Guido Cervone EOS 121 Lecture 3

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Chapter III

• Guido CervoneEOS 121 Lecture 3

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Energy Balance and Temperature

• Atmospheric influences on insolation
• The fate of solar radiation
• Energy transfer between the surface and the
  atmosphere
• Global temperature distribution
• Influences on temperatue

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Energy and the Seasons

• What is absorption and scattering of the
  atmosphere?
• Why is the sky blue?
• What is the difference between incoming and
  outgoing radiation?
• What is the greenhouse effect?
• What influences surface temperature?

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Variation of Surface Temperature

• Seasons

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Effect of Atmosphere on Solar Insulation

• We have seen that the direct normal illuminance
  (Edn), corrected for the attenuating effects of
  the atmosphere is given by
• where Eext is the sunlight at the top of the
  amotphere, c is the atmospheric extinction
  coefficient and m is the relative optical airmass.

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Atmospheric Influences on Insulation

• Sunlight must travel through the atmosphere to
  reach the surface
• The atmosphere absorbs, scatters, reflects and
  transfers sunlight
• The amount of sunlight reaching the surface is
  only a fraction of the energy at the top of the
  atmosphere

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Solar Energy through the Atmosphere
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Atmospheric Absorption

• Energy transfer
• Atmosphere gains energy and warms up
• The amount of energy reaching the surface is
  reduced
• The amount of absorption depends on the
  atmospheric composition
• It is highly localized (both altitude and
  position)
• O3 absorbed UV radiation in the stratosphere
• Visible light goes through with only minimal
  absorption
• Near IR (half or sunlight) is totally absorbed by
  CO2 and H2O

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UV Absorption through the Atmosphere
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Atmospheric Reflection

• Energy is simply redirected away without being
  absorbed
• The percentage of visible light being reflected
  is called the albedo
• Objects do not reflect all wavelength equally
• The source of illumination also changes the
  amount of reflected wavelengths

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Specular vs. Scattering Reflection

• Scattering relflection in many directions

• Specular reflection a beam of equal intensity

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Rayleigh Scattering

• Scattering particles are smaller than 1/10 of
  incoming radiation
• It does not affect all wavelengths, but is biased
  towards shorter waveleghts

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Rayleigh Scattering

• The intensity I of light scattered by a single
  small particle from a beam of unpolarized light
  of wavelength ? and intensity I0 is given by
• where R is the distance to the particle, ? is the
  scattering angle, n is the refractive index of
  the particle, and d is the diameter of the
  particle.

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Mie Scattering

• Scattering of small particles in the atmosphere,
  called aerosols
• Mie scattering is primarily forward, so that
  little radiation is reflected back into space
• It is not biased towards shorter wavelengths as
  in Rayleigh

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Atmospheric Scattering
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Why is the sky blue and red?

• Rayleigh scattering is inversely proportional to
  the fourth power of wavelength
• Shorter wavelength of blue light will scatter
  more than the longer wavelengths of green and red
  light
• Conversely, when one looks towards the sun, one
  sees the longer wavelengths such as red and
  yellow light, which were not scattered

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The sky appears blue because gases and particles
in the atmosphere scatter some of the incoming
solar radiation in all directions. Air molecules
scatter shorter wavelengths most effectively.
Thus, we perceive blue light, the shortest
wavelength of the visible portion of the spectrum.
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Blue vs. Red sky
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Sunrises and sunsets appear red because sunlight
travels a longer path through the atmosphere.
This causes a high amount of scattering to
remove shorter wavelengths from the incoming beam
radiation. The result is sunlight consisting
almost entirely of longer (e.g., red) wavelengths.
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Fate of solar radiation
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Shortwave Radiation vs. Longwave Radiation

• When sunlight reaches the Earth, it warms up both
  the surface and the atmosphere
• The Earth is emitting longwave energy back into
  space

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Earth Energy Balance
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Earth Energy Budget

• SW LW 0

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SW Vs. LW radiation
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Sensible Vs. Latent Heat

• Sensible Heat
• Temperature that can be measured
• Latent Heat
• Energy stored within the water molecules.
  Responsible for state changes

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Sensible Heat Increase

• Depends on two factors
• Specific Heat. Materials with higher specific
  heat warms up slower
• Mass. The more mass has an object, the more
  energy is required to warm it up

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Sensible Heat Increase
Heat energy required to raise two different
quantities of water 5 degrees Celsius.

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Greenhouse Effect
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Energy Budget with Greenhouse
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Annual (1987) pattern of solar radiation absorbed
at the Earth's surface
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Annual (1987) quantity of outgoing longwave
radiation absorbed in the atmosphere.
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Diurnal Cycle

• The incoming solar radiation peaks around high
  noon
• Outgoing radiation reaches its minimum around
  dawn
• Temperature attains its maximum about 3 to 4
  hours after noon, thus, not coincident with the
  radiation peak
• This lag is the result of several factors
• thermal uplifting
• winds that carry heat upwards and slow down the
  surface temperature rise

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Energy Distribution within the Atmosphere

• Conduction
• Temperature near the surface is warmer a few cm
  above the ground. A downward conduction energy
  transfer occurs
• Free Convection
• Warmer air masses rise and displace colder air
  masses that sink
• Forced Convection
• Mechanical turbulence. The flow breaks into
  several eddies.

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Boundary Layer
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Influences on Temperature

• Latitude
• Altitude
• Atmospheric circulation
• Proximity to large bodies of water
• Warm and cold ocean currents
• Local features
• Slope (North vs. South)
• Land coverage

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Average Annual Global Temperature 1982-1994
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Global Circulation
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Earths Deserts
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Surface Ocean Currents
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Surface Ocean Currents
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Great Ocean Conveyor Belt