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Atmosphere:
On the basis of chemical composition atmosphere can be divided into 2 parts.
1. Homosphere
Upto 90 km (Uniformity in chemical composition)
(i) Troposphere
(ii) Stratosphere
(iii) Mesosphere
2. Heterosphere

(i) Above 90 Km (non uniformity in chemical composition)
(ii) Ionosphere
(iii) Exosphere

Troposphere

(i) Average height is 16 Km-10 km at Poles & upto 18 km at Equator.
(ii) Greatest at Equator due to upward transportation of heat by strong conventional currents.
(iii) That is why height of troposphere at a given latitude is greater in summer.
(iv) Temperature decreases with height, roughly at 1 degree Celsius for every 165 meters i.e. Normal Lapse Rate.
(v) Lowest, Densest & contains 75 % of earth’s atmosphere with 90% of water vapours & dust particles.
(vi) All major atmospheric processes take place in this layer.

Tropopause

(i) Shallow transitional zone also known as unstable zone between Troposphere & Stratosphere i.e. Approx. 1.5 Km
(ii) Temperature stops falling in this layer
(iii) 80 degree Celsius over equator
(iv) 45 degree Celsius over poles

Stratosphere

(i) This layer rises up-to 50 km
(ii) Thicker at Poles than at Equator
(iii) Temperature remains constant in its lower portion upto 20 Km & then gradually increases to 0 degree Celsius till its upper limit i.e. Tropopause.
(iv) Increases mainly due to the presence of ozone gas, which absorbs sun’s UV rays.
(v) Practically no clouds, convection currents, Thundering or lighting, water vapours or dust particles hence airplanes fly in this region.
(vi) Few clouds called “Mother of Pearls / Narcreas” can be seen over Antarctica.
(vii) Its lower portion (15-35 Km) constitutes Ozone layer which prevents us from harmful UV rays.
(viii) amount of Ozone gas is found at Stratopause i.e. uppermost limit of Stratosphere.

Depletion of Ozone Layer

(i) Major cause CFCs i.e. mainly from Refrigerator, AC, Spray Cans, Plastic Packaging, Cleaning Fluids, Insulation materials
(ii) UV breaks CFCs & release Chlorine atom which reacts with Ozone & convert it into simple oxygen molecule, which is unstable to absorb UV rays.
(iii) Space probes are also responsible for depletion of Ozone layer, as every time a rocket is fired into space, 70 - 150 tons of Chlorine is injected into atmosphere
(iv) Another cause for ozone layer depletion is oxides of Nitrogen, esp. Nitric oxide, released from supersonic aircrafts, automotive exhaust, in form of nitrates in fertilizers etc.
(v) Already a major Ozone hole is formed over Antarctica with risk countries New Zealand, Australia, South Africa, Chile, Argentina etc.

Mesosphere

(i) Upto 80 ~ 90 Km, Temp, decreases gradually with height until - 100 *C at 80 km
(ii) Displays wispy clouds at high altitudes due to reflected sunlight from meteoric dust Particles.
(iii) Most weather balloons are placed in this region
(iv) Most meteors bum up in this layer ; Upper limit Mesopause

Ionosphere/ Thermosphere

(i) Extends upto 400 km, contains electrically charged particles (ions) with max. cone, at 250 Km.
(ii) Starts rising with increase in height because of ionization by solar radiations.
(iii) Zone of Earths Satellites.

Auroras

Are formed when Earth’s magnetic field traps solar winds in atmosphere, resulting in collision between solar wind & atmospheric charged molecules (ions).

1. Aurora Borealis

(i) Northern light in northern hemisphere (Arctic circle)

2. Aurora Australia

(i) Southern light in southern hemisphere (Antarctic Circle)
(ii) All radio waves are reflected in this layer (Radio transmission)
(iii) D layer-Reflects signal of low frequency & absorbs of medium & high frequency
(iv) E layer (Kennedy Heaviside layer)-Reflects medium & high frequency radio waves to earth
(v) F layer (Appleton layer)-Useful for long distance radio transmissions-reflects medium & high frequency radio waves to earth
(vi) G layer-Highest layer

Importance of Temperature

(i) Temperature influences the actual amount of water vapour present in the air & thus decides the moisture carrying capacity of the air.
(ii) It decides the rate of evaporation & condensation, & therefore governs the degree of stability of the atmosphere.
(iii) As relative humidity is directly related to the temperature of the air, it affects the nature & types of cloud formation & precipitation.

Factors influencing Temperature
1. Latitude

(i) Due to the earth’s inclination, temperature reduces from equator to poles.
(ii) Mainly due to direct & oblique sunrays falling differently on different latitudes

2. Altitude

(i) Since the atmosphere is mainly heated by conduction from the earth.
(ii) Hence places near to earth surface are wanner than those higher up.
(iii) Thus, temperature decreases with increasing height above the sea level

3. Continentality

(i) Land surfaces are heated more quickly than the water surfaces, due to higher specific heat of the water
(ii) Hence warmer summers & colder winters prevails in continental interiors as compared with maritime districts.

4. Ocean currents & winds

(i) Both ocean currents & winds affect temperature by transporting their heat or coldness into adjacent regions.
(ii) For e.g. the westerlies that come to Britain & Norway tend to be cool winds in summer & warm winds in winter.

5. Slope, Shelter & Aspect

(i) A steep slope experiences more rapid change in temperature than a gentle one.
(ii) Mountain ranges that have an eastward alignment like the alps show a higher temperature on the south facing sunny slope than the north facing sheltered slope.
(iii) The greater insolation of the southern slope is better suited for vine cultivation & has a more flourishing vegetative cover, consequently more settlements

6. Natural vegetation

(i) There is a definite difference in temperature between forested regions & open ground.
(ii) Thick amazon forest cuts off much of incoming insolation keeping the land surface of the jungle cool & few degrees lower than the open spaces in corresponding latitudes

7. Soil

(i) Light soils reflect more heat than the darker ones which are better absorbers of heat, which may give rise to slight variations in temperature of the region.
(ii) Dry soils like sand are very sensitive to temperature compared to wet clayly soils which retains moisture & warms up & cools down more slowly.


Precipitation


(i) Condensation of water vapour m air in form of water droplets or ice.
(ii) Their falling on earth surface is known as precipitation


1. Snowfall


(i) When condensation takes place below freezing point.
(ii) Means at 0*C, conversion of water vapour directly into solid state.
(iii) Precipitation occurs in forms of fine flakes of snow.


2. Sleet


(i) Sleet is frozen raindrops or refroze melted snow water.
(ii) When a layer of temp, above freezing point overlies a subfreezing layer near the ground, precipitation occurs in form of sleet.


3. Hail


(i) Sometimes, drops of rains after being released by the clouds become solidified into small rounded stone pieces of ice, known as hailstones
(ii) Formed by rainwater passing through colder layers hence have several concentric layers of ice, one over the another.


4. Rainfall


(i) Most common form of precipitation
(ii) Precipitation in form of water
(iii) Also known as cloud particles


Types of Rainfall



1. Convectional Rainfall


(i) Air on being heated becomes light & rises up as conventional currents
(ii) As it rises, it loses heat & consequently condensation takes place with the formation of cumulus clouds.
(iii) Under these conditions, heavy rainfall takes place along with thunder & lightening, but does not last for long.
(iv) Common in Equatorial & Tropical regions in summers daily.


2. Orographic or Relief Rainfall


(i) When a warm & moist air currents is obstructed by a mountain range, it is forced to ascent along its slopes.
(ii) It gets cooled while ascending & when its temp, falls below dew point, it causes rainfall on windward slope of mountain range.
(iii) However, when these winds cross mountain range & descend along its leeward side.
(iv) Here, they get warm & dry & causes only little rain (Rain shadow areas)
(v) This type of rainfall may occur in any season.


3. Cyclonic or Frontal Rainfall


(i) Rainfall associated with cyclone is known as cyclonic or frontal rainfall.
(ii) Occurs along the fronts of the cyclone viz. cold front & warm front.
(iii) At the warm front, the warm lighter wind rises gently over the heavier cold air, which being heavy stays close to the ground
(iv) As the warm air rises, it cools, and the moisture present in it condenses to form clouds altostratus clouds.
(v) This rain falls steadily for a few hours to a few days.


Planetary Winds


(i) Planetary winds are also known as permanent or prevailing winds
(ii) Blow from high to low pressure, over the earth surface & oceans throughout the year & in a particular direction
(iii) These winds are divided into 3 categories viz.

1. Trade Winds (Tropical Easterlies)
2. Westerlies
3. Polar winds (Polar Easterlies)


Trade Winds


(i) Winds blowing from subtropical high pressure area to equatorial low pressure area (Extremely steady winds).


(Tropical Easterlies)


(i) Since they travel from high latitude to low latitude area, they become gradually hot & dry and hence have a great capacity to hold moisture.
(ii) They cause considerable rainfall on eastern margins of the continents as they get moisture after blowing over oceans.
(iii) These winds converge near equator & form ITCZ, Here these winds rises & causes heavy rainfall
(iv) Absent in N Indian Ocean which is dominated by Monsoon winds.


Westerlies


(i) Winds blowing from subtropical high pressure belts towards subtropical low pressure belts.
(ii) Blow from S - W to N - E under Coriolis effect in Northern Hemisphere & from N - W to S - E in Southern Hemisphere
(iii) Blow from lower latitudes to higher latitudes
(iv) Cause considerable rainfall particularly on western margins of the continents
(v) More consistent in direction & blow with stronger force in S -Hemisphere due to lesser obstructions from continents.
(vi) Also known as brave winds or roaring forties, furious fifties & shrieking sixties according to the varying degree of storminess in the latitudes in which they blow.
(vii) It must be noted that not all the western coast of the temperate zone (30 degree - 60 degree) receive.
(viii) Westerlies throughout the year due to shifting of wind belts coz of earth’s inclination.
(ix) In June, when the overhead sun is over the tropic of cancer, all the belts move about 5 degree - 10 degree north of their average position. The Mediterranean parts of continents that comes under the effect of werterlies, receive rain in June & vice a versa in December, when sun is overhead tropic of Capricorn.


Polar Winds


(i) Winds blowing from polar high to sub polar low pressure belt.
(ii) Are very cold in nature as originate in polar areas & do not cause much rainfall.
(iii) These winds give birth to cyclones when they come in contact with westerlies.
(iv) Brings frequent change in weather conditions & causes heavy rainfall


Shifting of Wind Belts


(i) Wind belts described above keep on shifting northward & southward depending upon the movement of the sun.
(ii) March 21 & Sep 23 (Equinoxes).
(iii) Sun shines vertically over equator.
(iv) Equatorial low pressure belt lies between 5 degree North - 5 degree South.
(v) After March 21, sun moves northward & with it whole system of pressure belts moves northward.


June 21


(i) Sun shines vertically over Tropic of cancer & all the pressure belts move 5-10 degree northward from original position.


Dec 21


(i) Sun shines vertically over Tropic of Capricorn & all the pressure belts move 5-10 degree southward from original position.
(ii) Thus, shifting of world’s pressure belts also causes shifting of world’s wind system.


Periodic / Seasonal winds


(i) Winds which change their direction periodically.
(ii) Examples -Monsoon Winds, Land & Sea Breeze, Mountain & Valley Breeze.


Monsoon Winds


(i) Refers to system of winds which reverses their direction completely with change of seasons
(ii) Blow from sea to land during summers & land to sea during winters, due to differential in heating of continents & oceans-Halley’s law
(iii) In summers, sun shines vertically over Tropic of cancer resulting in high temp. & low pressure in central Asia, while pressure is sufficiently high at Bay of Bengal & Arabian Sea.
(iv) This induces air flow from Sea to land & induces heavy rainfall in India & neighbouring countries.
(v) In winters, sun shines vertically over tropic of Capricorn, hence North -West part of India grows colder than Arabian Sea & Bay of Bengal which results in reversal of monsoon in India.
(vi) Above theory of differential heating was replaced by shifting of ITCZ for monsoon in India & neighboring countries


Land and Sea Breeze


(i) Influence only a narrow strip of 20 - 30 km along the coast
(ii) During day sun shines hence sea breeze moves from sea to land (Sea Breeze)
(iii) In night it reverses its direction i.e. from land to sea (Land Breeze)


Mountain & Valley Breeze


(i) During day, mountain slopes gets heated more than valley floor hence air from valley floor blows up the slope (Valley Breeze)
(ii) After sunset pattern is reversed i.e. Mountain Breeze


Fohn Winds & Chinook Winds


(i) Both the Fohn & Chinook winds are local hot & dry winds experienced on the leeward side of the mountains when descending air become compresses with increased pressure.
(ii) Fohn wind is experienced in the valleys of northern Alps, particularly in Switzerland in spring.
(iii) Chinook winds are experienced on the eastern slopes of the Rockies in USA & Canada in winters.
(iv) While descending, most of the moisture of the wind is lost & hence it becomes dry & hot, which may lead to rise in temperature of leeward side.
(v) In North America, it is called Chinook which means the snow eater, as it melts the snow & causes avalanches.
(vi) It has blessings too, it enhances the growth of crops & fruits & thaws the snow covered pastures by raising temperature of the region quite quickly.


Cyclones


(i) A low pressure area surrounded by high pressure area from all from all the sides along with winds moving from all the sides towards central low.
(ii) Cyclones moves in Anti clockwise in North Hemisphere & in Clockwise direction in South - Hemisphere under the effect of westerlies due to coriolis effect.
(iii) No Cyclones at equator as coriolis force is 0 there.


Temperate Cyclone


(i) Also known as wave cyclones or Extra Tropical
(ii) Originate mainly in zones between 35 degree - 65 degree North & South of latitudes


Polar Front Theory


(i) Forms due to collision of 2 air masses of contrasting characteristics (in terms of temp. & humidity at about 60 degree latitude).
(ii) Here they do not meet each other readily but forms a front known as polar front
(iii) Cold air mass pushes the warm air mass upwards & a void is created due to decrease in pressure.
(iv) Air from surrounding area rushes to fill the void & a temperate cyclone if formed
(v) Average speed of extra tropical cyclone is 32 km/hr in summer & 49 km/hr in winters.


Tropical Cyclone


(i) Also known as Typhoons or Hurricanes
(ii) Originate mainly in zones between 5 degree - 30 degree North & South of latitudes
(iii) Are the violent storms that originate over oceans in tropical areas & move to coastal areas
(iv) Bring large scale destruction, caused by violent winds, heavy rainfall & storm surges
(v) Favorable conditions for formation of tropical cyclones are
(vi) Large sea surface with temp. > 27 degree Celsius
(vii) Presence of coriolis force
(viii) Small variation in vertical wind speed
(ix) Upper divergence above sea level
(x) Pre-existing weak low pressure area or low level cyclonic circulations
(xi) Energy that intensifies the storm comes from the condensation process in towering cumulonimbus clouds, surrounding center of the storm.
(xii) Hence, with constant supply of moisture from the sea, storm is further strengthened
(xiii) On reaching land, moisture supply is cut off & the stonn dissipates
(xiv) Place where tropical cyclone crosses the land is called landfall of the cyclone.
(xv) Central low pressure is known as eye of the cyclone —> Calm with subsiding air having lowest pressure & highest temp.
(xvi) Surrounding this area is zone of strong winds with clouds extending vertically.
(xvii) Surrounding the eye is eye wall, a place of strong spirally ascending winds to a height reaching tropopause, having max. wind velocity


Tropical Cyclone Distribution & its various names world wide



1. Cyclone


(i) Indian ocean, Arabian sea & Bay of Bengal


2. Hurricane


(i) Atlantic sea (West indies) & USA


3. Typhoons


(i) China sea + Japan sea


4. Willy Willies


(i) Western Australia



Difference between Tropical & Extra Tropical Cyclones


  Tropical Cyclones Extra Tropical Cyclones
 (i) Moves from east to west (i) Moves from west to east
 (ii) Wind velocity is very high & more destructive (ii) Low wind velocity & less destructive
 (iii) Originate only on sea &

dissipates on reaching land.

  (iii) Affect much larger area & can originate on land as well as sea.


Anticyclones


(i) An anticyclone is just opposite to a cyclone.
(ii) Basically it is a large-scale circulation of winds around a central region of high atmospheric pressure
(iii) Clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.
(iv) Anticyclones are form from air masses, cooling more than their surroundings, which causes the air to contract slightly making the air denser.
(v) Since dense air weighs more, the weight of the atmosphere overlying a location increases, causing increased surface air pressure.
(vi) Anticyclones herald fair weather, clearing skies, calm air with high temperature in summers & cold in winters.
(vii) Fog can also form overnight within a region of higher pressure.


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