Announcements
I will go over homework assignments for the semester
during the next two classes.
TODAY: Writing assignment options + assignment lists
WEDNESDAY: Lab experiment demonstrations by TAs
FRIDAY: Lab experiment kits distributed
I am in need of three notetakers for students in the
course. If you would be willing to do this, please see
me after class or during office hours. You will receive
recognition for doing this from the DRC.
Summary of Lecture 5
The three modes of heat transfer are conduction, convection, and radiation.
Radiation is energy propagated by electromagnetic waves and is emitted by all
objects so long as they have a temperature.
The type of radiation an object emits is dependent on its temperature. The
smaller the wavelength of radiation, the greater the energy and the higher the
temperature. The entire range of radiation types is given by the EM spectrum.
The total radiant energy emitted by an object is described by the StefanBoltzmann law.
The wavelength of maximum radiation emission is described by Wien’s law.
Solar radiation, or shortwave radiation, is that which comes from the sun and is
most intense in the visible part of the spectrum.
Terrestrial radiation, or longwave radiation, is that which comes from the Earth
and is most intense in the infrared part of the spectrum.
1st Review question from last lecture
Given that the sun’s temperature is around 6000 K, approximately
what is the wavelength of maximum radiation emission?
A) 5 × 10-3 m
B) 5 × 10-4 m
C) 5 × 10-5 m
D) 5 × 10-6 m
E) 5 × 10-7 m
2nd Review question from last lecture
How much more energy per unit area is emitted from an object
at 200 K vs. an object at 400 K?
A) About 2 times more
B) About 4 times more
C) About 8 times more
D) About 16 times more
E) About 32 times more
NATS 101
Section 6: Lecture 6
The Greenhouse Effect and
Earth-Atmosphere Energy Balance
Survey question
Thinking about the issue of global warming, sometimes called the
'greenhouse effect,' how well do you feel you understand this
issue? Would you say very well, fairly well, not very well, or not at
all?
A) Very Well
B) Fairly Well
C) Not Very Well
D) Not at All
E) I don’t understand the question.
Gallup Poll
March 13-16, 2006
Thinking about the issue of global warming, sometimes
called the 'greenhouse effect,' how well do you feel you
understand this issue? Would you say very well, fairly well,
not very well, or not at all?
Very Well 21%
Fairly Well 53%
Not Very Well 20%
Not at All 6%
My comments as an atmospheric
scientist:
Are the greenhouse effect and global
warming really the same thing?
If not, then NOBODY understands much
of anything “well”—including whoever
wrote this question!!
FOUR POSSIBLE FATES OF RADIATION:
1.Transmitted
2. Reflected
3. Scattered
4. Absorbed
The atmosphere does ALL of these…
SUNLIGHT
GLASS WINDOW
Transmitted:
Radiation passes through object
TRANSMITTED
SUNLIGHT
REFLECTED
SUNLIGHT
SUNLIGHT
MIRROR
Reflected:
Radiation turned back
SUNLIGHT
FROSTED GLASS
Scattered:
Path of radiation deflected
SCATTERED
SUNLIGHT
SUNLIGHT
BLACK BOX
Absorbed:
Radiation transferred to object
Blackbody: a perfect
absorber and emitter
of radiation in
equilibrium, with no
reflection or scattering.
SUNLIGHT
(SHORTWAVE)
BLACK BOX
Radiative equilibrium:
Absorption = Emission (Kirchoff’s Law)
INFRARED
(LONGWAVE)
EMISSION
SUNLIGHT
(SHORTWAVE)
GREY BOX
A “Grey Body” = Not all radiation absorbed
How the atmosphere behaves
SOME
TRANSMISSION
OF SUNLIGHT
THROUGH BOX
INFRARED
(LONGWAVE)
EMISSION
What Happens When
Radiation is “Absorbed”?
Internal energy increases by changes on the molecular and atomic
levels
ENERGY TRANSITIONS:
Rotational
Vibrational
Electronic: molecular
Electronic: atomic
ENERGY REQUIRED
Translational
Less Energy required
Changes on molecular level
Longer wavelength of radiation
More energy required
Changes on the atomic level
Shorter wavelength of radiation
Translational Energy
Gross movement of atoms
and molecules through
space.
The translational energy
reflects the kinetic
energy—and thus the
temperature.
Rotational Energy
Energy associated the rotation of the molecule. Takes on
discrete values (or quanta) dependent on the type of molecule.
Corresponds to energy changes shorter than 1 cm (far infrared).
Rotation of water molecules
Infrared photon
(Gedzelman 1980, p 105)
Rotational Energy of
Common Gases in the Atmosphere
Molecules have
rotational energy only if
they have a permanent
dipole moment, or
asymmetric charge
distribution.
(Hartmann 1994)
Vibrational Energy
Molecular energy stored in the vibrations (or stretching and
bending) of atomic bonds. Takes on discrete values (or quanta)
dependent on the type of molecule.
Corresponds to energy changes in the infrared spectrum.
Vibration of water molecules
Infrared photon
(Gedzelman 1980, p 105)
Vibrational Energy for
Common Gases in the Atmosphere
Most effective
absorbers are
molecules that have a
dipole moment and/or
are bent.
Carbon dioxide
creates a dipole
moment as a result of
its vibrational
transitions, so has
rotational energy as
well.
(Hartmann 1994)
Electronic energy: Photodissociation
(Molecular level)
Energy associated with breaking of atomic bonds of molecules.
Takes on discrete values (or quanta) dependent on the type of
molecule.
Corresponds mainly to energy changes in the ultraviolet spectrum.
UV
PHOTON
O
O
O
Oxygen molecule (O2)
O
Oxygen atoms (O)
Electronic energy: Excitation
(Atomic Level)
Energy associated with excitation of electrons in the outer shell of an
atom. Takes on discrete values (or quanta) dependent on the type of
molecule.
Corresponds to energy changes mainly in the ultraviolet.
UV
Photon
ee-
p+
Hydrogen atom
p+
Excited hydrogen atom
Electronic energy: Ionization
(Atomic level)
Energy associated with removal of electrons from an atom. Takes
on discrete values (or quanta) dependent on the type of molecule.
Corresponds to energy changes mainly higher than ultraviolet.
Short wavelength UV
or X-Ray photon
ee-
p+
Hydrogen atom
p+
Ionized hydrogen atom
Flashback: Why is there warming in
Thermosphere and Stratosphere?
THERMOSPHERE:
Warming because of
ionization of gases.
inversion
isothermal
6.5oC/km
STRATOSPHERE:
Warming because of
photodissociation of
ozone and oxygen.
Aurora Borealis or Aurora Australis
Caused by ionization of gases in the Earth’s atmosphere from the
solar wind, which appears as visible light. Occur only at the
poles because of the properties Earth’s magnetic field
SOLAR
TERRESTRIAL
Absorption of Radiation by
Common Atmospheric
“Greenhouse” Gases
The “greenhouse gases” are
called such because they absorb
and emit very effectively in the
infrared band (5-20 µm) at
selective wavelengths.
Water and CO2 are good
greenhouse gases
Oxygen (O2) and ozone (O3) are
absorbed in the UV.
Little or no absorption in some
places
λmax Sun
λmax Earth
Total Radiation Absorption Spectrum
by all Atmospheric Gases
TERRESTRIAL
Absorption %
SOLAR
Little or no
absorption.
A lot of absorption
because of greenhouse
gases, except in an
atmospheric window of
8-11 µm.
Atmospheric
Heating From
Below
LATENT HEATING
SOLAR RADIATION
TERRESTRIAL
RADIATION
Since there is little
or no absorption of
radiation in the
most intense part
of the solar
radiation band, the
heat supplied
comes from the
Earth itself.
Is the presence of the greenhouse
gases in our atmosphere good or bad?
The Importance of the Greenhouse Effect
The presence of the gases in our atmosphere that absorb and
emit infrared radiation help maintain the Earth’s average
temperature at about 59 °F.
Without these gases, ALL infrared radiation from the Earth’s
surface would be lost to space and the Earth would be a frozen
ice ball!
The Greenhouse Effect
DOES NOT EQUAL
Global Warming or Climate Change!
Global warming: The increase in Earth’s mean temperature that
would result because of the increase in greenhouse gases due
to human activities. This would enhance the greenhouse effect.
Climate change: Long-term change in global, regional, or local
climate resulting from both enhanced greenhouse gases and/or
other human activities.
Greenhouse Effect: Venus, Earth, and Mars
VENUS
(Same size as Earth)
EARTH
MARS
(Half size of Earth)
Pressure = 90 Atm.
Atmosphere composed
of 96% CO2
Temperature = 482 °C
Pressure = 1 Atm.
Atmosphere composed
of less than 1% CO2
Temperature = 15 °C
GREENHOUSE EFFECT
ON STERIODS!
GREENHOUSE EFFECT
JUST RIGHT
Pressure = 0.01 Atm.
Atmosphere composed
of 95% CO2
Temperature = -63 °C
VIRTUALLY NO
ATMOSPHERE TO HAVE
A GREENHOUSE EFFECT
We’ve talked so far about absorption and
emission of radiation in the atmosphere.
This process is most important for
terrestrial radiation.
Reflection and scattering are more
important for solar radiation.
Scattering: Radiation deflected
Rayleigh Scattering
Atmospheric gases: Molecules
preferentially scatter the smaller
wavelengths of visible light.
Mie Scattering
Cloud drops: Scatter all
wavelengths of visible light
equally.
Why is the sky blue?
Air molecules preferentially scatter smaller wavelengths
of visible light, which are blue!
Reflection: Radiation turned back
In meteorology the reflectivity is called the albedo.
Albedo = 1  All radiation reflected
Albedo = 0  All radiation absorbed
Snow : 0.7
Water: 0.1
Vegetation: 0.1 to 0.3
Sand: 0.15 to 0.45
Clouds: Depends on
type and location
NET ALBEDO OF EARTH = 0.3
Important Aside:
Clouds are a big part of the
radiation budget too!
ABSORPTION AND
REFLECTION OF
SOLAR RADIATION
VERY EFFECTIVE
ABSORBERS AND
EMITTERS OF
TERRESTRIAL
RADIATION
Now that we know what happens to
radiation in the atmosphere,
we’re ready to look at
the Earth’s average energy budget
Energy Budget: Shortwave Radiation
INCOMING
SOLAR AT TOA
EARTH’S ALBEDO
ATMOSPHERIC
ABSORPTION
SURFACE SOLAR ABSORPTION
(ABOUT HALF OF TOTAL INCOMING)
Energy Budget: Longwave radiation
and Sensible and Latent Heating
ATMOSPHERIC ENERGY
GAINED FROM SURFACE
ATMOSPHERIC
LONGWAVE
EMISSION
TO SPACE
( = 1- ALBEDO)
ATMOSPHERIC
LONGWAVE
EMISSION
TO SURFACE
SURFACE LONGWAVE
SENSIBLE AND LATENT
SURFACE LONGWAVE
SURFACE SOLAR
EMISSION
HEATING
ABSORPTION:
ABSORPTION
SUM OF SHORTWAVE + LONGWAVE
GREENHOUSE
EFFECT
ABSORPTION
( > 100)
Summary of Lecture 6
Radiation in the atmosphere has four possible fates: transmitted, reflected,
scattered, or absorbed. A perfect absorber and emitter of radiation is called a
blackbody.
The atmosphere selectively reflects, scatters and absorbs radiation at certain
wavelengths, which depend on the specific gas constituents.
Absorbed radiation increases the internal energy by changes on the
molecular and atomic level. Terrestrial radiation is associated with
translational, rotational, and vibrational energy transitions on the molecular
level. Solar radiation is associated with electronic energy transitions on the
atomic level.
Greenhouse gases are those which absorb and emit very effectively in the
infrared, like water and CO2. Because of them the atmosphere is very opaque
to terrestrial radiation and the Earth’s surface temperature is maintained.
Reviewed the atmospheric energy budget to prove the point.
Though the atmosphere is fairly transparent to solar radiation, scattering and
reflection of solar radiation is important. Scattering of visible light is why the
sky is blue!
Reading Assignment and
Review Questions
Ahrens, Chapter 3, pp. 55-63
Chapter 2 Questions
Questions for Review: 11,12,13,14,17,18,19,21
Questions for Thought: 7,9,10,11,12,13