Solar%20Energy%20and%20Energy%20Balance%20in%20the%20Atmosphere

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Solar Energy and Energy Balancein the Atmosphere
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Mid-chapter summary

• The temperature of a substance is a measure of
  the average kinetic energy (average speed) of its
  atoms and molecules.
• Evaporation (the transformation of liquid into
  vapor) is a cooling process that can cool the
  air, whereas condensation (the transformation of
  vapor into liquid) is a warming process that can
  warm the air.
• Heat is (thermal) energy in the process of being
  transferred from one object to another because of
  the temperature difference between them.
• In conduction, which is the transfer of heat by
  molecule-to-molecule contact, heat always flows
  from warmer to colder regions.
• Air is poor conductor of heat.
• Convection is an important mechanism of heat
  transfer, as it represents the vertical movement
  of warmer air upward and cooler air downward.
• All objects with a temperature above absolute
  zero emit radiation.
• The higher an objects temperature, the greater
  the amount of radiation, and the shorter the
  wavelength of maximum emission.

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Methods for heat transfer in the atmosphere

• Thermal conduction
• It requires physical contact, temperature
  difference
• The energy flow is from a body with high T to a
  body with low T.
• It is important only for a thin layer close to
  the ground.
• Convection (advection)
• Winds in the atmosphere can transport energy.
• Very efficient way of energy transport in the
  atmosphere.
• Radiation
• All bodies with Tgt 0 K emit EM waves.
• EM waves can propagate in vacuum.
• The EM spectrum of a body is the energy it emits
  at different wavelengths
• The higher the temperature the more energy is
  emitted (SB Law)
• The wavelength at which the peak of the EM
  emission of a body occurs is determined by its
  temperature (Wiens Law).

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Radiation and Temperature

• All bodies with Tgt0K emit radiation
  (electromagnetic energy).
• The origin of the emission is the transition of
  the atoms (molecules) from one energy state to
  another.
• The wavelength and the amount of energy emitted
  by the body depend on its temperature.
• Higher T -gt larger internal energy -gt atoms
  vibrate faster -gt the radiation has shorter
  wavelength and higher energy.

The Suns electromagnetic spectrum
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Temperature and Emitted Energy
Energy
The Sun emits (6000/288)4188,000 times more
energy than the Earth!!!
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Sun/Earth radiation

• Sun
• T6000 K
• lmax0.5 mm
• Maximum in visible

• Earth
• T288 K 15 C
• lmax10 mm
• Maximum in IR

The Sun emits (6000/288)4188,000 times more
energy than the Earth!!!
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Radiative Equilibrium
gt
Emission
Absorption
Cooling
lt
Emission
Heating
Absorption

Emission
Absorption
Equilibrium Tconstant
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The Earths Balancing Act Absorption, Emission
and Equilibrium

• First, suppose the Earth had no atmosphere at all
• Energy from the Sun would be absorbed during
  daytime
• Energy would be radiated away 24/7
• Average surface temperature at equilibrium
  255K-18C0F
• Why arent we freezing to death?

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The greenhouse effect of the atmosphere
With Atmosphere
No Atmosphere
T255 K
T288 K
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Selective Absorbers/Emitters

• Trees absorb visible energy from Sun, radiate in
  infrared
• Snow away from trees reflects visible sunlight,
  stays cold
• Snow near trees reflects visible sunlight, but
  absorbs the infrared radiation from nearby trees,
  heats up and melts.

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Atmospheric Greenhouse effect

• The gases in the Earths atmosphere are selective
  absorbers
• Greenhouse gases absorb the infrared radiation
  from Earth
• Also selectively radiate in the infrared, a
  fraction goes back to Earth
• The Earths greenhouse effect H2O 60 CO2 26
  rest 14

H2O, CO2
O2, O3
H2O, CO2
O3
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Fig. 2-11b, p. 41
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Fig. 2-11a, p. 41
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Fig. 2-11c, p. 41
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The greenhouse effect of the atmosphere
With Atmosphere
No Atmosphere
T255 K
T288 K
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The lower atmosphere is heated from below!!!
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The Incoming Solar Radiation

• Insolation Incoming Solar Radiation
• The solar constant the energy from the sun at
  the TOP of the atmosphere per unit surface per
  unit time (1367 W/m2)
• Scattered light scattering from molecules, dust
  particles, aerosols.
• The scattered (diffuse) light goes in all
  directions.
• Shorter wavelengths are scattered more
  efficiently by the atmosphere (the sky is blue,
  see Chapter 19).
• Reflected light some light is sent backwards.
• Albedo the percentage of the incident light that
  is reflected.
• The albedo is a measure of the reflectivity of
  the surface
• Absorption

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Table 2-3, p. 44
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The solar energy budget
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The Energy Balance

• The amount of incoming energy should be exactly
  equal to the amount of the outgoing energy!!!
• The Earth
• IN Solar radiationCloud IR emission
• OUT ConductionConvectionH2O Evaporation IR
  emissionReflection of solar light
• The atmosphere
• IN from the ground (convection,conduction,latent
  heat,IR emission)solar radiation
• OUT to the ground (IR emissionreflected
  light)to space(IR radiation)
• The Earth atmosphere
• IN Solar Radiation
• OUT IR radiation (groundatmosphere)Reflected
  light

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The Earth Energy Balance