Radiation in the Atmosphere Cont'

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Radiation in the Atmosphere (Cont.)
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Cloud Effects

• (2) Cloud effects occur only when clouds
• are present.
• (a) Absorption of the radiant energy by
• the liquid water droplets and ice
• crystals in clouds. A
• (b) Reflection of the radiant energy back
• to space by the liquid water droplets
• and ice crystals. R

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Cloudy sky
Clear sky
R
A
58F
46F
Surface of the Earth
In a clear sky more solar radiation reaches the
surface and warmer temperatures occur
In a cloudy sky less solar radiation reaches the
surface and cooler temperatures occur
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Cloud Effects (Cont.)

• The amount of solar radiation affected by clouds
  depends on the thickness of the clouds and their
  composition.
• Thick clouds tend to absorb and reflect more
  solar radiation than thinner clouds.
• Clouds with more liquid water tend to absorb and
  reflect more solar radiation than clouds with a
  higher percentage of ice crystals.

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Cloud Effects (Cont.)

• In general the net effect of clouds is to reduce
  the temperature in the daytime because they
  prevent some solar radiation from reaching the
  ground.

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Surface Effects

• (3) Surface effects occur when the solar
• radiation reaches the surface of the
• Earth.
• (a) Reflection of solar radiation is the
• primary surface effect.

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Surface Albedo

• The surface albedo is the percentage of solar
  radiation that is reflected by the surface.
• surface albedo
• reflected solar radiation x 100
• incoming solar radiation

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Surface Albedo (Cont.)
Reflected solar radiation 200 W/m2
Incoming solar radiation 800 W/m2
Surface albedo ((200 W/m2)/(800 W/m2)) x
100 Surface albedo 25
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Typical Surface Albedos

• Fresh snow 75 to 95
• Sand 15 to 45
• Grassy field 10 to 30
• Dry, plowed field 5 to 20
• Forest 5 to 15

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Measuring Solar Radiation

• A pyranometer is an instrument used to measure
  solar radiation

Glass dome to keep dust and other contaminants
off sensor. The solar radiation passes through
the glass unaffected.
Sensor measures the incoming solar radiation.
If you want to measure the reflected solar
radiation, you simply set out a second
pyranometer turned upside down.
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What happens to the incoming solar radiation?

• 19 is absorbed by gases and clouds
• 6 is scattered back to space
• 20 is reflected by clouds back to space
• 4 is reflected by the surface to space
• --------
• 49 of the solar radiation is not absorbed
• by the Earths surface
• 51 is absorbed by the Earths surface

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Other Important Surface Energy Transfers

• 1. Emission of terrestrial radiation by the
• surface.
• 2. Absorption of terrestrial radiation
• emitted by the atmosphere to the surface.
• 3. Latent energy transfer from the surface
• during evaporation.
• 4. Transfer of internal energy between the
  atmosphere and the surface.

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Emission of Terrestrial Radiation by the Earths
Surface

• Most of the terrestrial radiation
• (approximately 86 on average) emitted by
• the surface is absorbed by gases in the
• atmosphere.

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Terrestrial Radiation
Methane
Nitrous oxide
Ozone, oxygen
Carbon dioxide
Water vapor
Sum of all gases
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Absorption of Terrestrial Radiation

• Water vapor is an excellent absorber of
  terrestrial radiation in wavelengths less than
  7x10-6 m.
• Carbon dioxide is an excellent absorber of
  terrestrial radiation in wavelengths longer than
  12x10-6 m.
• Methane is an excellent absorber of terrestrial
  radiation in wavelengths near
• 7x10-6 m.

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Atmospheric Window

• The band of wavelengths between 7x10-6 m
• and 12x10-6 m is sometimes called the
• atmospheric window because few gases in
• the atmosphere absorb terrestrial radiation
• in these wavelengths very well.
• Thus, terrestrial radiation emitted in these
• wavelengths is likely to leave the Earths
• atmosphere when the sky is clear.

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Greenhouse Effect

• The absorption of terrestrial radiation by the
  gases in the Earths atmosphere is sometimes
  called the greenhouse effect.

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Greenhouse and Radiation

• Solar radiation passes easily through the glass
  used in the walls and roof of a greenhouse.
• Glass is a good absorber of terrestrial radiation
  emitted inside the greenhouse.

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The walls and sides of the greenhouse keep the
cooler outside air from mixing with the warmer
air inside the greenhouse.
Solar radiation easily passes through the glass
in the greenhouse.
Terrestrial radiation is absorbed by the glass
and the energy stays inside the greenhouse.
Temperature stays warm
Solar radiation is absorbed and stored as
internal energy inside greenhouse increasing the
temperature
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The Earths Greenhouse Effect

• Roughly half the solar radiation passes through
  the atmosphere and is absorbed by the Earths
  surface.
• Approximately 86 of the terrestrial radiation
  emitted by the surface is absorbed by gases in
  the atmosphere and only 14 escapes to space.

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Incoming solar radiation
Terrestrial radiation reaching space
Terrestrial radiation absorbed by the atmosphere.
Terrestrial radiation emitted by the atmosphere
back to the surface of the Earth
The Earths greenhouse effect occurs when
terrestrial radiation emitted by the surface is
absorbed in the atmosphere and then is radiated
back to the surface as terrestrial radiation
emitted by the atmosphere.
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Effect of Clouds on Terrestrial Radiation

• The liquid water droplets and ice crystals in
  clouds are excellent absorbers of terrestrial
  radiation.
• Even thin clouds are capable of absorbing a lot
  of terrestrial radiation that would otherwise be
  emitted out to space.

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The Effect of Clouds on Terrestrial Radiation
(Cont.)

• Clouds greatly enhance the greenhouse effect.
• Clouds absorb almost all terrestrial radiation
  emitted by the surface and then radiate the
  energy back to the surface.

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The Effect of Clouds on Terrestrial Radiation
(Cont.)

• The effect of clouds on terrestrial radiation has
  the biggest effect on temperature at night,
  especially in the winter.

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Cloudy Sky
Clear Sky
Terrestrial radiation is absorbed and radiated
back to the surface.
28F
10F
More energy is lost to space on clear nights and
the temperature decreases more.
Much less energy is lost to space on cloudy
nights and the temperature decrease less.
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Terrestrial Radiation Emitted by the Atmosphere

• The Earths atmosphere also emits terrestrial
  radiation.
• Most of the radiation is rapidly absorbed by
  surrounding gas molecules.
• The lower atmosphere emits terrestrial radiation
  back to the surface as part of the greenhouse
  effect.

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Terrestrial Radiation Emitted by the Atmosphere
(Cont.)

• Gas molecules and clouds emit terrestrial
  radiation upwards which is lost to space.
• The emission of terrestrial radiation to space is
  the major way the Earth and its atmosphere lose
  energy.

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Types of GOES Images

• Visible images
• Infrared images
• Water vapor images

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Visible Satellite Imagery

• Visible satellite imagery is constructed from the
  solar radiation reflected by an object to the
  sensor on the satellite. Objects that reflects a
  lot of visible light (i.e. have a high albedo)
  appear bright. These objects include fresh snow
  on the ground and thick clouds like cumuli and
  cumulonimbi.

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Cumuliform Cloud High Albedo
Snow
High Albedo
Bare soil (low albedo)
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SNOW
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Infrared Satellite Imagery

• Infrared satellite imagery is constructed from
  the terrestrial radiation in the longwave band 1.
  Cold objects, such as the tops of cumulonimbi or
  cirrus clouds, emit less radiation. Generally,
  cold objects, are depicted as bright regions on
  infrared images. Warm objects, such as the
  Earths surface or low clouds, emit more
  radiation and are generally depicted as darker
  areas on infrared satellite images.

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Cirrus T -50C
Cloud top T -45C
Cumulo- nimbus
Surface T 20C
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Snow
Low clouds
High clouds
Tall Thunderstorm clouds
High Surface Temperatures
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Water Vapor Imagery

• Water vapor imagery is constructed from emitted
  radiation in the upper level water vapor band.
  Regions of the upper troposphere with higher
  concentrations of water vapor emit more infrared
  (longwave) radiation in this band.

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Water Vapor Imagery (Cont.)

• Generally, more humid areas in the upper
  troposphere appear as bright regions on water
  vapor images and drier areas appear as darker
  regions.
• Looping of water vapor images provides an
  indication of the flow of air in the upper levels
  of the troposphere.

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