climate.pdf

Prelims 2020
Ghanshyam Gautam
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Last updated on 18/04/2020

 Mixture of different gases that envelopes earth all around.
 Regulates the entry of solar radiation.
 Proportion of gases changes in the higher layers of the atmosphere.
 Stabilization = Cambrian period
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 A variable gas = decreases with altitude.
 Decreases from equator towards poles.
 It absorbs not only the long-wave terrestrial radiation (infrared or heat
emitted by earth during nights), but also a part of the incoming solar radiation.
 Source of precipitation and clouds.
 On condensation, it releases latent heat of condensation —the ultimate
driving force behind all storms.
 The moisture – carrying capacity of air is directly proportional to the air
temperature.
 Thus, it affects stability and instability in the air.
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 Solid Particles like sand particles (from weathered rocks and also derived from
volcanic ash), pollen grains, small organisms, soot, ocean salts; upper layers of
the atmosphere may even have fragments of meteors which got burnt up in the
atmosphere.
 Hygroscopic (i.e. readily absorbing moisture from air) in character, and as such,
act as nuclei of condensation => an important contributory factor in the
formation of clouds, fog and hailstones.
 Generally concentrated in lower layers of atmosphere but convectional air
currents may transport to great heights.
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Q. Higher concentration of dust particles is found in subtropical and temperate
regions due to dry winds in comparison to equatorial and polar region.
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These dust particles also responsible for:
• Orange and red colours at sunset and sunrise
• For the length of dawn (the first appearance of light in the sky before sunrise)
• Twilight (the soft glowing light from the sky when the sun is below the horizon, caused by the
reflection of the sun’s rays by the atmosphere).
• Dusk: the darker stage of twilight.
• Blue colour of the sky is also due to selective scattering by dust particles.
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 Different layers with different temperature and density.
 Density = higher near surface of earth; ↓ with ↑ altitude.
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Heterosphere
Homosphere
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 Thickness is greater at the equator, because the heated air rises to greater
heights >> strong convection currents.
 fall in temperature with altitude = ‘lapse rate’
 temperature inversion, turbulence and eddies
 Dust/solid particles, water vapor here => all weather/climate phenomenon
occurs here but influenced by seasons and jet streams.
 called the convective region, since all convection stops at Tropopause.
Tropopause
 Top most layer of troposphere => boundary between troposphere and
stratosphere.
 This layer is marked by constant temperatures.
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 T = remains constant for some distance but then rises.
 This rise is due to the presence of ozone.
 almost free from clouds and associated weather phenomenon
=> most ideal for flying aeroplanes. {So aeroplanes fly in lower stratosphere,
sometimes in upper troposphere where weather is calm}
 Sometimes, cirrus clouds are present at lower levels in this layer.
Ozonosphere:
 (30-60km) spans the stratosphere and lower mesosphere.
 reflects the harmful ultraviolet radiation.
 also called chemosphere because, a lot of chemical activity goes on here.
 T ↑.
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 The temperature gradually falls to -100°C at 80 km altitude.
 Meteorites burn up in this layer on entering from the space.
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 T ↑ very rapidly with ↑ height.
 Person would not feel warm because of the thermosphere’s extremely low
pressure.
 The International Space Station and satellites orbit in this layer. (Though
temperature is high, the atmosphere is extremely rarified – gas molecules are
spaced hundreds of kilometers apart. Hence a person or an object in this layer
doesn’t feel the heat)
 Aurora’s are observed in lower parts of this layer.
A probable mains question 3.3 (Majid Hussain)
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 Part of Thermosphere.
 80-400 km and is an electrically charged layer. {coz atoms here absorb cosmic
rays, gamma rays, x-rays & shorter UV radiations}
 Thus ionization of atoms.
 Because of the electric charge, radio waves transmitted from the earth are
reflected back to the earth by this layer.
 Temperature again starts increasing with height because of radiation from the
sun => here onwards incoming space debris/vehicles, meteorites begin to heat
due to friction.
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 Uppermost layer (> 400 km)
• Air is extremely rarefied and T ↑
• Light gases like helium and hydrogen float into the space from here.
• Layer coincides with space.
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Is outer space hot or cold? WHY?

 Amount of heat received by different parts of the earth is not the same.
⟹ pressure differences in atmosphere occurs ⟹ transfer of heat from one
region to the other by winds
 Insolation: Energy received by the earth or total incoming solar radiation,
which is mostly in short wavelength range.
 What factors can affect the total solar energy received?
the rotation of earth on its axis
the angle of inclination of the sun’s rays;
the length of the day;
the transparency of the atmosphere;
the configuration of land in terms of its aspect.
The last two however, have less influence.
 The fact that the earth’s axis makes an angle of 66½ with the plane of its orbit
round the sun has a greater influence on the amount of insolation received at
different latitudes. {Earth’s inclination} 12
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Revolution of earth

 Second factor is inclination of the rays.This depends on the
latitude of a place.The higher the latitude the less is the angle
they make with the surface of the earth resulting in slant sun
rays.
 The area covered by vertical rays is always less than the slant
rays. If more area is covered, the energy gets distributed and the
net energy received per unit area decreases. Moreover, the slant
rays are required to pass through greater depth of the
atmosphere resulting in more absorption, scattering and
diffusion.
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 Red colour of rising and the setting sun and the blue colour of sky is result of
scattering of light within the atmosphere.
 Maximum insolation is received over the subtropical deserts, where the
cloudiness is the least.
 Equator receives comparatively less insolation than the tropics.
 Generally, at the same latitude the insolation is more over the continent than over
the oceans.
 In winter, the middle and higher latitudes receive less radiation than in summer.
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 Earth heated by Insolation ⟹ transmits to atmosphere in long wavelength.
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 The air in contact with the land gets heated slowly and the upper layers in
contact with the lower layers also get heated.This process is called conduction.
Conduction is important in heating the lower layers of the atmosphere.
 The air in contact with the earth rises vertically on heating in the form of
current and further transmits the heat of the atmosphere.This process of
vertical heating of the atmosphere is known as convection.The convective
transfer of energy is confined only to the troposphere.
 The transfer of heat through horizontal movement of air is called advection.
Horizontal movement of the air is relatively more important than the vertical
movement.
 In middle latitudes, most of dirunal (day and night) variation in daily weather
are caused by advection alone.
 In tropical regions particularly in northern India during summer season local
winds called ‘loo’ is the outcome of advection process.
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 The earth after being heated itself becomes a radiating body and it radiates
energy to the atmosphere in long wave form.This energy heats up the
atmosphere from below. This process is known as terrestrial radiation.
 The long wave radiation is absorbed by the atmospheric gases particularly
by carbon dioxide and the other green house gases.Thus, the atmosphere
is indirectly heated by the earth’s radiation.
 The atmosphere in turn radiates and transmits heat to the space. Finally the
amount of heat received from the sun is returned to space, thereby
maintaining constant temperature at the earth’s surface and in the
atmosphere.
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Variation in Net heat Budget as per Latitude
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 The latitude: T ∝ Insolation ∝ Latitude dependent
 The altitude: T decreases as H increases
 Distance from the sea : Compared to land, the sea gets heated
slowly and loses heat slowly. Land heats up and cools down quickly.
 Air-mass: warm air-masses ⟹ higher temperature and shit.
 Ocean currents: places located on the coast where the warm
 Ocean currents flow record higher temperature and shit.
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 Northern hemisphere the land surface area is much larger than in the
southern hemisphere. Hence, the effects of land mass and the ocean currents
are well pronounced.
 The presence of warm ocean currents, Gulf Stream and North Atlantic drift,
make the Northern Atlantic Ocean warmer and the isotherms bend towards
the north.
 Over the land the temperature decreases sharply and the isotherms bend
towards south in Europe.
 The effect of the ocean is well pronounced in the southern hemisphere. Here
the isotherms are more or less parallel to the latitudes and the variation in
temperature is more gradual than in the northern hemisphere.
 The highest range of temperature is more than 60° C over the north-eastern
part of Eurasian continent.This is due to continentality.
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 If T ↑ with ↑ in height = negative lapse rate.
 It only occurs at lower heights of troposphere. (Usually nearer to Earth surface
but may occur at higher reaches of troposphere).
Surface Inversion:
 Generally, in tropical & subtropical during long winter nights only.
 Also disappears with sunrise.
 Requirements:
-> long winter nights -> calm atm. = slow air movement
-> cloudless clear sky - > snow covered area helps
-> dry air = low humidity
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• For more on Temp. Inversion read Majid; Pg 3.8-3.11.
• This is also a probable mains ques.
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 Long Nights, so that, outgoing
radiation > Incoming.
 Clear skies, so that, unobstructed
escape of radiation.
 Calm and stable air, so that, no
vertical mixing at lower levels.

 The sun is directly overhead at noon on 21st June at: 23.5° N
 The atmosphere is mainly heated by the: Long wave terrestrial radiation
 Albedo: The percentage of visible light reflected by an object
 The main reason that the earth experiences highest temperatures in the subtropics
in the northern hemisphere rather than at the equator is :
Subtropical areas tend to have less cloud cover than equatorial areas.
 Why is the annual range of temperature high in the Siberian plains?
Contentiality and equalizing effect of sea is least.
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 Due to gravity the air at the surface is denser and hence has higher
pressure.
 Air pressure is measured with the help of a mercury barometer or the aneroid
barometer.
 At any elevation it varies from place to place and its variation is the primary
cause of air motion, i.e. wind which moves from high pressure areas to low
pressure areas.
 Why we do not experience strong upward winds?
Vertical pressure gradient is generally balanced by a nearly equal but opposite
gravitational force.
 Low pressure system is enclosed by one or more isobars with the lowest
pressure in the centre. High-pressure system is also enclosed by one or more
isobars with the highest pressure in the centre.
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 The horizontal winds near the earth surface respond to the combined effect
of three forces – the pressure gradient force, the frictional force and the
Coriolis force. In addition, the gravitational force acts downward.
 Pressure Gradient Force: strong where the isobars are close to each
other and is weak where the isobars are apart.
 Frictional Force: affects the speed of the wind. It is greatest at the surface
and its influence generally extends up to an elevation of 1 - 3 km. Over the
sea surface the friction is minimal.
 Coriolis Force: The rotation of the earth about
its axis affects the direction of the wind.
 The Coriolis force acts perpendicular to the
pressure gradient force.The pressure gradient
force is perpendicular to an isobar.
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 Higher Pressure gradient force = more is the velocity of the wind = larger is the
deflection in the direction of wind.
 As a result of these two forces
operating perpendicular to each
other, in the low-pressure areas
the wind blows around it.
 At the equator, the Coriolis
force is zero and the wind
blows perpendicular to the
isobars.The low pressure
gets filled instead of getting
intensified.That is the reason
why tropical cyclones are
not formed near the equator.
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When isobars are straight and when
there is no friction, the pressure
gradient force is balanced by the
Coriolis force and the resultant wind
blows parallel to the isobar.This wind is
known as the geostrophic wind.
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 Which of the following is essential for formation of clouds and
precipitation?
1. Convergence of Wind
2. Divergence of Wind
3. Orographic uplift
4. Some eddies
5. Conduction
6. Convection currents
7. Uplift along fronts
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 Pattern of planetary winds largely depends on:
 Latitudinal variation of atmospheric heating;
 Emergence of pressure belts;
 The migration of belts following apparent path of the sun;
 The distribution of continents and oceans;
 The rotation of earth.
 General circulation of the atmosphere also sets in motion the ocean water
circulation which influences the earth’s climate.
 The air at the Inter Tropical Convergence Zone (ITCZ) rises because of
convection caused by high insolation and a low pressure is created.The
winds from the tropics converge at this low pressure zone.
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 In mountainous regions, during the day the
slopes get heated up and air moves upslope
and to fill the resulting gap the air from the
valley blows up the valley.
 The cool air, of the high plateaus and ice fields
draining into the valley is called katabatic wind.
 The moisture in these winds, while crossing the
mountain ranges condense and precipitate.When
it descends down the leeward side of the slope the
dry air gets warmed up by adiabatic process.This
dry air may melt the snow in a short time.
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 Air remains over a homogenous area {Land or ocean} for a sufficiently longer
time it acquires the characteristics of the area a large body of air having little
horizontal variation in temperature and moisture.The homogenous surfaces,
are called source regions.
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 Front is a boundary surface that separates air masses of different
densities, one of which is usually warmer.
 Its like a transition zone of two masses with contrasting temperature
or humidity/density.
 As one air mass moves into the region occupied by other, minimal
mixing occurs along the frontal surface.
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Frontolysis.
Frontogenesis.

Warm Front Cold Front
Warm glides over cooler Cold air advances in to warm air
Steep slope Twice steeper compared to Warm
Faster 50% faster than Warm
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 Large sea surface with temperature higher than 27° C;
 Presence of the Coriolis force;
 Small variations in the vertical wind speed;
 A pre-existing weak low - pressure area or low-level-cyclonic
circulation;
 Upper divergence above the sea level system.
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 The energy that intensifies the storm, comes from the condensation
process in the towering cumulonimbus clouds, surrounding the centre of
the storm.
 With continuous supply of moisture from the sea, the storm is further
strengthened.
 On reaching the land the moisture supply is cut off and the storm
dissipates.
 The place where a tropical cyclone crosses the coast is called the landfall
of the cyclone.
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 Extra tropical cyclones form along the polar front. Initially, the front is
stationary.
 In NS = warm air blows from south and cold air from north of the front.
 When the pressure drops along the front,the warm air moves
northwards and the cold air move towards, south setting in motion an
anticlockwise cyclonic circulation.
 These cyclonic circulation leads to a well developed extra tropical
cyclone, with a warm front and a cold front.
 The warm air glides over cold air ⟹ sequence of clouds appear over
the sky ahead of the warm front and cause precipitation.
 The cold front approaches the warm air from behind and pushes the
warm air up ⟹ cumulus clouds develop along the cold front.The cold
front moves faster than the warm front ultimately overtaking the warm
front.
 The warm air is completely lifted up and the front is occluded and the
cyclone dissipates.
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Tropical
 Not present
 Originate only over the seas and on
reaching the land they dissipate.
 The wind velocity is much higher & it
is more destructive.
 Moves from east to west.
Extra tropical
 Clear frontal system.
 Originate over the land and sea.
 Not that much great.
 Move from west to east.
 Cover a larger area
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 Refer PMF IAS.
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 Humidity = Water vapour present in the air.
 Absolute humidity = actual amount of the water vapour present in the
atmosphere.
 Weight of water vapour per unit volume of air and is expressed in terms of grams
per cubic metre.
 The ability of the air to hold water vapour depends entirely on its temperature.
 Relative Humidity = The percentage of moisture present in the atmosphere as
compared to its full capacity at a given temperature.
 It is greater over the oceans and least over the continents.
 The air containing moisture to its full capacity at a given temperature is said to be
saturated. It means that the air at the given temperature is incapable of holding
any additional amount of moisture at that stage.
 Dew point = The temperature at which saturation occurs in a given sample of air.
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 The moisture in the atmosphere is derived from water bodies through
evaporation and from plants through transpiration.
 Exchange of water between the atmosphere, the oceans and the
continents through the processes of evaporation, transpiration,
condensation and precipitation.
 The amount of water vapour in the atmosphere is added or withdrawn
due to evaporation and condensation.
 Increase in temperature increases water absorption and retention
capacity of the given parcel of air.
 The greater the movement of air, the greater is the evaporation.
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Condensation = water vapor into water.
 Influenced by PVT.
 Takes place when = temperature of the air is reduced to dew point with
its volume remaining constant; (ii) when both the volume and the
temperature are reduced; (iii) when moisture is added to the air through
evaporation.
 The most favourable condition for condensation is the decrease in air
temperature.
 After condensation the water vapour or the moisture in the atmosphere
takes one of the following forms — dew, frost, fog and clouds.
 difference b/w mist and fog: mist contains more moisture than the fog.
 Mists are frequent over mountains as the rising warm air up the slopes
meets a cold surface.
 Fogs are drier than mist and they are prevalent where warm currents of
air come in contact with cold currents.
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Form Explanation Conditions
Dew Moisture on cooler
surfaces of solids in
form of water droplets
clear sky, calm air, high
relative humidity, and
cold and long nights.
Necessary: dew point
is above freezing
point.
Frost On cold surfaces when
condensation takes
below freezing points
{0C} i.e. dew pt. at or
less than freezing pt.
Same as above but air
temp < freezing point
Fog-Mist Condensation within
the water vapor on fine
dust particles
Temperature fall all of
a sudden
Clouds In free air at
considerable elevation
Low enough temp and
pressure to condense
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High clouds – cirrus, cirrostratus, cirrocumulus;
Middle clouds – altostratus and altocumulus;
Low clouds – stratocumulus and nimbostratus
clouds with extensive vertical development – cumulus
and cumulonimbus.
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Four basic types of Clouds:
Cirrus: (8,000 - 12,000m)
• Formed at high altitudes.
• Thin and detatched clouds having a feathery appearance.
• Always white in colour.
Cumulus: (4,000 - 7,000 m)
• Look like cotton wool.
• Exist in patches and can be seen scattered here and there.
• Flat base.
Stratus:
• Layered clouds covering large portions of the sky.
• These clouds are generally formed either due to loss of heat or the
mixing of air masses with different temperatures.
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Nimbus:
• Black or dark gray.
• Form at middle levels or very near to the surface of the earth.
• Extremely dense and opaque to the rays of the sun.
• Sometimes, the clouds are so low that they seem to touch the ground.
• Nimbus clouds are shapeless masses of thick vapour.
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Such rain is common in the
summer or in the hotter part of
the day. It is very common
in the equatorial regions and
interior parts of the continents,
particularly in the northern
hemisphere.
Guess the rainfall type?
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Between the latitudes 35o and 40o N and S of the equator, the
rain is heavier on the eastern coasts and goes on decreasing
towards the west. But, between 45o and 65o N and S of equator,
the rainfall is first received on the western margins of the
continents and it goes on decreasing towards the east. WHY?
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Further reading from Majid:
• Pg 3.25 = Local winds name and location; their significance.
• Pg 3.34 = Anticyclones.
• Pg 3.35 = Polar vortex and issues (Highly relevant for mains)
• Pg 3.48 = Ozone and issues.
• Also refer NCT XI, Pg 103, Ch.12 as this chapter has not been covered in these notes.
3/4/2018

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