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- Solar Luminosity (L)
- the constant flux of energy
put out by the sun
- L = 3.9 x
1026 W
- Solar Flux Density (Sd)
- the amount of solar energy
per unit area on a sphere centered at the Sun with a distance d
- Sd = L / (4 p d2) W/m2
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- Solar Constant (S)
- The solar energy density at
the mean distance of Earth
from the sun (1.5 x 1011
m)
- S = L / (4 p d2)
- = (3.9 x 1026
W) / [4 x 3.14 x (1.5 x 1011
m)2]
- = 1370 W/m2
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- Solar energy incident on the
Earth
- = total amount of solar energy can be
absorbed by Earth
- = (Solar constant) x (Shadow Area)
- = 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.
- The radiation emitted by the
Earth is called “terrestrial radiation” which is assumed to be like
blackbody radiation.
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- Blackbody
- A blackbody is something that
emits (or absorbs) electromagnetic radiation with 100% efficiency at all
wavelength.
- Blackbody Radiation
- The amount of the radiation
emitted by a blackbody depends on the absolute temperature of the
blackbody.
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- Solar and terrestrial radiations
are emitted at very different wavelengths.
- The greenhouse gases selectively
absorb certain frequencies of radiation.
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- Radiation energy comes in an infinite number of wavelengths.
- We can divide these wavelengths into a few bands.
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- Wien’s law relates an objective’s maximum emitted wavelength of
radiation to the objective’s temperature.
- 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
- lmax = 2898 mm K / 6000K
- = 0.483 mm
- Earth
- lmax = 2898 mm K / 300K
- = 9.66 mm
- 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 atmosphere is not a perfect blackbody, it absorbs some wavelength of
radiation and is transparent to others (such as solar radiation). è Greenhouse effect.
- Objective that selectively absorbs radiation usually selectively emit
radiation at the same wavelength.
- 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.
- The energy levels of atoms and molecular are discrete but not
continuous.
- Therefore, atoms and molecular can absorb or emit certain amounts of
energy that correspond to the differences between the differences of
their energy levels.
- è 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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- Distance from the Sun
- Albedo
- Greenhouse effect
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- On Venus è 510°K (very large!!)
- On Earth è 33°K
- On Mars è 6°K (very small)
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- Venus is too close to the Sun
- Venus temperature is very high
- Very difficult for Venus’s
atmosphere to get saturated in water vapor
- Evaporation keep on bringing
water vapor into Venus’s atmosphere
- Greenhouse effect is very large
- A “run away” greenhouse happened
on Venus
- Water vapor is dissociated into
hydrogen and oxygen
- Hydrogen then escaped to space
and oxygen reacted with carbon to form carbon dioxide
- No water left on Venus (and no
more chemical weathering)
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- Mars is too small in size
- Mars had no large internal heat
- Mars lost all the internal heat quickly
- No tectonic activity on Mars
- Carbon can not be injected back to the atmosphere
- Little greenhouse effect
- A very cold Mars!!
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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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- Polarward heat flux is needed to transport radiation energy from the
tropics to higher latitudes.
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40
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