TOPIC 3 GLOBAL CYCLES AND PHYSICAL SYSTEMS

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TOPIC 3 - GLOBAL CYCLES AND PHYSICAL SYSTEMS

• Atmosphere - envelope of gases surrounding Earth
  (Ch. 21 Env. Sci.)
• Structure and function (3.1)
• Depletion of Stratospheric Ozone (3.2)
• Tropospheric Ozone (3.3)
• Issue of Global Warming (3.4)
• Acid Deposition (3.5)
• Hydrosphere - liquid envelope (3.6)
• Lithosphere - solid earth (3.7)
• Soil System (3.8)

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ATMOSPHERE

• Composed mostly of gases but also some small
  solid particles (ash, dust, salt, pollen) and
  liquid solid water
• Main gases

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Atmospheric structure

• About 1000 km thick
• Held in place by gravity
• More than 99 of mass is in first 40 km of
  earths surface
• Above 100 km it is virtually a vacuum
• Five layers characterized by differences in
  temperature, composition, movement and density

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Five layers

• Troposphere (0 to 8 -16 km)
• Starts at earths surface
• Most dense layer - contains most of atmospheres
  mass, water vapour and dust
• Site and source of almost all weather
• Upper boundary called tropopause ranges in height
  from 8 km in high latitudes to 16 km above the
  equator
• Tropopause also varies with seasons - highest in
  summer and lowest in winter

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Temperature profile of the atmosphere

• Temperature decreases from 17 to -75oC in
  troposphere as the distance from earth increases
• lapse rate rate at which temperature declines
  with increasing altitude in the troposphere (6oC
  per km)

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More on the troposphere

• Contains most of water vapour and clouds in
  atmosphere
• Troposphere means region of mixing
• Wind speeds increase with height due to decreased
  friction

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2. Stratosphere (8-16 to 50 km)

• stratopause at 50 km
• Dry - very low water vapour content
• Lacks dust
• Less dense than troposphere
• Temperature remains constant up to 25 km then it
  increases to -3oC due to UV absorption by ozone
  layer (at 20-30 km)
• 90 of ozone in atmosphere is the stratosphere
  (10 ppm compared to 0.04 ppm in troposphere)
• Little vertical mixing - movement by diffusion
  only

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3. Mesosphere (50-80 km)

• Mesopause at 80 km
• Temperatures fall to -90oC due to decreasing
  density and inability to absorb energy

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4. Ionosphere (80-600 km)

• Ionosphere exosphere thermosphere
• Temperature increases with altitude up to 1200oC
  due to absorption of intense solar energy by O2
  and N2 which break apart into ions, releasing
  energy
• 5. Exosphere (600 - 1000 km)
• Relatively undefined
• Transitional zone between atmosphere and space
• Helium and hydrogen are major components

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Global warming and atmospheric layers

• http//www.ghcc.msfc.nasa.gov/MSU/atmos_layers.htm
  l

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Functions of the atmosphere

• Recycling of water and other chemicals
• Temperature regulation - moderates climate on
  earth (greenhouse effect)
• Protection from high energy short-wave radiation
  (UV) from sun
• Protection from small meteors
• Reservoir of oxygen, carbon dioxide and nitrogen

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Global atmospheric energy budget

• Solar radiation (short wave UltraViolet (UV)
  radiation) entering the atmosphere can take
  several paths
• 30 reflected by atmosphere - never makes it to
  earth
• 19 is absorbed by atmosphere
• 51 is absorbed by earths surface (land and
  oceans)

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Global energy budget
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Of the 51 absorbed by the earths surface

• 21 is re-radiated to space as long wave InfraRed
  (IR) radiation (directly or via the atmosphere)
• 23 is transferred to the atmosphere through
  evaporation (latent heat flux heat required for
  a material to change state)
• 7 is transferred by conduction and re-radiation
  (sensible heat flux heat transfer due to
  conduction and convection)
• Finally the atmosphere re-radiates heat as long
  wave (IR) radiation into space

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Weather

• Weather is caused by the transfer of energy
• Energy is transferred wherever there is a
  temperature difference between two objects
• Three main ways energy can be transferred
• Radiation e.g. visible light, UV, IR
• Conduction - contact between objects
• Convection - movement

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Energy movement

• Solar radiation (UV) enters the atmosphere - some
  is absorbed by the earth
• The energy is transferred by conduction to a thin
  layer of air above the earth
• Convection occurs because warmer air is less
  dense than colder air so it rises - taking the
  energy with it

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Unequal heating of earths surface by sun

• An area at the equator receives more solar
  radiation than an equal area at the poles because
  of
• The angle of exposure
• The distance the radiation travels through the
  atmosphere is less at the equator (the atmosphere
  absorbs part of the energy)

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Atmospheric circulation

• High solar radiation at the equator produces hot
  air which rises because of its low density
• In contrast, air at the poles is cold because of
  low solar radiation - the cold air sinks because
  of its high density which results in a high
  pressure zone near the poles
• Fluids flow from High Pressure Zones to Low
  Pressure Zones so we would expect surface winds
  to flow from the poles (high P) to the equator
  (low P) - single cell

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Coriolis Effect

• We dont see this simple pattern of air
  circulation (a single cell in each hemisphere)
  due to the CORIOLIS EFFECT
• Objects that are moving relative to the ground
  are deflected to the right in the northern
  hemisphere and to the left in the southern
  hemisphere
• The amount of deflection increases with speed of
  the moving object and with latitude (no Coriolis
  Effect at the equator)

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Global Wind Pattern

• Result of the effects of pressure (uneven heating
  of the earths surface) and Coriolis Effect
  (rotation of the earth)
• Heating at equator (0o latitude) causes air to
  warm - this air expands resulting in a low
  pressure zone
• Warm air rises, expands and cools (lapse rate!) -
  losing moisture by precipitation and resulting in
  a high pressure zone
• Air flows toward low pressure zone and as it
  becomes cooler and drier its density increases,
  so at 30oN and 30oS the air sinks

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• Much of this air returns to the equator along
  the surface becoming warmer and picking up
  moisture along the way - as it travels the air is
  deflected by the Coriolis Effect resulting in
  Northeast Trade Winds and Southeast Trade Winds
• These trade winds converge at the equator
  resulting in light and variable winds called the
  Doldrums where wind is being convected upwards
• Similarly there is little wind at 30oN and S
  (Horse Latitudes) because air is moving downwards

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Tricellular Model

• Hadley Cell - wind flow pattern of air rising at
  the equator and descending at 30oN and 30oS
• Ferrel Cell - at 30oN and 30oS some of the
  descending air travels pole-ward - these surface
  winds are deflected by the Coriolis Effect to
  become the Westerlies of both hemispheres
• Surface winds travel pole-ward until about 50oN
  and 50oS where they encounter cold dense air
  coming from the poles
• Air is forced upwards where it diverges so that
  some goes equator ward to complete Ferrel cell
  and some pole ward

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Tricellular model

• Polar Cell -upper air moving pole ward cools,
  becomes dense and sinks back to the surface at
  the poles

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Atmospheric circulation

• Is responsible for distributing energy (heat)
  from the equator to the poles
• Gives rise to climatic zones and biomes
• Tropical rainforests near the equator
• Deserts at 30o where dry air descends

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Tropical cyclones

• non-frontal low-pressure system over tropical or
  sub-tropical waters with organized convection and
  cyclonic surface wind circulation
• Vortex-like centres of very low pressure,
  associated with extremely high, often destructive
  winds blowing toward the centre or eye (IB)
• Disc shaped
• Vertical dimension - tens of kilometres
• Horizontal dimension - hundreds of kilometres
• http//www.aoml.noaa.gov/hrd/weather_sub/faq.html
• http//www.hko.gov.hk/informtc/nature.htm

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Tropical cyclones

• Tropical cyclones with maximum sustained surface
  winds of less than 17 m/s (34 kt, 39 mph, 63
  km/h) are called tropical depressions
• If maximum sustained wind is greater than 17 m/s
  then they are called tropical storms
• If winds reach 33 m/s CYCLONE / HURRICANE!
• Different names depending on location
• Hurricane - North Atlantic, NE Pacific
• Typhoon - NE Pacific
• Tropical cyclone - SW Indian Ocean
• Naming of hurricanes / cyclones
• http//www.aoml.noaa.gov/hrd/tcfaq/B2.html

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How do cyclones form?

• Requirements
• Warm water (26.5oC or more) - energy source
• Moist atmosphere
• Low wind shear - wind must be blowing in the same
  direction and at the same speed up to 9,000
  metres above the sea
• Air cools very quickly with vertical distance
• Minimum distance of 500 km from equator (Coriolis
  effect)
• Dissipate when they reach land as their energy
  source (warm water) is gone
• Cyclones and depressions have an important role
  in the distribution of energy

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Depression (temperate cyclone)

• A region of relatively low pressure (up to 2000
  km across) in middle latitudes in which winds
  spiral inwards and counterclockwise in the
  northern hemisphere and clockwise in the southern
  hemisphere (IB)
• Associated with windy, cloudy and wet weather