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Solar
Luminosity (L) |
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the
constant flux of energy put out by the sun |
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L = 3.9 x 1026 W |
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Solar
Flux Density (Sd) |
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the
amount of solar energy per unit area on a sphere centered at the Sun with a
distance d |
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Sd = L / (4 p d2) W/m2 |
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Solar
Constant (S) |
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The
solar energy density at the mean distance of Earth from the sun (1.5
x 1011 m) |
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S
= L / (4 p d2) |
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= (3.9 x 1026 W) / [4 x 3.14 x (1.5 x 1011 m)2] |
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= 1370 W/m2 |
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Solar
energy incident on the Earth |
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= total amount of solar
energy can be absorbed by Earth |
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= (Solar constant) x (Shadow Area) |
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= S x p R2Earth |
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The
Earth warms up and has to emit radiative
energy back to the space to reach a equilibrium condition. |
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The
radiation emitted by the Earth is called “terrestrial radiation” which is
assumed to be like blackbody radiation. |
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Blackbody |
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A
blackbody is something that emits (or absorbs) electromagnetic radiation
with 100% efficiency at all wavelength. |
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Blackbody Radiation |
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The
amount of the radiation emitted by a blackbody depends on the absolute
temperature of the blackbody. |
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Distance
from the Sun |
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Albedo |
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Greenhouse effect |
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On Venus
è 510°K (very large!!) |
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On
Earth è 33°K |
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On
Mars è 6°K (very small) |
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Venus is
very close to the Sun |
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Venus
temperature is very high |
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Very
difficult for Venus’s atmosphere to get saturated in water vapor |
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Evaporation keep on bringing water vapor into Venus’s
atmosphere |
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Greenhouse effect is very large |
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A “run
away” greenhouse happened on Venus |
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Water
vapor is dissociated into hydrogen and oxygen |
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Hydrogen
then escaped to space and oxygen reacted with carbon to form carbon dioxide |
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No water
left on Venus (and no more chemical weathering) |
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Mars is
too small in size |
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Mars had no large internal heat |
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Mars lost all the internal heat quickly |
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No tectonic activity on Mars |
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Carbon can not be injected back to the
atmosphere |
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Little greenhouse effect |
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A very cold Mars!! |
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The very strong downward emission of terrestrial
radiation from the atmosphere is crucial to main the relatively small
diurnal variation of surface temperature. |
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If this large downward radiation is not larger
than solar heating of the surface, the surface temperature would warm
rapidly during the day and cool rapidly at the night. |
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è a large
diurnal variation of surface temperature. |
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The greenhouse effect not only keeps Earth’s
surface warm but also limit the amplitude of the diurnal temperature
variation at the surface. |
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The amount of energy absorbed and emitted by
Earth geographically and seasonally. |
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Seasonal variations: the angle of inclination is
responsible for the seasonal variation in the amount of solar energy
distributed at the top of the atmosphere. |
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Latitudinal variations: the variations of solar
energy in latitude is caused by changes in: |
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(a)
the angle the sun hits Earth’s surface = solar zenith angle |
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(b)
albedo |
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(c)
the number of day light hours |
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Solar zenith angle is the angle at which the
sunlight strikes a particular location on Earth. |
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This angle is 0° when the sun is directly
overhead and increase as sun sets and reaches 90 ° when the sun is on the
horizon. |
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The larger the solar zenith angle, the weaker
the insolation, because the same amount of sunlight has to be spread over a
larger area. |
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The
larger the solar zenith angle, the larger the albedo. |
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When the zenith angle is large, sunlight has to
pass through a thicker layer of the atmosphere before it reaches the
surface. |
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The thinker the atmospheric layer, more sunlight
can be reflected or scattered back to the space. |
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Polarward heat flux is needed to transport
radiation energy from the tropics to higher latitudes. |
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Notes