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World Geography (Part - 3)
- Our planet is enveloped by a deep blanket of gases extending several
kilometres above its surface.
- Like the lithosphere and hydrosphere, the atmosphere too is an integral
part of a gigantic system i.e. earth.
- When compared with the radius of our planet, the atmosphere appears to
be only a very thin layer of gases.
- However, because of the force of gravity, it is inseparable from the
- Atmosphere contains life-giving gases, like oxygen for man and animal,
and carbon dioxide for plants.
- It also acts like a greenhouse and thus, keeps the Earth warmer than it
would otherwise be.
- The atmosphere, thus, acts like a blanket.
- It regulates the heat balance of the Earth and also protects us from the
harmful ultraviolet radiation of the sun.
- The atmosphere serves as a storehouse for water vapour, which leads to
precipitation and hence facilitates the hydrological cycle.
Structure of the Atmosphere Structure of the Atmosphere
- The atmosphere has a layered structure, because of density
stratification as a result of which lighter gases move up and denser ones
- The lowermost part of the atmosphere in which we live, and which is the
theatre for almost all the weather phenomena is known as the troposphere.
- Troposphere literally means the region of ‘mixing’ and has been derived
from the Greek word ‘tropos’, meaning mixing or turbulence.
- The height of the troposphere at the poles is about 8 km, while at the
equator it is about 16 km. This is because there is greater heating at the
Composition of Atmosphere
Above troposphere is the stratosphere, which is important
primarily because of the presence of ozone.
This layer of calm and clear air is preferred for high-
speed jet flights because of the absence of air pockets. In traces- Water
vapour, Dust particles, Helium, Ozone, Krypton, Xenon, Methane
Also, the near absence of water vapour in this layer
prevents the formation of clouds, thus providing pilots with better
Above stratosphere is mesosphere, which is more of a
Above mesosphere lies the ionosphere, which has
electrically conducting layers that help in radio communication.
There are two important layers in the ionosphere viz. E
Layer or Kennelly Heavyside layer that reflects the medium ‘ radio waves,
thus helping in short distance radio communication and F Layer or Appleton
layer that reflects the short radio waves and helps in long distance radio
communication. The outermost layer of the Earth’s atmosphere is known as the
exosphere. It is so highly rarified a region that its boundary is not clear.
Heating and Cooling of the Atmosphere
- The word ‘insolation’ is an acronym for INcoming SOLAr radiaTION, which
is received at the Earth’s surface at the rate of 1.94 calories per square
centimetre per minute.
- Interestingly, the solar radiation falling on the Earth’s surface in one
day would equal all the world’s industrial and domestic energy requirements
for the next 100 years based on current rates of consumption.
- Energy transfers in the atmosphere take place in three ways: radiation,
conduction and convection.
- The vast amount of energy coming to and leaving the Earth is through
- This, from the sun, is in the form of short waves.
- Radiation from the Earth is called terrestrial radiation and it is in
the form of long waves. The atmosphere is heated more by terrestrial
radiation than the incoming solar radiation and this also explains why the
atmosphere is heated from the ground up instead of vice versa, specially in
- The average temperature of Earth remains rather constant. It has been
possible because of the balance between the amount of incoming solar
radiation and the amount of terrestrial radiation returned to space.
- This balance of incoming and outgoing radiation has been termed Earth’s
Latitudinal Heat Balance
- At latitudes below 40 degrees, more solar radiation is received than is
lost to space by Earth.
- The opposite is true for higher latitudes where more heat is lost than
- Although Earth as a whole maintains a balance between the incoming and
outgoing radiation, its ratio is not uniform all over the Earth.
- The atmosphere and the oceans act as a giant thermal engine that
transfers heat from the tropics towards the poles.
- In this heat transfer, the air masses, winds and ocean currents play
- Most of the heat transfer takes place across the mid-latitudes hence,
much of the stormy weather is associated with this region.
- Thus, the transfer of surplus energy from the lower latitudes to the
deficit energy zone of the higher latitudes maintains an overall balance
over the Earth’s surface.
Inversion of Temperature
- Occasionally the temperature in the lower layers of the air increases
instead of decreasing with elevation.
- It occurs particularly on cold winter nights, when the sky is clear, the
air is very dry, and there is no wind.
- These conditions permit quick radiation of heat from the earth’s surface
as well as from the lower layers of the atmosphere.
- The upper air which loses its heat less rapidly remains comparatively
- Thus, there is reversal in the vertical distribution of temperature,
which is known as the inversion of temperature.
- This phenomenon is specially observed in the intermontane valleys.
- Temperature varies even along the same parallel of latitude, land and
water contrasts, prevailing winds and ocean currents.
- The difference between the mean temperature of any place and the mean
temperature of its parallel is called the temperature anomaly or thermal
- The anomaly is said to be negative when the temperature at a place is
less than the expected temperature of the latitude.
- The anomaly is positive when the temperature at a place is more than the
expected temperature of the latitude.
- Air is an extremely compressible gas, and as a result, atmospheric
pressure varies considerably with height.
- Since the pressure will vary according to the weight of the air above
it, the lowest layers of air are densest.
- The modern metric unit of pressure measurement is the millibar (mb), one
millibar being equal to the pressure necessary to support 0.75 mm of the
- The mean sea level pressure of the atmosphere is 1013.2 mb.
- Most of the atmospheric mass is concentrated in the lower layers:
- Below 5.6 km: 50% of atmospheric mass
- Below 16 km: 90% of atmospheric mass
- Below 32 km: 99% of atmospheric mass
The origin of Pressure
- Air has weight and it therefore exerts pressure, called atmospheric
pressure, on the earth’s surface.
- The pressure is not the same for all regions, nor is it always the same
for any one region all the time.
- Atmospheric pressure is affected by altitude, by temperature, and by
Influence of Altitude on Pressure
- Air pressure at sea level is higher than it is at the top of a mountain.
- Air pressure increases when air descends. When it descends its volume
decreases but the number of molecules in it remains the same.
- Similarly, when air rises, its volume increases and the outward pressure
of its molecules is spread over a larger area, and its pressure decreases.
Influence of Temperature on Pressure
- The temperature of air rises when its pressure rises.
- The temperature of air falls when its pressure falls
- The pressure of the air falls when its temperature rises.
- The pressure of the air rises when its temperature falls.
- Temperature: Low temperatures at the poles cause the air to contract -
high pressure develops.
- High temperatures along the equator cause the air to expand - low
pressure, called the doldrums low pressure, develops.
Vertical Distribution of Pressure
- Air being a mixture of gases is highly compressible.
- Its density is greatest at the lower layers where it is compressed under
the mass of air above.
- As a result, the lower layers of the atmosphere have high pressure.
- In contrast, the higher layers are less compressed and hence, have low
- Air pressure always decreases with increase in altitude.
Horizontal Distribution of Pressure
- It is the distribution of atmospheric pressure across the latitudes. Its
main feature is its zonal character known as pressure belts which number
- The leading function of the general circulation of atmosphere is to
redistribute heat and moisture across Earth’s surface.
- Atmospheric circulation accounts for about 87% of this heat
redistribution while oceanic circulation accounts for the remainder.
World Pressure Pattern in July
It is summer in the northern hemisphere with the sun
overhead along the Tropical of Cancer on 21st June. The doldrums extend well
into the northern hemisphere and link up with the low pressure areas over
the Asian land mass and south-west USA.
The sub-tropical high pressure of the northern hemisphere
exists as separate areas over the oceans. In the southern hemisphere (where
it is winter) it is almost continuous around the earth.
The temperate low pressure areas over the North Atlantic
and North Pacific Oceans are poorly developed.
World pressure pattern in January
It is summer in the southern hemisphere with the sun
overhead along the Tropical of Capricorn on 22nd December. The doldrums
extend into the southern hemisphere and are particularly well developed over
Australia and Central Africa.
The sub-tropical high pressure of the southern hemisphere
is no longer continuous. It forms separate areas over the oceans. In the
northern hemisphere it is continuous, having linked up with the high
pressure areas over the Asian and North American land masses.
The temperate low pressure areas over the North Atlantic
and Pacific Oceans are well developed.
Measurement of air Pressure
- Pressure varies with temperature and altitude and the instrument which
measures pressure is called a barometer. There are three types of barometer:
mercury barometer, aneroid barometer and barograph.
- Air pressure is usually measured in a unit of measurement called a
- The resulting temperature differences produce the global wind system.
- Horizontal movement of the air is called wind.
- It flows from areas of high pressure to those of low pressure.
- Since unequal heating of Earth’s surface is one of the main reasons for
the pressure differences, solar radiation may be considered the ultimate
driving force of winds.
- Winds are always named from the direction they blow.
- An east wind is one that blows from east to west and a south-west wind
is one that blows from the south-west.
- Prevailing or Planetary winds blow throughout the year from one latitude
to the other in response to the latitudinal differences in air pressure, for
example, the trade winds and the westerly winds.
- Westerlies of the southern hemisphere are stronger and more constant in
direction than those of the northern hemisphere because of the vast expanse
- They are best developed between 40° and 65° south latitudes. These
latitudes are hence often called Roaring Forties, Furious Fifties and
Shrieking Sixties - dreaded terms for navigators.
- Periodic winds reverse their direction periodically with season. For
example, monsoons, land and sea breeze and mountain and valley breeze.
- Local winds flow in comparatively small area and have special
characteristics. A lot of them are found in the Mediterranean lands and
their nomenclature derived from the regional language.
Besides major wind systems of the earth’ surface, there are certain types of
winds, eves; though on a much smaller scale, which are produced by the
topographical peculiarities & on local temperature differences.
Since these wind systems are generated by purely local
factors and their zone of influence is quite limited, they are termed as local
winds. They play an important role in the weather and climate of a particular
Monsoon Winds of SE Asia The powerful control exerted by the great landmass of
Asia on air temperatures and pressures extends to the surface wind systems as
well. Development of a summer low and a winter high over Asia in middle
latitudes creates a seasonally alternating system of pressure gradients and with
it a seasonally reversing set of surface winds which we call a monsoon system.
In summer, the low-pressure centre tends to develop in
the southern part of the continent. This is a low pressure associated with
heating of the lower levels of the atmosphere; it does not extend high into
the troposphere. Nevertheless pressure gradients at low levels are radially
inward from sea to land, so that a cyclonic circulation forms.
- This is the summer monsoon, which in southern and eastern Asia is
associated with the rainy season of the year.
- In winter, intense cooling produces the centre of higher pressure in the
northerly part of the landmass.
- An anti-cyclonic circulation is set up with surface winds blowing from
continental interior towards the coast. In southern and eastern Asia, this
winter monsoon is associated with a period of dry, cool weather.
Wind as an Energy Resource
- Wind power is an indirect form of solar energy that has been used for
- The windmill of the Low Countries (Belgium, Netherlands and Luxembourg)
of Europe played a major role in pumping water from the polders as they were
reclaimed from tidal land.
- Windmill was also used to grind grain in low, flat areas where streams
could not be adapted to water power.
- The total supply of wind energy is enormous.
- The World Meteorological Organization has estimated that the combined
electricitygenerating wind power of favourable sites throughout the world
comes to about 20 million megawatts, a figure about 100 times greater than
the total electrical generating capacity of the United States.
Latitudinal Shifting of Wind Belts
- When the sun shines vertically above the Tropic of Cancer, it is then
summer solstice, and when it is vertical above the Tropic of Capricorn, it
is winter solstice.
- With the shifting of the position of sun, the insolation belts make a
- This results in north-south displacement of the temperature belts.
- Since pressure and wind belts are largely controlled by position of the
sun, they are also displaced north and south with the apparent movement of
- Generally, winds are thought to be the prime factors in weather
- An air mass is a large body of air where physical properties like
temperature and moisture content are relatively uniform horizontally.
- An air mass extends over hundreds of kilometres and consists of several
layers, each having homogenous conditions. Thus, it has a stratified
- Most of the major weather changes in the middle and higher latitudes are
a result of advance and interaction between the air masses.
- The entire primary and secondary circulations in the atmosphere are
controlled by the air masses.
- However, the concept of air masses is more difficult to apply in low
latitude regions, where air masses contrasts are less marked.
Upper Air Circulation
- In the mid-latitudes, high-speed winds known as jet streams blow from
west to east in the upper troposphere near the tropopause, the interface
between troposphere and stratosphere.
- Jet streams are narrow meandering hands of swift winds that are embedded
in the prevailing westerlies and encircle the globe.
- Jet streams play an important role in the possible formation, steering
or intensifying weather phenomena such as monsoons, cyclones, anticyclones
and other weather conditions.
- They are also used by aviators if they have to fly in the direction of
the flow of the jet streams.
- Cyclones, hurricanes and typhoons, differing only in name, transfer
energies equal to several megaton nuclear bombs, from the ocean to the
atmosphere every year.
- Nearly 70 to 90 cyclonic systems develop all over the globe every year.
- The Coriolis force, the deflecting force caused by the rotation of the
Earth about its own axis, is maximum at the poles and progressively
decreases to zero at the equator.
- It is this force that compels the surface winds to spiral towards the
low pressure system.
- As Coriolis force is negligible in the equatorial belt between latitudes
5° N and 5° S, cyclonic systems do not develop in this region.
- Cyclonic systems do not develop or intensify all of a sudden. They build
up over a period of time.
- The topography and the intensity as well as frequency of cyclones that
could strike a coast decide the vulnerability of the place.
- Tropical cyclones are the most destructive and violent type of storms.
- Damages to life and property caused by them fall into three categories:
(a) damages caused by high velocity winds, (b) damages caused by flooding,
and (c) damages caused by a storm surge.
- Torrential rains that occur in a hurricane inundate the low-lying areas,
cause floods and landslides resulting in a great loss of life and property
- According to many weather scientists, by far the greatest damage and
loss of life and property from intense tropical cyclones are due to storm
- The rise in sea level because of the passage of a hurricane is called
the storm surge.
Moisture in the Atmosphere
- Although water-vapour content forms only a small proportion by volume of
the atmosphere, it is the most important constituent of air in deciding
weather and climate.
- Water may be present in the atmosphere in all the three forms.
- The amount of water vapour in the air is called humidity.
- The humidity of the air depends upon the temperature, e.g. if the
temperature rises then air can hold more water vapour.
- When air can hold no more water it is said to be saturated.
- The actual amount of water vapour in a given volume of air at a
particular temperature is called the absolute humidity.
- The ratio between the absolute humidity of a given mass of air and the
maximum amount of water vapour that it could hold at the same temperature is
called the relative humidity.
- Changes in humidity account for production of precipitation.
To measure humidity
- Two ordinary thermometers are used to measure the humidity, and they are
the Stevenson Screen wet bulb thermometer and hygrometer.
Different Measures of Humidity
Absolute Humidity is the weight of actual amount of water
vapour present in a unit volume of air, expressed as grams per cubic metre
Specific Humidity is the weight of water vapour per unit
weight of air, expressed as grams per kilogram of air.
Relative Humidity is the ratio of the air’s actual water
vapour content to its water vapour capacity at a given temperature,
expressed as percentage. Air containing moisture to its full capacity at a
given temperature is said to be saturated (when relative humidity = 100%).
The temperature at which saturation occurs is known as dew point.
- A cloud is a mass of small water droplets or tiny ice crystals.
- Clouds form when moist air rises and cools.
- Heat from the sun turns water in the oceans, rivers and moist soil, into
- The water vapour expands as it rises and becomes cooler.
- Cool air cannot hold as much moisture as warm air and soon reaches a
- The water vapour condenses into tiny water droplets forming clouds.
- Their form, shape, height and movements tell us a great deal about the
sky conditions and the likely weather.
- For meteorological purpose, the amount of cloud cover in the sky is
expressed in eighths or oktas.
These clouds are 6000 to 12000 m above sea level.
- Cirrus: These are composed of small ice crystals; white, wispy, fibrous
or feather-like in appearance; in bands or patches.
- Cirrocumulus: These are also composed of ice crystals but they are
globular or rippled (like ripples in the sand on a sea shore) in appearance;
forming a thin cloud.
- Cirrostratus: Looks like a thin white almost transparent sheet which
causes the sun and moon to have ‘halos’.
- Altocumulus: These are composed of water droplets in layers or patches,
globular or bumpy-looking with flattened bases arranged in lines or waves.
- Altostratus: Composed of water droplets; forming sheets of grey or
watery-looking clouds, partly or totally covering the sky.
- Stratocumulus: Large globular masses: bumpy looking; soft and grey in
appearance forming a pronounced regular and sometimes wavy pattern.
- Nimbostratus: Dark grey and rainy-looking; dense and shapeless; often
give continuous rain.
- Stratus: These clouds are low, grey and layered, almost fog-like in
appearance; bring dull weather and often accompanied by drizzle.
Clouds of Great Vertical Extent
- Cumulus: Round-topped and flat-based forming a whitish-grey globular
mass; consists of individual cloud units.
- Cumulonimbus: This is a special type of cumulous cloud of great vertical
extent; white or black globular masses whose rounded tops often spread out
in the form of an anvil; summits rise like towers and mountains; often
indicate convectional rain, lightning and thunder.