World Geography Physical
Atmosphere (Part -1)
- Our planet is enveloped by a keep blanket of gases extending several
kilometers 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 cycles.
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 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
- Above troposphere is the stratosphere, which is important primarily
because of the presence of zone.
- This layer of calm and clear air is preferred for high-speed jet flights
because of the absence of air pockets.
- Also, the near absence of water vapour in this layer prevents the
formation of clouds, thus providing pilots with better visibility.
- Above stratosphere is mesosphere, which is more of a transitional layer.
Composition of Atmosphere
Carbon Dioxide 0.03%
In traces-Water vapour, Dust particles, Hellum, Ozone, Krypton,
- 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 of
Kennelly Heavy side layer that reflects the mediumradio 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
- 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 Insolation
- 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 fromthe ground up instead of vice
versa, specially in the troposphere.
- 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 latitude 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 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 night, 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 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 layers:
I. Below5.6 km: 50% of atmospheric mass
II. Below16 km: 90% of atmospheric mass
III. Below32 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 lowpressure, develops.
Vertical distribution of pressure
- Air being a mixture of gases is highly compressible.
- Its density is greatest at the lower layers where 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 atmospheric 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
World pressure pattern in January
Measurement of air pressure
- 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 developed between 40º and 65º 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’s surface, there are
certain types of winds, even though on a much smaller scale, which are produced
by the topographical peculiarities or 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 locality.
Monsoon Winds of SE Asia
- The powerful control exerted by the great landmass of Asia on air
temperatures and pressures extends to the surface and systems as well.
Development of a summer low and a winter high over Asia
in middle latitude 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 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.
||Coastal Southern California
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- 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
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 kilometers 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 bands 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 Carioles 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 Carioles force is negligible in the equatorial belt between latitudes
5º N and 5º S, Cyclonic systems do not develop in this region.
- Cyclonic systems do not develop or intensify all of 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 damage.
- 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 of air.
- 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 water vapour.
- The water vapour expands as it rises and becomes cooler.
- Cool are cannot hold asmuchmoisture 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.
- Formeteorological purpose, the amount of cloud cover in the sky is
expressed in eights or oktas.
2/8 Quarter covered
4/8 Half covered
8/8 Completely overcast
> 2 km
< 1 km
Near zero visibility
- Clouds are classified according to their appearance, form and height.
There are four groups.
A High clouds
6000 to 12000 m
B Middle clouds
2100 to 6000 m
C Low clouds below
D Clouds of great
1500 to 9000 m vertical extent.
- The different types of cloud are given Latin names which are all
combinations of the following words.
1. Cirrus means looking like a feather, it is used to describe the very high
2. Cumulus means looking like a heap. It is used to describe clouds which have
flat bases and rounded tops. There are patches of blue sky between the clouds.
3. Stratus means lying in level sheets. It is used for layer-type clouds.
4. Nimbus means rain cloud.
5. Alto means high.
- 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 bumpylooking 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
- On the basis of its origin, precipitation may be classified into three
main types- convectional, orographic, and cyclonic or frontal.
A. Convectional Precipitation
- It is caused by convectional ascent of warmand humid air to great
- It is heavy but highly localized, occurs mostly during the days and is
associated with minimum amount of cloudiness.