|
|
|
|
Solar Energy Absorbed = Terrestrial Energy
Emitted |
|
|
|
|
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 |
|
|
|
|
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 |
|
|
|
|
|
|
|
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. |
|
|
|
|
|
|
|
|
Distance
from the Sun |
|
Albedo |
|
Greenhouse effect |
|
|
|
|
|
|
On Venus
è 510°K (very large!!) |
|
On
Earth è 33°K |
|
On
Mars è 6°K (very small) |
|
|
|
|
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 |
|
|
|
|
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!! |
|
|
|
|
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. |
|
|
|
|
|
A
portion of the longwave spectrum can pass through the atmosphere unimpeded. |
|
This
range of wavelengths, 8-15μm, match those radiated with greatest
intensity by the Earth’s surface. |
|
This
range of wavelengths not absorbed is called the atmospheric window. |
|
|
|
|
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. |
|
|
|
|
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. |
|
|
|
|
The most energetic photons (with shortest
wavelength) are at the top of the figure, toward the bottom, energy level
decreases, and wavelengths increase. |
|
|
|
|
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 |
|
|
|
|
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. |
|
|
|
|
|
Involves gases, or other scattering agents that
are smaller than the energy wavelengths. |
|
Scatter energy forward and backward. |
|
Partial to shorter wavelength energy, such as
those which inhabit the shorter portion of the visible spectrum. |
|
Responsible for (1) blue sky in clear days, (2)
blue tint of the atmosphere when viewed from space, and the redness of
sunsets and sunrises. |
|
|
|
|
|
|
Short wavelengths (blue and violet) of visible
light are scattered more effectively than are 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. |
|
|
|
|
|
Radiation energy comes in an infinite number of
wavelengths. |
|
We can divide these wavelengths into a few
bands. |
|
|
|
|
|
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. |
|
|
|
|
|
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. |
|
|
|
|
|
Atmospheric reflection averages 25 units, 19 of
which are reflected to space by clouds and 6 units which are back-scattered
to space from atmospheric gases. |
|
The atmosphere absorbs another 25 units. |
|
Remaining 50 units are available for surface
absorption and reflection. |
|
|
|
|
|
|
|
Polarward heat flux is needed to transport
radiation energy from the tropics to higher latitudes. |
|
|
|