches the earth penetrating the atmosphere through
this process. Again by radiating heat through this process, the earth becomes cold.
Conduction : This is a process by which heat can be transferred through a close
contact. In this way heat is transferred from one part of an object to another. The crust
of the earth becomes heated by the process of transference of heat.
Convection : The heated particles transfer heat from hot areas to the cold areas
through the process of convection. This process is also found to occur in case of liquid and gas. In this case, the liquid or gas becomes lighter being heated and rises up and the surrounding cold liquid or gas occupies the vaccum space. The temperature of the atmosphere is transferred this way.
So, the sun’s rays through radiation, after penetrating the atmosphere, reaches the
earth’s surface. Through conduction, the earth’s surface is heated and by convection,
there is a exchange of temperature between water and the atmosphere.
Distribution of temperature : Due to the influence of latitude and altitude, the
temperature of different places differ in different seasons. There is a change of
temperature of any locality due to diurnal and annual motion of the earth. Besides,
variation of temperature occurs due to wind movement.
So, though the temperature differs in different places, yet we get an average
distribution pattern of seasonal temperature throughout the world. Isotherms are used to show the distribution of average temperature. The imazinary lines joining the points of equal temperature on a map is known as Isotherm. The isotherms show the
temperature of all points or the average for many days or several months of a year for
any particular time. The horizontal distribution of temperature of the earth for the months of January and July is shown by isotherms. With the change of season, the highest temperature is also changed in the northern and southern hemispheres. For example, the lowest and the highest temperature is recorded in the
month of January and July respectively in the northern hemisphere. But during that
time, completely, opposite condition prevails in the southern hemisphere.
Air Pressure
Like any other matter, the air has its own weight. The pressure of the air thus created
due to the weight of the air is known as air pressure.
Pressure Belts
Due to the variation of temperature in different latitudes and the rotaion of the earth, several pressure zones have been formed at the lower layer of the atmosphere which
are known as pressure belts
1. Equatorial Low Pressure Belt : This Equatorial Low Pressure Belt occupies an
area lying on both sides of the equator extending from 0° to 5°. This region receives vertical sun’s rays throughout the whole year. The air pressure is low over this region as the air in comparison to its surrounding areas is hot and light. Moreover, this region has more waterbodies. As a result, low pressure which is known as Equatorial Low
Pressure Belt has been formed.
1. Tropical High Pressure Belt :
More the equatorial hot, humid and
light air goes up, more it becomes cold.
Thus warm air continuously rises up but
it cannot come down in the equatorial
zone. As a result, the upper air starts
moving towards north and south. In this
way, the upper cold and heavy air starts
coming down in the tropical region
between 25° to 30°. Thus two heavy
pressure belts are formed in the mid
region between 25° to 35° North and
South latitudes. These two pressure
belts are known as the Tropical High
Pressure Belts (One over the Tropical of Cancer and the other one over the Tropic of
Capricorn).
3. Subpolar Low Pressure Belt : The rotational force of the earth at two polar
regions is very high. As a result, the wind of these two regions moves towards the
tropics. During that time, the pressure of the wind decreases at two arctic regions
(between 60° to 70° latitudes) creating two low pressure belts. These two are known
as Subpolar Low Pressure Belts.
4. Polar High Pressure Belt : The wind nearer to the two poles is very cold and
heavy due to the location in the cold regions. So, two high pressure belts have been formed at two polar regions. These two are known as Polar High Pressure Belts. The wind from these two high pressure belts moves towards the subpolar low pressure belts.

The value of pressure is shown as milibar (mb). At the sea level, this pressure of the
air is about 6.7 kg. per 6.45 square centimetres. The pressure of the wind decreases with the increase of altitude. Sea level height is internationally accepted as zero (0) height to measure the air pressure. The pressure of the wind is measured by an
instrument known as Barometer. The lines joining the places on a map having equal
average pressure is known as Isobar. The air moves from high pressure area towards
low pressure area.
Due to difference in temperature and pressure the air moves from one part of the earth to another. This movement of the air is known as wind. The wind has certain specific characteristics as follows :
1.  The cold and heavy air moves from the areas of high pressure to low pressure area.
2.  According to Ferrel’s Law, the air moves towards right in the northern hemisphere and to wards left in the southern hemisphere.
The wind system of the earth can be divided into four types. These are Planetary
wind, Seasonal wind, Local wind, and Irregular wind.
Planetary wind : These winds being controlled by the pressure belts, blow towards the same direction throughout the whole year. The planetary wind is of three types such as : the trade wind, the westerlies and the polar wind.
The trade wind : The sun’s rays fall vertically over the equatorial region, so the air becomes hot and goes upwards. Since the pressure is less upward, the rising air gets room for expansion and consequently the air becomes cool and dense. The cool air
could not come down directly due to warm air at the bottom. As a result, the air moves
towards north and south directions through the upper atmosphere. Moving upto 30°
latitudes some part of this air finds its way to come downward and blows towards the
equatorial low pressure belt. This part of the air is known as the Trade Wind.
According to Ferrel’s Law, the trade wind blows from north-east in the northern
hemisphere and south-east in the southern hemisphere. The trade wind of northern
hemisphere is known as North-East Trade Wind and that of southern hemisphere as
South-East Trade Wind. The sky remains clear and the weather is hot and dry as this
trade wind originates in the high pressure zones. The big deserts of the world are
situated nearer to this area. For example, the Sahara desert, the Lybian desert, the
Arabian desert in the northern hemisphere and the Kalahari desert in the southern
hemisphere can be mentioned.
The Westerlies : Some part of the air from 30° latitude blows towards the poles over
the surface of the earth and after reaching 60° latitude faces the cold and dense air
coming from the poles. So, comparatively the warm and light air from the tropical
areas rises above the dense cold polar air and a part of it blows towards the polar low
pressure belt. This is known as West Wind. In the northern hemisphere, it blows from
south-west and in the southern hemisphere from north-west. As the area of the
landmass is greater in the northern hemisphere, so locally, some changes occur in the air motion. However, in the southern hemisphere, the maximum area is covered by the waterbodies and hence the west wind can move uninterrupted. The velocity of the westerlies reaches at the highest between 40° to 47° South latitude. This region is called Roaring Forties. This air movement is known as Brave West Winds. Two more airmasses regularly move from north and south polar high belts towards the subpolar lows. These are known as north-east and south-east polar winds. In this, the circulation of planetary wind system (the trade, the westerlies and the polar) exists between the equator and the poles.
Temporary wind : The wind which is caused due to the difference of temperature of land and water occurring during at specific time of the day or during a particular season of the year is known as temporary wind such as : the monsoon, the land and the sea breezes.
The Monsoon : In Arabic ÔMonsoon’ means season. The most important characteristics
of this wind is the changes in the wind direction with the seasons. The monsoon is also a regional air. This mainly prevails in south and south-east Asia. Besides, that the
monsoon is also observable in North Australia, part of Africa and in the United States of America. During summer a low pressure is formed due to excessive heat along the mid￾latitudes or in the north-west of the Indian sub-continent. Hot and moist air from the adjacent oceans blows towards the continent. According to Ferrel’s Law, after crossing the equator this wind is transformed into the south-west monsoon. This wind being obstructed by the high mountains, causes heavy shower on the land.
During winter the interior part of Asia remains very cold and so there creates a high pressure cells. The cold air blows from the north-east high pressure area of the
landmass towards the low pressure zone of the ocean. As this air originates in
landmass, it is dry and it doesn’t contain any water vapour. This is why the Central
and South Asian regions remain dry in winter. However, when this blows over the Bay of Bengal it acquires great amount of water moisture and ultimately causes shower to the coast of Sri Lanka and Tamil Nadu. This air, however, when crosses the equator according to Ferrel’s Law blows over North Australia as the north-east monsoon and acquires large amount of water moisture from the Indian Ocean causing heavy shower to northern Australia.
Land and Sea Breezes : In the coastal regions, the temperature of local landmass
increases with the advancement of the day and a low pressure is established locally
and comparatively the cool air from the adjacent sea blows towards the land 
This is known as sea breeze. The velocity of this air reaches to its maximum in the
afternoon. The air blows from the high pressure zone formed over the ocean towards low pressure area over the landmass as the temperature in the afternoon increases.
After the sun set, the landmass cools quickly in comparison to the ocean. During that time, the high pressure prevails over the landmass. So, the air moves from the high pressure area formed over the landmass towards the ocean. This is known as the Land Breeze.
Local Wind : The wind which is created due to the difference in diversity of
landscapes and in temperature is known as local wind. There are about several
hundreds of such local winds such as the valley and the mountain breeze.
Valley and Mountain Breeze : A special type of wind movement is found to occur in
the mountainous regions due to the relief of the land. During the day time, the sides of
the mountains become more warmer
in comparison to the bottom of the
valley. As a result, the high pressure
prevails at the bottom of the valley
and low pressure on the sides of the
mountains. The wind which is found
to rise upwards along the slopes of
the mountains during day time is
known as valley wind.
During night time, the slopes of the
mountains cools down due to
radiation. At this time, the valley
floor remains comparatively hot.
So, the cool and heavy air comes
down the slopes of the mountain.
The wind which during night time
moves along the slope of the
mountains towards the valley floor is known as the mountain breeze.
Irregular Wind : In some places, the air pressure decreases and low pressure is
formed due to high temperature. Again, high pressure is localised in some places due to much coldness. The wind movements thus formed is known as Irregular Wind such as : cyclones and anti-cyclones.
Atmospheric Humidity and Precipitation
Humidity is the amount of water vapour present in the air. The humidity of the air can be expressed in two ways such as, absolute humidity and relative humidity. Absolute humidity refers to the amount of water vapour per unit volume of air.
On the other hand, relative humidity represents the amount of water vapour actually present in the air (absolute humidity) compared with on the other hand, relative humidity is the ratio of the amount of water vapour actually present (absolute humidity) in the air to the amount of water vapour required to saturate the same amount of air at a given temperature and pressure.
The humidity of the air mainly depends on the water vapour. The air receives the
water vapour through evaporation of water from the open waterbodies (sea, rivers and
small waterbodies) as well as from vegetation.
The air is said to be saturated when at a particular temperature, the air cannot absorb more water vapour.
If the air starts cooling it cannot hold the water vapour it had, then some of the water
vapour is transformed into water particles. This is known as condensation temperature
at which the water vapour is condensed is known as Dew Point.
If the temperature is 0° Celsius or remains below freezing point, then the water vapour becomes hard and falls on the earth’s surface as ice crystals or snow. But if the condensation temperature or dew point remains above the freezing temperature then the water vapour after condensation becomes dew, fog or rains.
Snow : In the cold regions, if the temperature falls below the freezing point then the water vapour of the air condenses like the carded cotton and falls on the earth. This is known as snow.
Dew : After radiating the temperature, the earth’s crust gets cold at night. The
atmospheric stratum adjacent to the earth’s crust also becomes cold coming in contact
with the cold earth. So, the water vapour containing capacity of the air is also
decreased and hence the extra amount of water vapour which the air cannot carry is
transformed to water and rest on the earth as water drops. This is known as dew. In the
cold regions when the temperature of the earth’s crust goes below the freezing point
then the dew is condensed to frost.
Fog : In the winter season due to close contact with earth crust, the temperature in the lower atmospheric strata is decreased. Consequently, centering the dust particles of the air, condensation starts in the lower strata of the atmosphere which is ultimately transformed to tiny water particles. These water particles remain in the air in a floating state. This is known as fog.
Rainfall : Saturated air going up becomes cool and condensed and transformed into
cloud. A cloud contains innumerable water and ice particles. These water and ice
particles coalesce together to form a bigger size of water particles which due to
gravitational force fall on the earth’s surface as rain. The rainfall is of four types such as : convectional, orographic, cyclonic and frontal.
Convectional rain : In the low pressure
region, the air being heated goes high into
the atmosphere and expands. So, it easily
cools down. The rainfall which is caused
due to condensation of the water vapour is
known as Convectional rain
The rising air, in the equatorial low
pressure zone, contains plenty of water
vapour and on condensation causes regular
Convectional rain there.
Orographic rain : The moistureladen wind is forced to rise up the slope of the
obstructed mountain, is cooled down
and on condensation causes rain on the
windward side of the mountain. This
type of rainfall is known as Orographic
rain. No rainfall is found to
occur on the other side of the mountain
as the air remains dry. This side is
known as Leeward Slope. The south￾west monsoon being obstructed by the
hills of Meghalay gives abundant
orographic rain in Sylhet.
Cyclonic rain : The air from the centre of the cyclone rising upward becomes cool.
At this moment, the excess water vapour of the air condenses and causes rainfall. This
type of rainfall is called as Cyclonic rain.
Frontal rain : When the cold and warm airmasses meet together, then it coming in
contact with cold air, the temperature of the warm air decreases and reaches the
condensation level. This condensation is deepened and gives rain in the conflict area. This is known as Frontal rain. This type of rain is found in the temperate regions.

Monsoon Climate
Characteristics of the Monsoon : It has been discussed earlier that the monsoon is a
seasonal wind in which there is a complete or almost complete reversal of wind
direction. Along with the change of direction of wind, there is also a change in the characteristics of the monsoon wind. Due to the reversal nature of the wind direction,
the winter remains dry and abundant rain comes in summer. The air is dry in winter as it originates in the landmass. During winter season, the monsoon wind starts blowing from the landmass and while crossing ocean it absorbs moisture and brings abundant rainfall on the landmass it comes across. The winter monsoon causes rainfall in the north-east coast of Sri Lanka, in the coast of Tamil Nadu, in West Japan and in the
east coast of the Philippines. The summer monsoon, as it blows from the ocean
towards the landmass, carries sufficient moisture with it. As a result, heavy shower
occurs. It is due to the effect of the monsoon, that the variation of temperature is less
between the winter and the summer in the low latitudes. However, the intensity of
winter is well felt in the mid-latitudes. During this time, the cold continental airmass
can reach the mid-latitudes. The mid-latitudinal regions come under the influence of
tropical airmass in summer. So, hot weather prevails there.
Geographical locations of the Monsoon : Most of the monsoon regions are located
in Asia, particularly in South and South-East Asia. The vastness of Asia, the situation
of the Himalayas and the location of the warm water in south-eastern part create great difference in temperature between the landmass and the waterbodies. This difference
of temperature is the cause for the formation of the monsoon.
The monsoon climate of South and South-East Asia covers India, Bangladesh,
Pakistan, Nepal, Myanmar, Thailand, Laos, Cambodia, Vietnam, Malaysia and the
Philippines. Besides, the monsoon climate is also found to prevail in China, Taiwan,
Japan, Korea and the northern coast of Australia.
Monsoon climate of South Asia : Three seasons are found in Bangladesh, Sri Lanka,
Pakistan and India due to the influence of monsoon climate. These are :
1. Winter season : From October to the end of February.
2. Summer season : From March to the end of May.
3. Rainy season : From June to the end of September.
Among the three seasons, the rainy season is most important. The air blows from the
land to the ocean in winter and from the ocean to the land in summer. South Asian
countries experience more or less rainfall in all the seasons in different localities. But
about 80 per cent of the total rainfall occurs in the rainy seasons. The winter season in
this region starts to prevail just after the rainy season. The durability and intensity of winter differs with the variation of the regions. As for example, the intensity of winter is moderate in central Bangladesh and is simply unbearable in northern India during
the same period. Temperature shows an increase from March and reaches to the
maximum in May. A low pressure develops during this period over the area causing
the maritime air to blow towards this region. By June this maritime air reaches the
landmass with its fullest intensity and in this way the Ôburst of the monsoon’ starts
which brings the rainy season.
The Monsoon Climate of South-East and East Asia : The winter monsoon of
South-East Asia is influenced by the Siberian airmass and so it is cold and dry. The
wind first blows towards the east and then takes a turn towards the equatorial region.
As this wind blows over the ocean, it causes abundant rain in the western coast of
Japan, in the Philippines, and in South China. Due to the formation of the low
pressure in Central Asia during summer (May to September), the air from the adjacent
oceans starts to blow towards East and South-East Asia. Consequently, it gives rain
first in South-East Asia and later in East Asian countries.
Importance of the Monsoon : The impact of monsoon is very much important in the
agricultural sector, particularly in South and East Asia. The crop calendar of these
regions is mostly controlled by the monsoon climate. For example, the agriculture of
Bangladesh is hard hit by drought if monsoon rain does not start in time and if the
rainfall is not in required quantity.
Atmosphere
What we learnt from this aspect :
The coverage of the air which surrounds the earth is known as atmosphere.
Atmosphere is formed by the mixture of different gases. The main two constituent
elements of fresh and dry air is nitrogen and oxygen.
The atmospheric layers are divided into four divisions such as : Troposphere,
Stratosphere, Mesosphere and Thermosphere.
Weather and climate : The weather is a daily average condition of atmospheric,
temperature, pressure, rainfall, humidity, and the air movement for a certain place.
Again the climate is the average condition of weather for 30 to 40 years.
Solar energy : The amount of energy received by the earth through radiation in the
form of small waves is the solar energy. The solar energy is transformed through the
process of radiation, conduction and convection.
Air movement : Due to difference in pressure and temperature, the air moves from
one part of the earth to another. The movement of air is known as the wind. The wind
moves from high pressure to low pressure zone. The wind system of the earth is
divided into four types. These are Planetary wind, Seasonal wind, Local wind and
Irregular wind.
Land and sea breeze : In the coastal areas with the advent of the day, the temperature
of the air over the landmasses is increased. So, to neutralize the pressure, cold and
dense air from the sea starts blowing over the land. This is known as sea breeze.
Just after the sun sets, the temperature over the landmass cools down quickly. But at
that time, the ocean remains comparatively warm. So, the air blows from the land
towards the sea. This is known as land breeze.
Rainfall : The moistureladen air goes high into the sky and cools down and is
transformed into cloud. The cloud contains innumerable water and ice particles. These
water and ice particles coalesce together to form a bigger size of water particles. So,
by the gravitational attraction of the earth, these water particles fall on the earth as
rain. Rainfall can be divided into four types such as : Convectional, Orographic,
Cyclonic and Frontal.



The Hydrosphere
Definition : The hydrosphere is the mantle of water which covers the lower parts of
the surface of the globe. The hydrosphere covers 71 per cent of the surface of the
globe. Its area is about 36.25 lakh square kilometres.
The hydrosphere is formed of oceans, seas, gulfs, lakes etc. The great stretch of open
water is known as Ocean, the smaller areas less than ocean are the seas. The open
stretch of water surrounded on three sides by land is the bay while the gulf is
surrounded by land almost on all sides. The lakes are the stretches of water
surrounded by land.
The Oceans
There are five oceans in the world. These are : the Pacific Ocean, the Atlantic Ocean,
the Indian Ocean, the Arctic Ocean and the South Ocean. Of these, the Pacific is the
largest ocean having an area of 60 crore and 60 lakh square kilometres. The average
depth of the Pacific is the highest being about 4,270 metres. The Atlantic Ocean is the
second largest ocean having an area of 8 crore and 24 lakh square kilometres although
its place is third in respect of average depth. There are many semi-circles seas in the
Atlantic having indented coastline. The Indian Ocean is about 7 crore and 36 lakh
square kilometres in area and its average depth is 3,962 metres. The Arctic Ocean or
the North Ocean surrounds the north pole. The southern part of this ocean is
surrounded nearly all over by North America and Eurasia. The North Ocean is 1 crore
and 50 lakh square kilometres in area and the average depth is 824 metres. The South
Ocean is 1 crore 47 lakh square kilometres with average depth of 149 metres
Definition of the ocean currents : The movement of water in a regular and fixed course
from one part of the ocean to another gives rise to ocean currents.
Causes of the ocean currents : The causes of the flow of the ocean currents are
mentioned below :
1. Wind movement;
2. Difference of temperature;
3. Difference of salinity;
4. Difference of evaporation;
5. Variation in depth;
6. Rotation of the earth;
7. Location of the landmass.
1. Wind movement : The strong and regular wind blows over the surface of the ocean to
a particular direction. So the wind system is the major cause of the ocean currents. In the
belts of the Trade Winds, ocean currents flow from the east to the west while in the
westerly wind belts they flow from the west to the east.
2. Difference of temperature : The
temperature of the surface water of the
ocean is different in different latitudes.
Strong heat in the equatorial and
tropical regions causes the water to
become warmer which in turn results in
the expansion of the water in volume
and thereby the water becomes lighter
and the density also decreases. But in
upper and mid-latitude regions, the
water is heavy due to less heat. The hot
and light water in the equatorial regions
flows poleward as warm surface
currents while the colder water from the
poles creeps as under currents to fill the
vaccum.
3. Difference of salinity : Density of
water depends on the salinity. Water
becomes lighter with the decrease of
salinity while the water becomes
heavier with the increase of salinity.
Light water flows as surface currents
while the heavy water flows as under
current (Fig. 50).
4. Differences of evaporation : Much
evaporation occurs in the hot regions of
the ocean resulting in the decrease of
water level. Comparatively cold water
from the neighbouring less hot areas of
the ocean move towards more hot areas
to maintain equilibrium in the ocean
water.
5. Variation in depth : The shallow ocean water becomes hot and light very quickly
that causes up-rise of the warm water resulting in a downward flow of the cold water
current to occupy the place of the warm water. As a result, of the upward and
downward movement of the ocean water as currents are created.
6. Rotation of the earth : The earth rotates from west to east. As a result, the ocean
currents are deflected to the right in the northern hemisphere and to the left in the
southern hemisphere.
7. Location of the landmass : Landmasses deflect the flow of ocean currents
resulting in the formation of a new course. It is, for example, due to the Cape of Saint
Rock in the east of Brazil that the South Equatorial current in the Atlantic Ocean is
divided into two branches. One is flowing through the north coast of South America
and the other through south-east coast.
Various ocean currents : The ocean water does not remain stationary in one place.
The water of all the oceans and seas are flowing either as surface or under current.
Hence, the ocean currents are divided into three on the basis of the location of the
oceans. These are :
(a) The Atlantic Ocean Currents;
(b) The Pacific Ocean Currents;
(c) The Indian Ocean Currents.
The Atlantic Ocean Currents : The continents of Europe and Africa are in the east
of Atlantic while North America and South America are in the west. The equator has
divided the Atlantic Ocean into north and south part. So, the currents can be divided
into two parts.
A. South Atlantic Ocean Currents; and
B. North Atlantic Ocean Currents.
A. South Atlantic Ocean Currents
1. The Antarctic Current : The cold waters from the Antarctic move under the
influence of the strong westerly wind from west to east. This is known as the
Antarctic current and it enters into the Atlantic by the south of South America.
2. Benguela Current : The branch of the Antarctic current being deflected near the
Cape of Good Hope turns to the north and flows by the west side of South Africa.
This current is known as the Benguela current. The current is cold since it comes from
the cold current. It turns westward under the influence of the south-east trade winds
and joins the South Equatorial current.
3. South Equatorial Current : The South Equatorial Current originates from the
extended part of the Benguela current. This current marches towards north-west upto the equator being influenced by the rotation of the earth and the south-east trade
winds. This current is known as the South Equatorial Current as it flows to the south
of the equator. This is warm current.
4. Brazil Current : The branch of South Equatorial current known as Brazil current
which flows through the east coast of Brazil to the south-west. The current is warm
since it originates from the warm current and flows through the tropical region,
crossing the Tropic of Capricorn, the current turns eastward gradually under the
influence of westerly wind and meets the Antarctic current.
5. Falkland Current : A branch of the Antarctic current on entering the Atlantic
Ocean turns north and flows northward along the coast of Falkland Island and
Argentina. This is known as Falkland Current and it is a cold current.
B. North Atlantic Ocean Currents
1. North Equatorial Current : The warm currents flowing from east to west along
the north of the equator under the influence of the rotation of the earth and the north￾east trade winds is known as the North Equatorial Current. This is warm
current. After crossing the Mid-Atlantic, the northern branch of the South Equatorial
current meets the North Equatorial current. The North Equatorial current is divided
into two branches. The first branch flows into the Gulf Stream taking a turn to the
north. The second branch flows first into the Carribean Sea and then into the Gulf of
Mexico.
2. The Equatorial Counter Current : Between the North and South Equatorial
currents, there is a weak current flowing from west to east is known as the Equatorial
Counter Current. This is a warm current.
3. The Gulf Stream : The Gulf Stream, practically, has originated from the extended
part of South and North Equatorial currents. This mixed current enters into the
Carribean Sea and is divided into two branches being obstructed by the islands. One
of the branches enters into the Gulf of Mexico. The rush of water from the Mississippi
river of the United States of America enhances the speed of this current and flows
through the narrow strait of Florida to North Atlantic. It is known as the Gulf Stream
since it has originated in the Gulf of Mexico. The extent of the Gulf Stream varies
from 64 to 80 kilometres at the entrance of Florida strait, the depth is 914 metres, the
average speed is 8 kilometres per hour and the temperature is 30° Celsius. The colour
of the current is deep blue.
At the Mid-Atlantic, the depth and the temperature of the current comes down while
the width gets increased and this current flows here at a speed of 2.5 kilometres per
hour. While flowing north-east along the east coast of the United States of America,
the Gulf Stream is divided into three branches at the mid of the North Atlantic due to
the influence of the westerlies.
(a) North Atlantic Stream;
(b) West Greenland Current; and
(c) Canaries Current.
(a) North Atlantic Stream : The first branch of the Gulf Stream being called North
Atlantic Stream flows along the coast of West Europe, the British Islands and Norway
into the North Sea as a warm current.
(b) West Greenland Current : The warm West Greenland current, the second branch
of Gulf Stream curving northward proceeds along the south of Iceland and Greenland
and then flows through Davis Strait between Greenland and Baffin Island to the north.
(c) Canaries Current : The third branch of Gulf Stream known as the Canaries
current turning southward flows along Portugal and the west coast of West Africa. The
current is divided into two branches. The first branch curving south-west under the
influence of Trade Wind flows into North Equatorial Current. This is a cold current.
4. Guinea Current : The Guinea current, the second branch of Canaries Current,
flows along the coast of Guinea of West Africa to the south upto the Equator. Then
joining the Equatorial Counter Current, it flows into the Bay of Guinea. This is a cold
current.
The flow of different currents and cross currents along the sides of North Atlantic
Ocean has caused in the centre of the ocean an area of stagnant sea often full of drifted
branches of plants, grass, sea-weeds etc. and is called the Sargasso Sea.
5. Labrador Current : Two cold currents from the North Ocean flows along the east
and west of Greenland into the Atlantic Ocean. These two currents join in the north of
Labrador Peninsula and taking the name of the Labrador Currents. It flows along the
east coast of Newfoundland and the United States of America to the south. The
Labrador current flowing south to New York gets deflected by the warm Gulf Stream.
The deep blue water of the Gulf Stream and the green water of the Labrador flow side
by side in the opposite direction and the border of these two cross currents is called
the Cold Wall.
The impact of ocean currents on trade
1. The ocean water in the mid-tropic and high tropic region freezes in winter.
Naturally the mercantile ships cannot ply in those seas. But the warm current keeps
the sea ports ice free where it enters the sea. This makes possible the free trafic of the
commercial ships in the seas.
2. A ship can sail to a port of destination in a short time in favour of the current but it
takes a lot of money, energy and time if the ship goes against the current.
3. It is safe to sail ships in the passage of the warm currents. Cold currents bring
icebergs. The ships are damaged if hit by an iceberg and sinks into the ocean. This
way the Titanic ship sinks into the Atlantic Ocean.
4. The icebergs that come floating with the cold current are quickly melted and the loads
such as pebbles, sand, mud etc. are deposited on the sea bed creating shoals. Plenty of
fish food is available in these shoal areas, and as such are good for fishing ground.
Hydrosphere
What we learnt from this aspect
Hydrosphere : The hydrosphere is the envelope of water which covers about 71 per
cent of the surface of the earth. It covers an area of 36 crore and 25 lakh square
kilometres.
The land and waterbodies : The water area covers 71 per cent and land area covers
29 per cent of the total area of the surface of the earth.
The Oceans : The wide open mantle of water is called the Ocean. There are five
oceans in the world namely, the Pacific Ocean, the Atlantic Ocean, the Indian Ocean,
the North Ocean and the South Ocean.
The area of the Oceans : The Pacific Ocean is 16 crore and 60 lakh square
kilometres, the Atlantic Ocean is 8 crore and 24 lakh square kilometres, the Indian
Ocean is 7 crore and 36 lakh square kilometres, the North Ocean is 1 crore and 50
lakh square kilometres, and the South Ocean is 1 crore and 47 lakh square kilometres.
The Ocean Currents : The movement of water from one part of the ocean to another
in a regular and fixed course is known as ocean current.
Ocean currents are of three types (1) The Atlantic Ocean Current, (2) The Pacific
Ocean Current, and (3) The Indian Ocean Current.
Warm current generally flows as surface current, while the cold current moves as
under current.
The causes of Ocean Currents : The wind system, the difference of temperature
depth, salinity, evaporation, rotation of the earth and location of land area cause ocean
currents. Of these factors, the wind system plays a vital role to create ocean currents.
South Atlantic Ocean Currents : The South Atlantic Ocean currents flow towards
left (leftward or anti-clockwise). The Antarctic current, Falkland current, Benguela
current, South Equatorial current, and Brazil current flow into the Atlantic Ocean.
Among all these currents, the Antarctic current, Falkland and Benguela currents are
cold currents and others are warm currents.
North Atlantic Ocean Current : The currents of the North Atlantic Ocean are
flowing towards right (southward or clockwise). The North Equatorial Current, the
Gulf Stream, North Atlantic Stream, West Greenland Current, Canaries current,
Guinea Current, Labrador Current, and Equatorial Counter Current are the principal
currents of the North Atlantic Ocean. The Labrador, Canaries, and Guinea currents are
cold currents.
There are different currents and cross currents on the sides of North Atlantic Ocean
and in the whirlpool of water in the centre, there is no current resulting in the deposit
of moss and drifting weeds in the stagnant water. This is called the Sargasso Sea.
The impact of the Ocean Currents : The seas in the high latitudes are frozen during
winter and the mercantile ships cannot ply. Naturally, the commercial transactions are
disturbed in the ports during winter on the high latitudes. But the seas where warm
currents flow do not freeze in winter and mercantile ships can move freely. There are
possibilities for the ships to get damaged by the icebergs that the cold currents bring.
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