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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 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 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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- The single factor that determine how much energy is emitted by a
blackbody is its temperature.
- The intensity of energy radiated by a blackbody increases according to
the fourth power of its absolute temperature.
- This relationship is called the Stefan-Boltzmann Law.
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- Sun
- Es = (5.67 x 10-8
W/m2 K4) * (6000K)4
- = 73,483,200 W/m2
- Earth
- Ee = (5.67 x 10-8
W/m2 K4) * (300K)4
- = 459 W/m2
- 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.
- č 204 = 160,000
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- Distance from the Sun
- Albedo
- Greenhouse effect
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- On Venus č 510°K (very large!!)
- On Earth č 33°K
- On Mars č 6°K (very small)
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- Venus is very 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 liquid water left on Venus
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- Saturation vapor pressure describes how much water vapor is needed to
make the air saturated at any given temperature.
- Saturation vapor pressure depends primarily on the air temperature in
the following way:
- č
- Saturation pressure increases exponentially
with air temperature.
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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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- Solar and terrestrial radiations
are emitted at very different wavelengths.
- The greenhouse gases selectively
absorb certain frequencies of radiation.
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- The single factor that determine how much energy is emitted by a
blackbody is its temperature.
- The intensity of energy radiated by a blackbody increases according to
the fourth power of its absolute temperature.
- This relationship is called the Stefan-Boltzmann Law.
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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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- Radiation energy comes in an infinite number of wavelengths.
- 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.
- The sun radiates more energy than the Earth.
- 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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- Solar radiation is often referred to as “shortwave radiation”.
- 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.
- 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.
- 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 very strong downward emission of terrestrial radiation from the
atmosphere is crucial to main the relatively small diurnal variation of
surface temperature.
- 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.
- č a large diurnal variation of surface
temperature.
- 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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- Absorption
- - convert insolation to heat
the atmosphere
- Reflection / Scattering
- - change the direction and
intensity of insolation
- Transmission
- - no change on the direction
and intensity of insolation
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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.
- Scattering: light is split into a larger number of rays, traveling in
different directions.
- Although scattering disperses light both forward and backward
(backscattering), more energy is dispersed in the forward direction.
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- Involves gases, or other scattering agents that are smaller than the
energy wavelengths.
- Scatter energy forward and backward.
- Violet and blue are scattered the most, up to 16 times more than red
light.
- Responsible for (1) blue sky in clear days, (2) blue tint of the
atmosphere when viewed from space, (3) why sunsets/sunrises are often
yellow, orange, and red.
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- Short wavelengths (blue and violet) of visible light are scattered more
effectively than longer wavelengths (red, orange). Therefore, when the
Sun is overhead, an observer can look in any direction and see
predominantly blue light that was selectively scattered by the gases in
the atmosphere.
- At sunset, the path of light must take through the atmosphere is much
longer. Most of the blue light is scattered before it reaches an
observer. Thus the Sun appears reddish in color.
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- Larger scattering agents, such
as suspended aerosols, scatter energy only in a forward manner.
- Larger particles interact with
wavelengths across the visible spectrum.
- Produces hazy or grayish skies.
- Enhances longer wavelengths
during sunrises and sunsets, indicative of a rather aerosol laden
atmosphere.
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- Water droplets in clouds,
typically larger than energy wavelengths, equally scatter wavelengths
along the visible portion of the spectrum.
- Produces a white or gray
appearance.
- No wavelength is especially
affected.
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- The amount of energy absorbed and emitted by Earth changes
geographically and seasonally.
- 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.
- Latitudinal variations: the variations of solar energy in latitude is
caused by changes in:
- (a) the angle the sun hits
Earth’s surface = solar zenith angle
- (b) the number of day light
hours
- (c) albedo
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- Solar zenith angle is the angle at which the sunlight strikes a
particular location on Earth.
- 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.
- When the zenith angle is large, sunlight has to pass through a thicker
layer of the atmosphere before it reaches the surface.
- 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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- The difference between the daily maximum and minimum temperature is
called the daily (or diurnal) range of temperature.
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- The diurnal cycle (the daily range of temperature) is greatest next to
the ground and becomes progressively small as we move away from the
surface.
- The diurnal cycle is also much larger on clear day than on cloudy ones.
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- Sunlight warms the ground and the
air in contact with the ground by conduction.
- Air is a poor heat conductor, so
this heating is limited to a layer near the surface. Air temperatures
above this layer are cooler.
- Wind stirring can reduce this
vertical difference in air temperatures.
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- Both the ground and air above
cool by radiating infrared energy, a process called radiational cooling.
- The ground, being a much better radiator than air, is able to cool more
quickly.
- Shortly after sunset, the earth’s surface is cooler than the air
directly above.
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- The thermometer has to be mounted 1.52m (5 ft) above the ground.
- The door of the instrument shelter has to face north in Northern
Hemisphere.
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