Solar Radiation, Earth's Atmosphere, and
the Greenhouse Effect.
Take away ideas and understandings:
- You should understand the different ways radiant energy can interact
- How energy is transferred from radiant energy to matter and visa versa.
- What processes control the shape of the vertical temperature profile of
the atmosphere between the ground surface and 100 km above the surface.
- Why the sky is blue and Sunsets are red.
- Radiant energy can interact with matter in three extreme modes. Most often
its behavior is a combination of two or more of these modes, but for the sake
of explanation we will look at them individually. If matter does not interact
with the incident radiation, that is, there is no change in the matter because
of the radiant energy that strikes it and it does not let the energy pass
through it (i.e. it is opaque to the radiant energy), then it reflects
the energy. Reflection only changes the direction of the beam of radiant energy,
not its wavelength or amplitude. If matter allows radiant energy to pass through
it unchanged, the matter is described as transparent to the
incident radiation. Again, as with reflection, there is no change in any of
the properties of the radiant energy. On the other hand, if there is some
interaction between the incident radiation and the matter (eg. some energy
is transferred from the radiant beam to the matter resulting in an increase
in molecular energy of the matter), then we describe this transfer of energy
from the radiant beam to the matter as absorption.
- Black bodies by definition absorb and re-radiate radiant energy equally and
completely at all wavelengths they intercept. Thus, as a perfect reflector
reflects all radiation it intercepts, and a perfectly transparent substance
transmits unchanged all radiant energy striking it, so a black body absorbs
all radiation that it intercepts. Gases on the other hand are not black bodies;
they absorb and re-radiate only at very specific wavelengths. Our atmospheric
gases absorb different narrow bands of incoming solar radiation. Each absorption
band is a response to a different mechanism of energy transfer. The smaller
molecules of oxygen and nitrogen absorb very short wavelengths of solar radiation
while the larger molecules of water vapor and carbon dioxide absorb primarily
longer infrared radiant energy.
II. Earth's limb (Figures 2, 3, 4).
III. Effect of Earth's atmosphere on incoming solar radiation.
- Reflectivity: 35% of incoming solar radiation
is reflected back to space).
- Clouds; Twenty-four percent of incoming solar radiation is reflected
by clouds, 4% by the Earth's surface.
- Scattering; Seven percent of incoming solar radiation is scattered back
to space. Particles in the atmosphere can scatter incoming solar radiation.
This process works as follows: a particle momentarily traps some part of the
solar spectrum that strikes it and then releases that same energy in all
directions. Consequently one half of the radiation scattered is returned to
space and the other half is sent down to the Earth's surface.
The wavelengths scattered depends on the size of the scattering particle.
Haze and smog particles are relatively large and they scatter all wavelengths.
The presence of particles of smog and haze (small water droplets) gives the
sky a milky appearance. Contrast the color of the sky on a hot humid summer
day with its appearance on a cold clear winter day.
Small particles, such as air molecules (molecules of nitrogen or oxygen),
scatter a larger proportion of short wavelength light (blue and violet)
rather than longer wavelengths (red). This preferential scattering of
blue light is what gives the sky its blue color. This effect is also
responsible for red Sunsets. At Sunset, if you look directly at the
Sun, the Suns rays have traveled through a much greater thickness of
the Earth's atmosphere than they do when the Sun is directly overhead
at noon. Consequently, because of the preferential scattering of blue
light by the atmosphere, only red and yellow light reach your eyes,
hence red Sunsets.
- Absorption: about 17% of incoming solar radiation is absorbed at various
levels in the atmosphere. While reading this refer to figures 6 and figure 7.
Absorption is the process by which radiant energy is transferred
to matter. If the matter is a gas, radiation can effect it in a number of
ways. The ways it can absorb energy depends on the size and complexity of
the gas molecule. The gas molecule can be rotated and a variety of vibratory
modes can be excited depending on the nature of the molecule. If the energy
is strong enough the molecule can be broken apart. Each mode of energy absorption
occurs at a specific narrow band of the solar spectrum. Gases, therefore,
are not like black bodies that absorb equally and completely at all wavelengths.
Rather, they absorb only at specific, often narrow ranges of wavelengths.
Diatomic molecules such as nitrogen and oxygen (most of our atmosphere)
can absorb energy by increasing the vibration of the bond between the two
atoms. If the energy absorbed is great enough it may break the bond resulting
in two free wheeling oxygen or nitrogen atoms traveling at high speeds.
O2 + ultraviolet light = O + O
This occurs in the uppermost regions of the atmosphere, above one hundred
kilometers (refer to figure 8). The most energetic (shortest
wavelength) part of the solar spectrum is involved in this process. Nitrogen
absorbs only in the extreme ultraviolet of which there is very little
in the Sun's radiation. Oxygen absorbs more strongly than nitrogen and
over a wider range of wavelengths in the ultraviolet. Oxygen molecules
are therefore broken into oxygen atoms in the highest regions of the atmosphere.
By an altitude of about 100 kilometers much of the radiation that is energetic
enough to do this breaking of molecular bonds is used up and this process
diminishes. Hence their is heating of the uppermost atmosphere (fast moving
atoms of nitrogen and oxygen) and as the altitude decreases to about one
hundred kilometers the atmosphere cools. For some distance above and below
80 kilometers there is little absorption of solar energy and consequently
little heating of the atmosphere so the temperature reaches a minimum.
Descending below eighty kilometers the atmosphere is heated by another
process. Here the atmosphere gets denser (thicker) with decreasing altitude;
the molecules of oxygen and nitrogen are closer together. Now if the bond
of an oxygen molecule is broken and the two atoms go flying off, there
is a higher likelihood that one of these atoms will strike an oxygen molecule.
If it does it may form an ozone molecule. Above 50 kilometers the heating
is primarily due to the break up of oxygen molecules by ultraviolet radiation
with wavelengths between .12 and .18 microns, while between 50 kilometers
and 10 kilometers the heating is due to the absorption by ozone of ultraviolet
radiation with wavelengths between .18 and .34 microns.
O + O2 = O3
Ozone can in turn be broken up by ultraviolet light resulting in this
O3 + ultraviolet light = O2
Both the breaking up of oxygen molecules above fifty kilometers and ozone
molecules at fifty kilometers and below causes heating of the atmosphere
that peaks at about 50 kilometers (the stratopause). Between 50 and 10-15
kilometers (the stratosphere) the solar energy energetic enough to break
up ozone (ultraviolet radiation) is used up and the atmosphere cools.
IV. Effect of atmosphere on Earth radiation.
- Below ten kilometers the atmosphere warms in a linear way to the Earth's
surface. This final heating is dominated not by solar radiation but rather
by radiation from the Earth's surface. The Earth, being much cooler than the
Sun, emits much longer wavelength radiation. At a solar temperature of nearly
6000 Kelvin, the peak of solar output is in the visible (light) part of the
electromagnetic spectrum while the Earth, at a temperature of 278 Kelvin,
emits most of its energy in the infrared (heat) portion of the electromagnetic
spectrum (figure 7). Energy of this
wavelength cannot be absorbed by tiny oxygen and nitrogen molecules, but it
can be absorbed by the larger and more complex molecules of water vapor (H2O)
and carbon dioxide (CO2). These complex molecules have a number
of vibratory and rotational modes which absorbs energy in the infrared portion
of the electromagnetic spectrum (figure
7). These together with other so called greenhouse gasses (methane CH4,
and nitrous oxide N2O) cause the Troposphere to warm as the Earth's
surface is approached (greenhouse effect). Man can and probably is enhancing
this effect by adding to the atmospheric concentration of these greenhouse
gasses. Most notably we are enhancing carbon dioxide, which has natural sources
in the decay of plant and animal remains, through the burning of fossil carbon.
Methane, which has natural sources in swamps and grazing ruminant animals,
is being enhanced through the proliferation of rice paddies (artificial swamps)
and cattle herds (cattle burp methane, more on that later). The atmosphere,
heated by the absorption of Earth radiation by these greenhouse gasses, in
turn radiates heat back to the Earth's surface increasing the Earth's surface
- Transparency: The Earth's atmosphere is effectively transparent to solar
radiation between .34 and .7 microns. Consequently 22.5 percent of incoming
solar radiation goes directly to the surface of the Earth and is absorbed.
- Transfer of radiation through a planet's atmosphere.
- A planet and its atmosphere, in our solar system, can radiate back to space
only as much energy as it absorbs from incoming solar radiation.
The solar constant x (1 - the albedo)
- The amount of radiation it radiates back to space is determined by
its effective temperature.
Planetary radiation = σTe4
- The wavelength of this planetary radiation is determined by the
planet's effective temperature.
λ = a/Te Wein's
- How strongly a planet's atmosphere absorbs the planet's radiation
depends on the optical properties of its atmosphere and the nature of
the planet's radiation.
- A planet's atmosphere has a bottom and a top; it radiates both
upward and downward.
- Some atmosphere's are so strongly absorbing that some fraction of the
atmospheric thickness can absorb all radiation received from the ground
or adjacent atmospheric layers. From now on we will assume the planet
does not absorb any incoming solar radiation i.e. the atmosphere is
transparent to it.
- We can divide such a planet's atmosphere into a series of layers,
each layer being just thick enough to absorb all the radiation
received from adjacent layers. In this kind of atmosphere, each
layer is heated by the overlying and underlying layers or the ground
below the lowest layer (figure
- The topmost layer must radiate to space the same amount of energy
it receives from the Sun. If this were not the case and the topmost
layer radiated less, the planet would heat up, or if it radiated
more, the planet would cool off. As a consequence, the planet's
temperature will change until it radiates as much as it receives.
- The topmost layer will radiate downward as much energy as it radiates
upward. Consequently, it must receive from the layer below, or the
ground, if it is the lowermost layer, twice as much energy as it radiates
to space (remember, we are assuming this atmosphere is transparent
to incoming solar radiation). Think about this. Lets assume that the
ground lies below the second layer. How much energy must this second
layer receive from the ground? Remember it is receiving energy from
the layer above and from the ground below. Keep in mind the conservation
of energy law; it makes the problem a lot easier than if energy were
- Think about the surface temperature of a planet that has an
atmosphere so strongly absorbing that it can be divided into 5
|The budget of solar radiation is as
|Absorbed by atmosphere
|Scattered to the Earth from blue sky
|Scattered to the Earth from clouds
|Radiation going directly to Earth's surface