ATMOSPHERE

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ATMOSPHERE
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Earth as a System

• http//www.teachersdomain.org/resource/ess05.sci.e
  ss.earthsys.hologlobe/

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STRUCTURE OF THE ATMOSPHERE

• Thermosphere
• Mesosphere
• Stratosphere
• Troposphere contains 75 of the mass of the
  atmosphere and almost all of the moisture and
  dust.
• http//www.teachersdomain.org/resource/ess05.sci.e
  ss.watcyc.vertical/

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Troposphere

• Temperature decreases by 6.4oC every 1000m
  lapse rate
• Solar radiation heats the air by conduction
• Contains most of atmospheric water, vapour,
  cloud, dust

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Stratosphere

• Steady increase in temp. caused by increasing
  concentration of ozone O3 which absorbs
  ultraviolet radiation.
• Winds light and increase with height
• Pressure falls and air is dry

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Mesosphere

• Temperature falls rapidly as no water vapour,
  cloud dust or ozone to absorb incoming radiation
• Lowest temperature 90o C and strongest winds

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Thermosphere

• Temperatures rise rapidly with height 500o C
  due to increasing proportion of atomic oxygen
  which absorbs incoming UV radiation (like ozone)

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Global Heat Budget
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THE GLOBAL HEAT BUDGET

• The atmosphere system involves inputs and
  outputs.
• Incoming solar radiation is balanced by outgoing
  terrestial energy from the earth.
• The balance between input and output is usually
  referred to as the Global Heat Budget.

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• Input begins with solar energy (insolation). Some
  insolation is
• Reflected by clouds and scattered by gas particles

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• Absorbed by water vapour, dust and clouds.

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• Output is in the form of long wave radiation
  emitted from the earth this balances the input
  of energy from the sun
• 94 of this radiation is absorbed by water vapour
  and CO2 in the atmosphere.
• 6 is radiated back into space.

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LATITUDINAL VARIATIONS

• As well as a vertical transfer of energy between
  earth and space there is also a horizontal
  transfer of energy between high and low latitudes.

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• These energy variations are more extreme between
  the tropics and the poles. Such marked contrasts
  are referred to as the global temperature
  gradient and are the result of a number of
  factors

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• The curvature of the earth
• Due to the curvature of the earth, the equator is
  closer to the sun than the poles and as a result,
  insolation at the equator is more concentrated.

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• Atmosphere penetration
• The suns energy also passes through a greater
  depth of atmosphere at the poles causing a lot of
  energy to be diffused.

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• The Albedo effect
• Ice and snow reflect more solar radiation back
  into space making them cooler whereas areas of
  dense vegetation absorb radiation making them
  warmer.

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Seasonal Variations

• Seasonal variations in amount of radiation
  received by the earth with latitude
• Sun appears to be at Tropic of Cancer mid-June,
  so northern hemisphere receives more insolation
• Mid-December the sun appears to be at the
  Tropic of Capricorn and so the southern
  hemisphere receives the maximum insolation

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ENERGY TRANSFER

• Such an imbalance in energy receipt could
  theoretically result in the lower latitudes
  becoming warmer and the higher latitudes becoming
  even colder. In reality however, energy is
  transferred from areas of surplus to areas of
  deficit by atmospheric circulation and by ocean
  currents.

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ATMOSPHERIC CIRCULATION

• The three cell model is a useful tool in
  describing atmospheric circulation and energy
  transfer.
• Circulation 1
• Circulation 2

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THE HADLEY CELL

• This extends from the equator to about 30 N and
  S of the equator. The intense heating at the
  equator causes the air to expand and become
  lighter, producing an area of low pressure.

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• This warm, rising air contains large amounts of
  moisture which condenses to form cumulonimbus
  clouds and heavy rainfall. The rising air then
  spreads polewards and sinks to the sub-tropics.
  The sinking air produces high pressure resulting
  in clear skies

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• And little precipitation (these areas correspond
  with the desert areas of the world)

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THE POLAR CELL

• Air over cold surfaces will become cold, contract
  and become heavy. It will therefore sink and
  produce an area of high pressure. The sinking
  air moves towards lower latitudes where it will
  expand and rise back up creating a cell.

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THE FERREL CELL

• The Ferrel Cell lies between the Hadley Cell and
  the Polar Cell. The HC and the PC are thermally
  direct cells (powered by temperature differences).

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• The Ferrel Cell is a thermally indirect cell
  because it is powered by the other two. The FC
  transfers warm air from the Hadley cell to the
  high latitudes and transfers cold air form the PC
  to the low latitudes for warming.

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ATMOSPHERIC CIRCULATION The Fuller Picture

• Atmospheric circulation is a lot more complicated
  than is suggested by the three cell model.
  Recent research questions the existence of the
  Ferrel Cell.

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• In place of the Ferrel Cell it is now argued that
  there are
• Alternating patterns of high and low pressure
  which travel at relatively low levels.
• A series of high level, horizontal wavelike
  motions called Rossby waves.

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• Rossby waves are very large, high velocity belts
  of wind operating in the upper atmosphere. They
  have a distinct wave like motion as they snake
  their way across the globe. At their core are
  long, narrow cylinders of very fast flowing air
  called jet streams.

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JET STREAMS
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• Teachers' Domain The Effect of Jet Streams on
  Climate

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GLOBAL WIND CIRCULATION

• Whatever the actual workings of energy transfer
  and atmospheric circulation, broad global
  patterns of winds and pressure can be identified

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MODEL OF GLOBAL WIND CIRCULATION
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• The model of Global Wind Circulation is more
  complicated than it would appear due to several
  factors

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• The earths tilt and consequential seasonal
  contrasts.

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• In June in the Northern Hemisphere, the earths
  axis is tilted towards the sun and the sun
  appears directly overhead in the Tropic of Cancer.

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• In December, the earths axis tilts away from the
  sun and the sun appears to be overhead at the
  Tropic of Capricorn.

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• The apparent movement of the overhead sun is
  important because it controls the belt of maximum
  heating which moves with the sun. This called
  the thermal equator.

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• The distribution of land and sea.
• The wind pattern is more consistent in the
  Southern Hemisphere especially above latitude 30
  degrees south as there are virtually no land
  masses to interrupt the winds and heating and
  cooling properties of the oceans means that a
  relatively consistent wind pattern results.

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• Over the Northern Hemisphere, the large land
  masses result in an altered wind pattern due to
  the more extreme temperature differences
  experienced over the continents in summer and
  winter.

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• Low pressure is dominant in the summer due to the
  intense heating of continental interiors (winds
  therefore spiral inwards anticlockwise towards
  the centre of the low pressure).

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• High pressure dominates in winter and winds blow
  outwards in a clockwise direction.

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OCEAN CURRENTS

• Sea water has a high thermal capacity, so the
  oceans are an effective store of thermal energy.
  In contrast with the land, the seas warm to a
  greater depth and also move and so redistribute
  this energy.

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• Upper ocean currents are generated by prevailing
  winds blowing across the surface of the ocean.
  These are influenced by the rotation of the earth
  and the distribution of the land masses. The
  currents largely flow in loops called gyres.
• http//www.teachersdomain.org/resource/ess05.sci.e
  ss.watcyc.gulfstream/

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• In addition to the surface ocean currents of the
  world, there is also and oceanic conveyor belt ,
  or deep ocean circulation, that corresponds to
  the atmospheres climate.

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• Antarctica is important in this pattern of
  movement, here vast amounts of water freeze into
  ice, this loss of fresh water causes the
  remaining sea water to become more saline and
  therefore more dense. This denser water
  consequently sinks and makes its way northwards
  towards the equator where it is warmed and
  returns southwards.

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• Cold ocean currents flow from the poles.
• Warm ocean currents flow from the equatorial
  regions.
• http//www.teachersdomain.org/resource/ess05.sci.e
  ss.watcyc.convey2/

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• Below latitude 30 degrees, the west coast of
  continents have contact with cold ocean currents
  e.g. Peru, and the east coast of continents have
  contact with warm ocean currents e.g. Brazil
  current.

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• Above 45 degrees, the position is reversed west
  coast in contact with warm currents e.g. NAD, and
  east coast in contact with cold current e.g.
  Labrador current.

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• In the Pacific and Atlantic oceans large loops
  (gyres) appear which are associated with cells of
  sub-tropical high pressure.

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Rainfall and Vegetation in Africa

• Rainfall Vegetation

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ITCZ

• Critical to our understanding of the varying
  rainfall totals and their seasonal distribution
  in tropical Africa is the seasonal movement of
  the ITCZ.

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• The ITCZ is a belt of low pressure produced by
  the combination of equatorial heating and the
  convergence of trade winds, and migrates in
  response to the changing location of the thermal
  equator. (see hand out)

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The Inter tropical Convergence Zone
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ITCZ
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• As the airflows converge at the ITCZ, they rise
  and create a zone of clouds and rainfall. Once
  the air ascends it diverges and flows polewards,
  descending over a wide area centred around 30
  degrees N and S. As it descends it is warmed and
  results in dry, cloudless conditions.

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• The descending air at the subtropics will be
  affected by the air mass at the Earths surface.
  The most important air masses which affect Africa
  are Tropical Continental and Tropical Maritime.

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• The Tropical Continental air mass (Harmattan) is
  a hot and dry air mass.
• The Tropical Maritime is a hot and wet air mass.

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• In July, the ITCZ has reached its most northerly
  extent and it pulls in hot, moist tropical
  maritime air bringing the Wet Season to West
  Africa.

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• By January, in response to the changing position
  of the thermal equator, the ITCZ has migrated to
  the Tropic of Capricorn. Most of Africa north of
  the equator will experience its dry season at
  this time.

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• West Africa is also influenced by Tropical
  continental air at this time bringing dry, dusty
  conditions.

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ITCZ in Africa

• http//people.cas.sc.edu/carbone/modules/mods4car/
  africa-itcz/index.html

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Continental tropical air
Maritime tropical air
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IN JANUARY
N
S
Hot dry cT air
Moves this way
Harmattan wind
Gulf of Guinea
Coastal areas- equatorial climate
Inland areas- savanna climate type
Sahara- Desert climate type
Copy diagram
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Continental tropical air
Maritime tropical air
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Continental tropical air
Maritime tropical air
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IN JULY
S
N
Compare the January and July diagrams.
Hot dry cT air
Wet warm mT air
Moves this way
Harmattan wind
HEAVY RAINS
LIGHT RAINS
Gulf of Guinea
Coastal areas- equatorial climate
Inland areas- savanna climate type
Sahara- Desert climate type
Copy diagram
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http//www.srh.weather.gov/srh/jetstream/tropics/i
tcz.htm
Continental tropical air
Maritime tropical air
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CLIMATIC CHANGE

• Causes of climatic change
• Natural causes
• Man-made causes

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Natural Causes

• Variations in solar energy sun spot activity
  occurring in cycles.
• Milankovitchs Cycle (wobble, roll and stretch
  theory)

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• Composition of the Earths atmosphere volcanic
  activity can add dust particles into atmosphere
  increasing the absorption and scattering of
  incoming solar radiation.

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Man Made Causes

• Increased CO2 levels
• Deforestation increases CO2 levels
• Flatulent cows - increased population pressure
  has led to increased food production cows
  produce a lot of methane gas.

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• Deforestation, soil erosion etc have increased
  the albedo effect.
• Increased use of CFCs in aerosols etc.

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Consequences

• Predicted temp rise of 1.5 to 4.5 degrees C
  this would threaten wildlife, affect agricultural
  areas, tropical diseases would spread

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• Sea levels would rise many low lying areas
  would be flooded.
• Increase in extreme weather conditions.

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Solutions

• Reduce CO2 emissions
• Reduce use of nitrogen fertilisers
• Less intensive livestock production
• Ban CFCs