CHAPTER NO.6 FORMATION OF ATMOSPHERE
CHAPTER NO.6 FORMATION OF ATMOSPHERE
In our daily life, we often discuss various
characteristics about weather.
Generally such discussion is limited upto good, bad,
dry hot and cold weather. While student of geography has acruisty to gather
more and more information about these aspect. Gathering information about
surroundings includes knowing reasons,responsible for these changes and
descriptions of the aspects related to atmosphere at different places of the
world.
In this series, our discussion begins with the basic
information about the important sphere of our mother earth, which is known as
‘Atmosphere’. It is also as important as Lithosphere and Hydrosphere are
Our Earth is engulfed by a mixture of various gases
around it which remian glued with Earth becasue of its gravitational power.
These gases are tasteless, colourless,odourless and invisible. Apart from this
these are always moving, elastic and compressible also. This mixture is not
dense and solid as land or liquid like ocean
regions but it is a mixture of gases which also has
mass/weight, like all physical bodies and has air pressure also. The
interesting fact of information about
its existance is that
it comes to be known or is falt only when it
horizentally means when wind blows.Definition : Atmosphere is combination of
two words “Atmo’ and ‘sphere’. It means that region of Earth which has ‘air’.
Various scientists have defined ‘Atmosphere’ in their own respective ways.
Gathering of information about Atmosphere is harder
as compared to that of hydro and lithospheres because of its inaccessibility.
Especially about its upper layers because these extend upto hundreds of
kilometers. Presently we have been able to extract quite
reasonable information about Atmosphere with the
help of technology but earlier this was not possible. Systematic study of
atmosphere is knwon as atmospheric science.This is further divided into two
parts weather science and climatology.
Weather science is a branch in which study of
characteristics and conditions of atmosphere are included.Investigation of
forecasting on scientific lines is also a part of it.
On the same lines, purpose of climatology is to
study the factors, differences and other characteristics pretai ning to Nature
in relation to those of human beings.
Historical Timeline :Information about atmospheric
science, like other branches of sciences also begins with the views and
principles given by ancient Roman and Greek philospheres. Greek
thinkers were first who published documents about
atmoshphere in 400 B.C. large information about the existence of wind, effect
of climate on health, division of world in different spheres etc. is included
in ancient scriptures. There was a ‘deadlock’regarding information due to dark
age which was broken in 15th and 16th century.
Invention of devices which are used to measure
different aspects of weather included;Galileo invented thermometer in 1593,
Torricelli invented barometer in 1643 to measure the atmospheric pressure. In
1668, a space scientist Edmund Halley came forward with his views about winds.
In 1735 Hedley presented his views about the effect of rotation on the
direction of winds. Regular recording of figures about winds
started in 18th and 19th century. On the bases of
these figures, of weather maps
started coming upin Europe. Weather scientist of
Germany, Valadimir Koppen brought the thought of climatic classification in
1900. During first and second world wars emphasis was given on collection of
information about weather and present day weather predictions find their birth
in those days.
After 1960 information sector crossed national
political boundaries and carved its niche at world level platform as weather
sattelites provided regular information about atmosphere data at world level.
Information provided by weather sattelites like TIROS, NOAA and GOFS helped the
scientists in solving atmosphere issues.Moreover, the information provided by
these sattelities helps to understand the
changes in atmosphere.
Significance : Our mother Earth is most important
celestial body in our solar system because it supports life while atmosphere
plays a vital role in this aspect. Atmoshpere is present as life saving layer
between outer space and land surface. It is the source of important gases which
are important for the existance and continuity of life. It also filters the
harmfull rays travelling towards Earth and also protects Earth from extensive
heat for which might be generated by ultrovidet (UV) rays. It provides
essential energy for life cycle also.Atmosphere helps in distributing solar
energy on earth vertically and horizontally as
well. Atmospheric conditoins have biggest effect on
flora and fauna of Earth. Presently atmospheric changes at local, regional and
global level owing to natural and man made conditons extend the importance of
its deep study.
Use of air force in second world war brought
revolution in the field of air transport and has reduced the world to be a
global village which has resulted in making every minutes information about
atmosphere crucial for the safety of aeroplanes in particular and
mankind in general.
Extent : With the advancement in the field of
science, today, we are able to move thousand kilometers above the ground. As
compared to the past, we have more information about atmosphere presently but
still scientists have various divergent opinions about related facts with the
help of sattelities the height of atmosphere has been settled varying between
16000 to 32000 Kms. while portion of atmosphere upto
ten kilometers part closest to the Earth 1s most
important for weather and human life.
Aeroplanes, Baloons and man made sattelities are
indirect sources, which provide informaton about atmosphere.
Density of atmosphere decreases as we move away from
the surface of Earth
Structure of Atmosphere :Definition of atmosphere is
addresed as a combination of gases but along with those gases large number of
very fine solid and liquid materials, aerosols and water vapours are the part
of atmosphere.
Amount of suspended particles, water vapours and
some gases change with time and place but amount of major gases like N, O,, Av
remain same in dry air. Contribution of various gases in the atmosphere is as
under :
Nitrogen : Amount of nitrogen (78.03%) is highest in
atmosphere as compared to
other gases. Although its elements do not match with
other particles but it is very important for living organisms because it is an
important element of Amino acids which form protein. Living organisms donot use
it directly. Its main function in atmosphere
1s toregulate combustion by diluting oxygen.
Oxygen : Amount of oxygen in atmosphere 1s 20.11
percent chemically it is very active and combines with other elements at fast
rate. Our mother Earth is supporting life because of presence of oxygen. All
the living organisms use it for breathing. It helps in combustion and itis a
source of energy. In living organisms it helps in the convertion
of food into energy.
Carbondioxide : It covers 0.03% part of atmosphere
and is produced by
combustion. It plays a vital role in absorption of
energy moving towards atmosphere from land and also keeps the lower atmosphere
warm. Plants use this gas for the process of photo synthesis. Its amount has
been increased from 290 PPM to 350 PPm 1.e, 0.029% to 0.0350% in last 100
years.
Ozone : It is a life saving gas and another form of
oxygen. Its amount is very less in atmosphere (0.0005% by volume). Its maximum
amount is found between 20 to 30 kms above the earth surface. Itis present in a
form of layer and protects Earth from harmful rays of sun by absorbing that
part their of.
Since the last decade of 20th century scientists are
very concerned about the depletion of ozone layer. Majority of scientists
presume that halogenated gases such as chlorofloro carbons, halons and nitrogen
oxides are responcible for this depletion as
these gases change ozone atom into oxygen atom and
oxygen molecule, scientifically we put it as O,= 0,40.
Apart of it, hydrogen, Helium Neon, Zenan, Crypton
etc. are also present although in less amount in atmosphere. These do not
affect the weather of a particular place. On the other hand Oxygen,
Carbondioxide and Ozone react very fast therefore these are
known as active gases. These gases play an important
role in settling the weather of a particular place.
Water Vapours : Water vapours are gaseous form of
water present in atmosphere.Although it is a gas but due to its importance in
atmosphere it must be discussed as different aspect. Spatial and Temporal
variations of water vapours formed by evaporation of water from the water
sources situated on Earth, are found in atmosphere. The amount of water vapours
is higher in lower layers of atmosphere present near to the Earth. This amount
decreases as the height increases.
About 90% of water vapours are found in region
extending upto five kilometers above the Earth and vanish fully at and above
height of eight kilometers. Distnbution of water
vapours varies honzentally also like 0.02% of volume
of air water vapours are found in cold polar regions and 5% of volume of air
water vapours are found in moist temperate zones. Atmosphereic humidity is
important for every type of life. Water vapours absorb the long wave radiations
of Earth and increase the temperature of lower atmosphere. These are also known
as Green house gases. At the time of condensation they release large amount of
energy, which act as power of storms.These are the sources of various type of
precipitation and condensation .e. clouds, fog,dew, rain, hail and snow. Water
vapours are closely related to temperature. The capacity of wind to acquire
water vapours increases with rise in temperature.
Aerosols : Solid and liquid particles of different
sizes suspended in air are known as Aerosols. These particles are very minute
and are visible to naked eye in light streaks at dark places only. Generally,
these are invisible to naked eye. These particles enter in atmosphere from
natunal and human sources e.g. dust particles from volcanic eruption, salt
particles from sea, polination of various plants and flowers, bacteria and
virus from living organisms, smoke and dust
particles, particles produced from meteors etc.
The amount of different particles decreases with the
increase in height. Amount of aerosols varies from place to place and it may be
100 per cubic centimeter of air to lakhs per cubic centimeter of air. Although
aerosols are pnncipally found in lower layers
of atmosphere yet Aerosols may be found in upper
layers of atmosphere because of
meteors, nuclear experiments, volcanic blasts and
high altitude aircraft generating them there. Atmospheric circulations help
them all these particles everywhere in atmosphere.
The radiation rays origrating from sun also spread
these particles at some wave lengths which results into changing colour of sky
at dusk, dawn and noon namely red, orange and blue.
Some particles absorb and reflect small amount of
radiation rays of sun. These also play an important role in weather, they act
as center for humidity acquisation. Water vapours gather around these particles
in the form of condensation and reach on Earth
in various forms of precipitation and effect the
division of water on Earth.
Structure of Atmosphere : Now it is clear that
atmosphere starts from Earth surface but scientists have different views about
its maximum height. On the basis of inofrmation provided by radars, baloons,
rockets, sattelities at present large number of scientists agree on a point
that the height of atmosphere is 29000 kilometer and layer upto 800 kilometer
is considered as important part of atmosphere.
On the bases of air pressure, temperature and
weather phenomena, out of the total amount of major gases in atmosphere 97% are
found upto 29 km of height from Earth while 50% are found upto 5.6 kilometers
only.
In 20th Century with the help of advanced technical
devices, scientists have succeded in solving major mysteries about atmosphere.
Teisserence de Bort, S. Petteisun, Sir Napier shaw, Picardy, Kennelly
Heaviside, Ferrel etc. have played animportant role in this field. Atmosphere
may be divided into layers or spheres on the basis of height,every layer having
its ownimportance. Boundaries of these layers are also arbitranly established
as other natural boundaries.
Structure of atmosphere can be divided in two ways;
On the basis of temperature and on basis of Chemical Composition.
On the basis of temperature S. Peterson
has divided atmosphere into five layers vertically :—
1. Troposphere
2. Stratosphere
3. Mesosphere
4. Inosphere
5. Exosphere (Thermosphere)
1. Troposhere : This is the lowest layer of
atmosphere and it is very important for all the living organisms because all
the activities and aspects related with weather e.g.Evaporations, condensation,
precipitation, storm, lightening etc. originate in this sphere. Around 75% mass
of gases and nearly all water vapours and aerosols are present in this layer.
Teissesence de Bort was the first scientist who used
this name *...T..ro..po.... sphere’ it has been derived from Greek word
“Tropos’ which means mixing or Turbulence,overall we can say ‘region of mixing’
One of the important feature of this sphere is that
in this part of atmosphere,temperature decreases with increase in height. In
normal conditions the rate of decrease is 6.5 degree celsius per kilometer or
3.6 degree F per 1000 feet. This is also known as mean temperature fall rate.
This happens because of decrease in gases with increase in height, airpressure,
decrease in aerosides with increasing height.However, due to local reasons, at
some places this phenomena reverses also.
The height of Troposhere changes with change of
weather. The change is registered lesser towards poles as compared to tropical
regions and in winters as compared to summers. According to scientists the
height of this sphere on Equator is 16 km and
on poles itis 8 km. Interesting fact is that the
temperature decreases at the upper part of this sphere at Equator not on poles.
At average its height is upto 12 kilometer from ground.
Tropopause : The region between Troposphere and
Stratosphere which is 1.5 kilometer high, known as boundary of troposphere .
The fall in temperature comes to an end in this region named as tropopause.
Turbulent mixing of gases, winds, and radiation etc. none of the weather
activities take place in this region. The minimum temperature in winters, at Equator
(At the height of 17 kilometer) is 70° C and in summers over the poles itis -
45°C (At the height of 9 kilometer). Air pressure in this region is also
between 100 millibar over Equator and 250 millibar over poles.
2. Stratosphere : This sphere starts from 12 to16
kilometers above the Earth surface and extend upto SO kilometers known for
isothermal characterstics also. Although scientists have different views about
the height, width and temperature but they all agree on one point that at the
lower layer of this sphere temperature does not change
with increase in height but at the height of 20 KM
in upper stratosphere temperature starts increasing because of absorption of
ultra violent rays by ozone gases. At the height of 50 KM it becomes © celsius
(32°F).
Region where ozone gas is present in abundance is
also known as ozone layer. This gas 18 present in abudance between 15 to 35
kilometers above the Earth surface, which originates when oxygen molecule
disintegrates into two atoms of oxygen and then integrating back into a
molecule. Molecules of ozone layer are not stable, their origin and
disintegration is a gradual and continuous process.
It has been proven that the ozone layer is dipleting
day by day because of human
activities like spray and syntehtic chemicals used
for air conditioning which are stable compounds of clorine, florine and carbon
on Earth but when these reach upto outer layers of atmosphere by vertical
currents of air they affect the ozone layer.
Because of depletion of ozone ultra violent rays are
increasing the temperature of Earth. Moreover they cause skin problems,
blindness and also kill senstive organisms.Apart from all this, these rays also
decrease the process of photosynthesis, supersonic
Jets fly at the height of 20 km above the land
surface and they emit nitrogen oxide which also affects the ozone layer.
2. Mesophere : Above the upper boundary of
stratosphere, which extends from 50
kms. above surface of Earth, is mesosphere, upto 80
kms. In this layer temperature again starts decreasing with the increase in
height and at the upper boundary of this layer it touches down to-80°C point.
Air pressure is very low in this layer. At the height of 50 kms it is 1 mb and
at height of 80 kms it drops to 0.01 mb.
4. Ionosphere : The height of this sphere extends
from 80 to 640 kms above surface of Earth. Credit of discovering this layer
goes to scientists Kennelly and Heaviside who, with the help of radio waves
proved that the atoms and molecules of the gases present in this layer get electrically
changed by the addition and removal of electrons
due to the effect of ultroviolent, Xray and Gama
rays.
Short wavelength rays of solar energy (UV, X, L) get
absorbed by molecules and atoms of nitrogen and oxygen because of which
temperature increases upto 1000°C in this scanty pressure layer of atmosphere.
Because of scanty air pressure,temperature here is quite different as compared
to the temperature felt at the surface of Earth. On the basis of height
ionosphere may be further divided into following layers :
5. Exosphere : This is the outermost layer of
atmosphere and we donot have much information about this layer. It lies above
the height of 640 kms. Only Helium and hydrogen are found in this layer.
Moreover the density of this layer is very low.
On the basis of chemical composition
atmosphere has been divided into two
parts :
a) Homosphere : It starts from the surface of Earth
and extends upto 90 kms. In this sphere all the gases are found in their
accurate ratio.
b) Hetrosphere : Atmosphere above the homosphere is
not constant on the aspect
of gaseous composition. On the basis of presence of
gases this sphere has been further divided into four layer :-
Oxygen layer = 90 to 200 km
Nitrogen layer = 200 to 1100 km
Helium layer = 1100 to 3500 km
Hydrogen layer = 3500 km onwards, upto outermost
limit of atmosphere.
Insoloation Sunis main source of energy for relief
and atmosphere of Earth. It lies around 15 crore kilometer (93 million miles)
away from Earth. Surface temperature of this star 1s 6000°C (10,800°F) which
radiates its energy through low frequency electromagnatic
radition rays. The highest layer of atmosphere
receives only 1/2 billionth part of energy produced by sun.The small part of
energy that reach at Earth generates the life cycle on the planet. The average
energy is
1.94. calorie/cm/minute and yearly
itis 520X10* Joules. About 51% part of this energy
reaches upto Earth and increases the temperature of relief. This energy is
known as Insolation and in other words it is a smaller form of
incoming solar radiation.
The structural shape of earth is GEOID and it is
quite similar to sphere. Earth is inclined at the angle of 66 1/2° and it
rotates on its own axis and also revolves around sun. This rotation and
revolution makes daily and yearly difference in the amount and intensity
of heat gained by earth.
Apart from above mentioned reasons transparency of
atmosphere and relief of Earth also effect the heat gaining capacity at a
particular place which brings difference between the height at different places
of Earth creating difference between air pressure
which makes possibility of shifting and transfer of
temperature with the help of winds.The winds move the water cycle on Earth in
the form of evaporation and precipitation resulting to prove very helpful for
the life cycle on Earth.
Heating and Cooling of Atmosphere :
Energy gained by Earth from sun is continously
changing. As we know that the temperature of a particular thing represents its
heat, increase and decrease in heat means rise and fall of temperature.
Difference between the temperature of two things shows us mobility or change in
heat. Heat or temperature moves from high temperature to low temperature.
Earth recievs heat from sun by way of radiation and
then starts heating atmosphere through convection. The mobility of heat in this
case has three important steps namely.Radiation, Conduction and Convection.
Radiation : This is adirect process of transfering
energy. No medium is required for the transfer of heat from one thing to
another. Heat reaches upto Earth from sun through space and after gaining the
heat Earth reflects it towards the atmosphere through waves. Because of this
the temperature of lower layers of atmosphere rises first.
Conduction : This is a process in which heat moves
from a body having high
temperature to a body having low temperature, until
the temperature of both bodies comes to equal or two bodies are not seperated.
The lowest layer of atmosphere adjoining to earth surface gains heat and they
upper layers of atmosphere gains heat from lower layer. Temprature shifts
through this process.
Convection : Transfer of heat takes place through
convection in liquid and gaseous forms. Convection is the process which
represents such transfer. Even in atmosphere,warm air heats upper layers of
atmosphere through vertical waves of convection.
Advection : This is an additional form of transfer
of heat in which tempratural effect is transfered horizontally. Equatorial
region and polar regions transfer heat and cold respectively to adjoining
regions through this process.
Yet other important process is evaporation and
condensation which creates indirect energy. The heat energy gained by water
vapours while evaporation process at a place, are released in atmosphere after
vapours turn into rain drops on condensation,
transfering heat in atmosphere.
Factors Controlling Temperature :
The average global surface temperature is 14°Celsius
which vanes with time at different places. The spatical and temporal differences
of temperature are very important because they affect the weather, atmosphere,
vegetation and activities of human life.Temperature of a place depends upon the
heat gained by that place from sun. The temperature of a place depends on
different.
Latitude: The heat gaining capacitity of a
particular place depends on the latitude of that place. For example the lower
latitudes situated near Equator receive more heat from such as compared to
poles because of this air pressure decreases toward poles.
Altitude : Indirectly the temperature of lower part
of atmosphere increases due to long waves of heat released by Earth. This is
well established fact that convection flows from high to low temperature means
from Sun to Earth in short and long waves respectively. As the height increases
the temperature decreases at a particular rate i.e.
6.5° Celsius per kilometer. Because of this the
temperature of mountaineous regions is low as compared to the plain areas
situated near sea level.
Distance from Coast : The relation to seas, distance
from sea also affect the
temperature of any place. e.g. if two regions
acquiring same level and amount of heat from sun namely relief and water, get
heated and cooled at different ranges.Temperature of hydrosphere is low as
compared to lithosphere because the heat moves down deep due to transparency of
water and moreover it mingles also due to
mobility of water. Regions situated near sea have
moderate temperature on the other hand internal parts of continent are aloof
from this affect because of which they face extreme heat and extreme cold.
Prevailing winds and Ocean Currents : Horizontally
flowing wind on the surface of Earth and the speed of water shifts the
temperature/heat at high level. This brings a big change in the temperature of
those regions which are affected by these two factors.We can feel the effect of
winds in the inten or parts of continent on the other hand the effect of ocean
currents is limited to the coastal regions. Hot winds increase the
temperature and on the other hand cold winds
decrease the temperature. Winds
blowing from coastal regions to land, carry moderate
effect with them while wind blowing from land areas to coasts take extreme
effects or what so ever the cause be,with them. Apart from facts mentioned above,
lower layers of atmosphere being more dense gain more water vapours and dust
particles as compared to upper layers.
Type of Relief : Colour, vegetation and land use of
relief also effects the temperature.Various land surfaces reflect solar heat
reaching itin different manners, the process is
known as Albido. If the surface is covered with
snow, it reflects 70 to 80 percent of solar heat reaching it whereas lands with
dark soil absorbs more heat as compared to regions with lighter soil and thick
vegetation. Soils of lighter colour and vegetation
reflect heat move as compared to dark coloured soil
regions.
Clouds : Presence of clouds at a particular place
brings change in local temperature.Temperature during clear day is higher than
that of acloudy day. On the other hand temperature during cloudy night is
higher than that of aclear night. This happens because on a clear night heat
escaping from the Earth at night moves toward atmosphere freely giving ample
chance of cooling to lower part of atmosphre whereas on clody night, escaping
heat stays in lower part of atmosphere because of hindrance
of clouds.
Relief : Like clouds, relief also effects the
temperature of a particular place. We can feel this effect specially in the
mountaineous regions. e.g. Mountai neous regions situated in northern
hemisphere like the southern slopes of Himalayan mountains receive more solar
heat and that also for longertime of a year as compared to the northern slopes.
Distribution of Temperature : To understand the difference in temperatures all over the world, study of temperature variation of months of January and June is very important. At any places these are the coldest and hottest months of a year.Generally in temperature maps the effect of latitude is very clear because isotherms run almost parallel to latitudes. Similarly, the effect of water and land is very clear on the distribution of
isotherms because of which isotherms are almost straight in southern hemisphere
as most part of this hemisphere is covered by Oceans. Whereas in northern
hemisphere during summer isotherms divert towards Equator while travelling from
continental region to oceans and tend to divert a bit towards Equator while
showing their voyage from oceanic to continental regions 1n winter.
Similary the effect of Ocean currents is also very
clear. Specially in North Atlantic Ocean where temperature increases due to hot
water current, Gulf Stream and Isotherms shift northwards. While moving further
they divert southwards as these enter continental parts of Europe having low
temperatute. This effect is particularly clear in the plains of Siberia.
Facts about distribution of temperature
in January :
1. Isotherms of January are irregular and very close
to each other in the Northern hemisphere.
2. In Southern hemisphere isotherms are almost
straight lines because of abundance of water bodies.
3. The coldest place in the world is North-Eastern
Siberia and the second coldest place is situated in Greenland.
4, The closely situated isotherms over continental regions
in Northern hemisphere show steep variation in temperature.
5. Variation in temprature of eastern and western
parts of any continent is also found.Heat gradient on eastern regions 1s 3
times more than that of western regions e.g. 1.5°C
per latitude & 5°C per latitude.
6. In Southern hemisphere the highest temperatute
has been recorded at 30° southern latitude.
Facts about distribution of temperature
in July :
1. During the month of July, Nothern hemisphere has
summer season and Southern hemisphere has winter season.
2. Areas between North Africa to North Western parts
of India have highest
temperature (32°C). Similarly North Western parts of
U.S.A. also have quite high temperature.
3. In Northen hemisphere, the temperature of
continents is higher than the parts of ocean.
4, Heat gradient of eastern regions of continents in
Northern hemisphere grows weak where as reglegiable variation is found in
western regions while western regions donot experience much variation.
Air Pressure
Distribution of temperature is not similar at all
the places on the Earth. Because of difference in temperature, air pressure
also varies immencely.
Weight of airis known as air pressure. Airis a
composition of various gases therefore it has specific weight. Weight of air on
any unit of area on Earth is known as air pressure while it is respresented in
Millibar unit. Air expands in summer due to high temperatre
and in winter it shrinks due to low temperature. High temperature causes scanty air and less air pressure while low temperature brings thick air and higher air pressure. Thus difference between air pressure creates air movement from high pressure areas to low pressure areas which is known as wind.
Temperature and Air pressure cause expansion and shrinking of air
which
further results into distribution of heat and
moisture in the atmosphere. In
normal circumstance average air pressure at sea
level is 1013.2 milibar.
Instrument used to measure air pressure is known as
Barometer.
Factors affecting air pressure :—
Temperature : As the temperature increases, air
expands because of which its density decreases which results in low pressure.
On the other hand air shrinks due to low temperature because of which its
density increases which creates high pressure. The relation between air pressre
and temperature is defined with following quote : “When
the mercury of thermometer rises, mercury of
barometer falls”. Equatorial regions have low pressure because of high
temperatures. On the other hand Polar regions have high pressure due to low
temperature.
Height from sea level : Air pressure is created
dueto weight of air therefore sea level has highest air pressure. As we move
upward from sea level leaving behind the heavy gases at lower layers of
atmosphere, air pressure decreases because the upper air is light and its
density is low.
There is no fixed rate of fallin air pressure with
increase in height but it decreases with increase in height. Air pressure is
reduced to half at the height of S Kms from seal level
and at the height of 11 kms it is reduces to one
fourth. It is because of low pressure in mountai neous regions that breathing
gets hard.
Moisture in air (Humidity) : Conversion of water
from liquid state to gaseous state because of evaporation is knwon as
atmospheric humidity. Water vapours are light in weight therefore they rise up
and pressure of humid air decreases as compared to dry air. Amount of water
vapours changes with time and place and because of this the pressure of air
also varies.
Gravitation of Earth : Atmosphere glues around the
Earth due toits gravitation. The intensity of gravitational pull decreases as
we get away from core of Earth. Another fact is that as Earth rotates round its
axis, average distance of polar regions and equatorial
regions varies from the core of Earth. For example
Polar regious are nearer to core of the Earth as compared to Equatorial regions
and hence have higher air pressure.
Rotation of Earth : Rotation of Earth produces
centrifugal force which has more effect in Equatorial region while lesser
effect on Polar regions. Centrifugal force Pushes things away from its core.
Same is the effect on air pressure which results into lesser
pressure in Equatorial regions as compared to that
in polar regions.
Distribution of Air Pressure : As air is present all
around the Earth, it may be
distributed in to ways namely; horizontally and
vertically.
Horizontal Distribution : Air pressure of a
particular place changes with day and night, with summer and winter but average
air pressure conditions remain same generally. If we study the division of air
pressure on Earth then we will come to know there are various high and low
pressure regions. On maps these divisions are shown with help of isobars.
Generally air pressure is divided into two types :-
1. High pressure.
2. Low pressure
On the basis of combined affect of various factors
affecting the air pressure on different latitudes seven air pressure belts are
found on the Earth.
Equatorial low pressure belt region extending betwen
S°N latitude of Equator to S°S latitude is known as Equatorial low pressure
belt. Following are the reasons which are responsible for its origin :
(1) Rays of sun fall vertically in this region whole
the year long and because of this temperature is high which creates low
pressure.
(u) Owing to high temperature, evaporation process
is also very fast while large number of water vapours decrease the weight and
density of air resulting in reduction of air pressure.
(ii) Rotation of Earth has its maximum effect on
Equator and sois effect of contnfugal force which results into reduction in air
pressure.
At about 30°N and 30°S latitudes high pressure
regions are found in both hemispheres which are formed with decending winds on
this latitudes risen up at hot Equatorial regions. Further at 60°N and 60°S
latitudes low pressure regions are found. At poles high pressure regions are
formed because temperature remains low for whole of the year.
It 1s important to mention that these belts are not
stable. They shift according to the situation of sun as on earth. e.g. During
winters (December) in northern hemisphere,sun rays fall vertically on the
tropic of capricorn. During these pressure belts shift towards south. On the
other hand situation is opposite in summer.
Vertical Division : As we already know, air pressure
on Earth exists due to pressure of upper layers. Atmosphere extends upto the
height of hundred kilometers from the Earth surface. Air pressure is highest at
sea level because the density of gases is highest
in lower layers. Density changes due to the
expansion and the air shrinks because of low temperature. Air pressure and
height are reversly proportional to each other,which means airpressure
decreases due to increase in height. But the rate of fallin air pressure decreases
with increase in height.
Fast blowing winds originate due to the difference
between air pressure at earth surface but on the other hand air pressure
decreases as the height increases inspite of this we donot feel fast blowing
upward winds. The reason behind this is that the gravitational force of Earth
also decreases with increase in height.
sobars : Isobars are those lines drawn on a map
which enjoins places wit
same air pressure. This air pressure is measured,
taking average air pressurd
at sea level as base so that difference created by
increase in height ma
vanish. For this purpose air pressure for isobars is
deducted 34 milbars fo
each 300 meters height.
EXERCISE
1. Answer the following questions in
one or two words :
(a) What name is given to the air cover which
engulfs the Earth ?
(b) Thermometer was invented by whom and when ?
(c) What is full name of UV rays originating from
sun ?
(d) Which gas contains principal portion among
atmospheric gases ?
(e) How much %age of atmospheric gases do Oxygen and
Carbondioxide combined contain ?
(f) Upto which height, vapours are found in
atmosphere ?
(g) What are the upper and lower height limits of
Stratosphere ?
(h) What name is given to heat transfer process in
liquid and gaseous matters ?
i) What degree centrigrade is temperature of Sun
Surface ?
J) Which gas is most important to keep up Green
house effect ?
2. Answer the following questions in
one or two sentences :
(a) Write two characteristics of atmospheric gaseous
mixture.
(b) Name two satellites responsible for deceminating
atmospheric data.
(c) As what source does oxygen works for human body
?
(d) In what forms do Water Vapours fall on Earth ?
(e) What are any two activities related to weather
taking place in atmosphere.
(f) Between which two lyers of atmosphere,
Tropopause lie ?
(g) What ailment to human body may excess x-rays
cause ?
3. Answer the following in 60 to 80
words :
(a) If a baloon goes upto height of 700 kms, in what
sequence shall it cross various layers of atmosphere ?
(b) How Troposphere got this name, write a short
note.
(c) Write a note on need of Ozone layer citing its
depletion also.
(d) Classify atmosphere on basis of its chemical
constitution.
(e) What is insolation ? Write a short note.
(f) Write three causes responsible for creation of
Equatorial low pressure belt.
(g) Write a note on Isobars.
4. Answer the following in 150 to 250
words :
(a) Write in detail, factors effecting air pressure.
(b) Write characteristics of temperature
distribution in months of January and July.
(c) Write notes on four types of temperature
mobility on the Earth.
(d) What are permanent factors effecting upon
distribution of temperature on Earth ?
(e) Describe formation of atmosphere on basis of
temperature and write notes on each layer.
(f) Various isoplteths are used in representing
chmatological features. name any 10 isopleths descnibing their representation
characteristic also.
