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Virtually all the exchange of energy between the
Earth and the rest of the universe takes place by radiation transfer. |
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Radiation transfer is also a major way of energy transfer
between the atmosphere and the underlying surface and between different
layers of the atmosphere. |
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Radiation energy comes in an infinite number of
wavelengths. |
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We can divide these wavelengths into a few
bands. |
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All objectives radiate energy, not merely at one
single wavelength but over a wide range of different wavelengths. |
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The sun radiates more energy than the Earth. |
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The greatest intensity of solar energy is
radiated at a wavelength much shorter than that of the greatest energy
emitted by the Earth. |
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The single factor that determine how much energy
is emitted by a blackbody is its temperature. |
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The intensity of energy radiated by a blackbody
increases according to the fourth power of its absolute temperature. |
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This relationship is called the Stefan-Boltzmann
Law. |
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Sun |
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Es
= (5.67 x 10-8 W/m2 K4) * (6000K)4 |
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= 73,483,200 W/m2 |
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Earth |
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Ee
= (5.67 x 10-8 W/m2 K4) * (300K)4 |
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= 459 W/m2 |
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Sun emits about 160,000 times more radiation per
unit area than the Earth because Sun’s temperature is about 20 times higher
than Earth’s temperature. |
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č 204
= 160,000 |
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Wien’s law relates an objective’s maximum
emitted wavelength of radiation to the objective’s temperature. |
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It states that the wavelength of the maximum
emitted radiation by an object is inversely proportional to the objective’s
absolute temperature. |
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Sun |
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lmax
= 2898 mm K / 6000K |
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= 0.483 mm |
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Earth |
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lmax
= 2898 mm K / 300K |
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= 9.66 mm |
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Sun radiates its maximum energy within the visible portion of the
radiation spectrum, while Earth radiates its maximum energy in the infrared
portion of the spectrum. |
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The hotter the objective, the shorter the
wavelength of the peak radiation. |
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The
Planck function relates the intensity of radiation from a blackbody to its
wavelength. |
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Solar radiation is often referred to as
“shortwave radiation”. |
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Terrestrial radiation is referred to as
“longwave radiation”. |
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The atmosphere is not a perfect blackbody, it
absorbs some wavelength of radiation and is transparent to others (such as
solar radiation). č Greenhouse effect. |
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Objective that selectively absorbs radiation
usually selectively emit radiation at the same wavelength. |
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For example, water vapor and CO2 are strong
absorbers of infrared radiation and poor absorbers of visible solar
radiation. |
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Radiation energy is absorbed or emitted to
change the energy levels of atoms or molecular. |
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The energy levels of atoms and molecular are
discrete but not continuous. |
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Therefore, atoms and molecular can absorb or
emit certain amounts of energy that correspond to the differences between
the differences of their energy levels. |
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č Absorb or emit at selective frequencies. |
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The energy of a molecule can be stored in (1) translational (the gross movement of
molecules or atoms through space), (2) vibrational, (3) rotational, and (4)
electronic (energy related to the orbit) forms. |
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The most energetic photons (with shortest
wavelength) are at the top of the figure, toward the bottom, energy level
decreases, and wavelengths increase. |
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The shorter the wavelength of the radiation, the
larger the amount of energy carried by that radiation. |
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What happens to incoming solar radiations? |
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Absorption |
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Reflection |
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Scattering |
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Transmission (through the atmosphere) |
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Reflection: light bounces back from an objective
at the same angle at which it encounters a surface and with the same
intensity. |
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Scattering: light is split into a larger number
of rays, traveling in different directions. |
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Although scattering disperses light both forward
and backward (backscattering), more energy is dispersed in the forward
direction. |
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We assume the atmosphere is opaque for longwave
radiation and transparent to shortwave radiation. |
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We divide the atmosphere into many layers. |
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We assume energy is balance at each atmospheric
layers. |
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We can determine the temperature of each
atmospheric layers. |
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The radiative equilibrium temperature calculated
from the energy balance model is hydrostatically unstable. (meaning the
lapse rate is larger than the dry adiabatic lapse rate). |
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As a result, convections occur. |
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č The atmosphere becomes stable with a
radiative-convective equilibrium temperature. |
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By measuring the amounts of solar and infrared
radiation, satellites can give us information of the atmosphere, such
as temperature, cloudiness, water
vapor,.. |
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There are two types of weather satellites: |
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GEO (Geostationary Earth Orbit) satellites |
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LEO (Low Earth Orbit) satellites |
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GEO satellites orbit the earth as fast as the
Earth spins. |
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They hover over a single point above the Earth
at an altitude of about 36,000 km. |
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To maintain their positions, GEO satellites must
stay over the equator. |
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They have continuous high-quality views of the
tropics and mid-latitudes. |
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They have a poor view of the polar regions. |
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Satellite instruments measure radiation that the
Earth and the atmosphere reflect, scatter, transmit, and emit. |
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These instruments are called radiometers. |
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There are two types of radiometers: |
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(1) Visible: measure the amount of visible light
from the Sun that is reflected back to space by the earth’s surface or by
clouds. |
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(2) Infrared: measures the amount of infrared
radiation emitted by Earth’s surface or clouds. |
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A visible satellite image represents sunlight
scattered or reflected by objectives on Earth. |
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Dark areas represent geographic regions where
small amounts of visible light from the Sun are reflect back to space, such
as oceans. |
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White areas represent snow or clouds. |
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The infrared radiometers on satellites measure
radiations with wavelengths between 10-12 micrometers (the “atmospheric
window”). |
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The infrared radiometers measure HEAT. č They
provides information on the temperature of land, water, and clouds. |
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Cold objectives are white and hot surfaces are
black. |
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