Solar and terrestrial radiation
Earth: A Dynamic Planet A
Solar and terrestrial radiation
Aims
To understand the basic energy forms and principles of energy transfer
To understand the differences between short wave and long wave radiation.
To appreciate that the wavelength of radiation impacts the interactions of energy with
matter
Objectives
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To identify basic forms of energy
To distinguish heat and temperature
To state three mechanisms of heat transfer
To define specific heat
To be able to state the first two laws of thermodynamics
To describe the electromagnetic spectrum
To define reflection, transmission, absorption and emission
To be able to state the basic electromagnetic radiation laws
To distinguish between solar and terrestrial radiation
To give a suitable definition for the solar constant and a currently accepted value
To define planetary albedo and explain its significance
To give values for the shortwave albedo for some surface types
To discuss the role of clouds
To list the most important radiation absorbing gases in the atmosphere and their
wavelengths of absorption
Earth: A Dynamic Planet A, Lecture 12
Solar and terrestrial radiation
Outline
Introduction
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Solar radiation - the main energy supply to Earth
Energy and radiation
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Energy, energy forms
Heat, temperature
Radiation
Reflection, albedo, absorption, transmission, emission
Laws of thermodynamics
Radiation laws I
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Stefan-Boltzman law
The electromagnetic spectrum
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Electromagnetic spectrum
Some properties of light
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Diffuse light, scattering, refraction
Radiation laws II
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Planck’s law
Wien's displacement law
Incoming solar radiation and its absorption
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Spectrum
Albedo, Clouds, Planetary albedo
Solar radiation in the atmosphere
Emission and absorption of terrestrial radiation
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Long wave radiation emission and absorption
The atmospheric window of longwave radiation
Venus and Mars
Earth: A Dynamic Planet
Earth: A Dynamic Planet A
Solar and terrestrial radiation
Topics
• Basic energy forms, principles of energy transfer
• Wavelength of radiation.
• Interactions of radiation with matter
Outline
• Energy and radiation
• Laws of thermodynamics
• Radiation laws I
• The electromagnetic spectrum
• Some properties of light
• Emission, absorption, transmission
• Radiation laws II
• Incoming solar radiation and its absorption
• Emission and absorption of terrestrial radiation
Earth: A Dynamic Planet
Solar and terrestrial radiation
Bullets
Introduction
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Solar radiation is the main energy supply for the Earth system
What is radiation? How does it interact with matter?
How is it distributed within the Earth system, in particular the atmosphere?
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energy is the ability to do work
potential energy - energy due to position in a force field, e.g. water behind a dam, or
object at some height
kinetic energy - energy due to movement, weaker or harder collisions due to velocity
what is with a hot object?
internal energy of a macroscopic body - potential and kinetic energy of its molecules
heat is one way to change the internal energy of a body
temperature - “heat intensity” - average kinetic energy of the molecules
cup <-> bathtub
heat flux (an energy flux!) in direction of temperature difference
how does a car dashboard get hot? Radiation, another energy flux
radiation travels through vacuum, equally in all directions, intensity drops with the
square of the distance
mechanism how solar energy reaches Earth
objects can absorb radiation and heat up due do absorption
absorption increases kinetic energy of molecules
objects need to emit energy - otherwise they would heat up indefinitely
albedo, the reflectivity, is the fraction of radiation which is not absorbed
specific heat of matter, the amount of energy needed to heat up that matter by 1oK
water and soil have a very different specific heat - gives rise to local circulations - fog
1. law of thermodynamics - absorbed energy by a body at rest is either used to do
external work or to increase the internal energy - total energy is conserved.
2. law - heat flows occur along the temperature gradient
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electromagnetic spectrum
wavelength, frequency
UV, visible light, PAR, infrared, Colors
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scattering - reflection of light in various directions from little objects (gases, aerosols)
diffuse light - direct light
scattering can be wavelength dependent
clouds - cloud droplets of about 20μm scatter all wavelengths of visible light more or
less equally -> clouds white
Mie scattering - equal scattering
air molecules are selective scatterers - oxygen and nitrogen scatter shorter wavelengths better than longer ones
blue sky is blue because blue is best scattered and thus comes from all directions
sunsets are reddish because most of the blue has been scattered away due to longer
atmospheric path
if the atmosphere contains many fine particles which are a little larger than the air
molecules (aerosols, e.g. SO2 from volcano eruptions), then the yellow light is scattered away, too, and the sunsets are even more red
Laws of thermodynamics
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The electromagnetic spectrum
Some properties of light
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Earth: A Dynamic Planet
Solar and terrestrial radiation
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Raleigh scattering - selective scattering
light bends (changes direction) if it enters another substance at an angle - refraction
twinkling or flickering of lights at a distance results from that effect - as the light has to
pass through various layers of air with different densities and the movement of air.
Radiation laws II
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blackbody - absorbs all incident radiation - a perfect absorber and emitter
Stefan-Boltzman law
all objects emit energy - the hotter the more
solar constant, temperature of the sun
Planck’s, Wien's displacement law
Incoming solar radiation and its absorption
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at top of the atmosphere: 99% between 0.15μm and 4μm, 9%UV (λ<0.4μm), 49%
visible (0.8μm<λ<0.8μm), 42% infrared (λ>0.8μm)
comparison to blackbody radiator at ~6000oK
selective absorbers - absorb only at certain wavelengths O2, O3, H2O and CO2
glass - hot dashboard - green house
UV absorption within the atmosphere - O2 and O3 - of vital importance since they
absorb the high energy radiation from the sun which can do a lot of damage to life
Venus and Mars
planetary albedo, absorption by Earth, albedo of surface types
30% of solar radiation reflected, 19% absorbed by atmosphere, 51% by surface
Emission and absorption of terrestrial radiation
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heating of the lower atmosphere mainly through backradiation of Earth's surface and
absorption of long wave
hardly any heating of the troposphere by short wave
short wave <-> long wave
short wave albedo, long wave albedo
during days more sun light reflected from clouds - cooler
during nights more long wave radiation radiated back to the surface (water!) - warmer
the atmospheric window for longwave radiation
more CO2 might decrease long wave loss through the atmospheric window and thus
lead to a warmer atmosphere and consequently a warmer surface, too
delicate balance of absorption and emission maintains current climate; important to
understand it
Links
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http://cwx.prenhall.com/bookbind/pubbooks/aguado2/chapter2/deluxe.html
Radiation laws: http://csep10.phys.utk.edu/astr162/lect/light/planck.html
Heat and Change of Phase: http://fermi.bgsu.edu/~stoner/p201/heat/
Heat Transfer http://fermi.bgsu.edu/~stoner/p201/transfer/
Ideal Gases http://fermi.bgsu.edu/~stoner/p201/idealg/index.htm
Earth: A Dynamic Planet
Solar and terrestrial radiation
GY1003 - Earth: A Dynamic Planet A, Lecture 12, Jörg Kaduk
Solar and terrestrial radiation
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Energy and radiation
• Potential energy
energy due to location in a force field
• Kinetic energy
energy due to movement
(arth: A Dynamic Planet
Solar and terrestrial radiation
• Hot body?
internal energy: energy due to potential and
kinetic energy of molecules of a body
• Heat - internal kinetic energy
• Temperature - average internal kinetic energy
Heat capacity of matter: energy needed to heat up that matter by 1oK
Heat transfer
• Conduction: Heat flux due to direct contact of matter
• Convection: Heat transport by water vapor and warm air/water.
Energy Sun -> Earth ?
Gk
Solar and terrestrial radiation
Radiation
• another form of energy transfer
• travels through vacuum
• equally in all directions
• intensity drops with the square of the distance
• mechanism how solar energy reaches Earth
If radiation reaches a body it can be
• reflected
• transmitted
• absorbed
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Solar and terrestrial radiation
Interaction of radiation with matter
Reflection
albedo,
the reflectivity, is the
fraction of radiation
which is reflected back
Transmission
Radiation passes
through the body
without interaction
Absorption
matter can absorb radiation
heat up due do absorption
absorption increases kinetic energy of molecules
Everyday example?
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Solar and terrestrial radiation
Laws of thermodynamics
First law
Energy absorbed by a body at rest is either used to do external work
or to increase the internal energy - total energy is conserved.
Energy cannot be created nor can it be destroyed, only transformed.
Second law
Heat never passes from a cooler to a hotter body. In a body of uniform
temperature there will never be a spontaneous change in temperature.
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Solar and terrestrial radiation
Consequence
Earth receives Energy from the sun all the time, so:
First law: Earth would heat up all the time unless it loses energy
Loss must be to space - only possible via radiation
Emission
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objects need to emit energy - otherwise
they would heat up indefinitely
Solar and terrestrial radiation
• Blackbody
Radiation laws I
• absorbs all incident radiation
• a perfect absorber and emitter
Stefan-Boltzman law
For a Black body E energy flux (radiation) emitted from the object
T temperature
E=σT4
σ Stefan-Boltzmann Constant σ=5.67 x 10-8 Wm-2K-4
All objects emit energy - the hotter the more
• radiative equilibrium temperature
temperature at which radiative energy loss equals absorbed radiation
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Solar and terrestrial radiation
Sum up I
• Conduction, convection and radiation are energy transfers
• Radiation travels through the vacuum
• Reflection, transmission and absorption of radiation
• Heat is an expression of total internal energy
• Temperature is an expression of average internal energy
• Energy can only be transformed, never destroyed (1. Law)
• Absorbed energy is either used to do external work or to increase the
internal energy (1. Law)
• For a body at rest radiation absorption leads to heating (1. Law)
• All objects with positive temperature emit radiation - the hotter the more
• Black bodies - idealized absorbers - radiate energy according to the
Stefan Boltzman Law
• Do not confuse radiation, temperature and heat!
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Solar and terrestrial radiation
Earth’s surface temperature?
5 min activity
Get together in groups of four
Assume:
Radiation heating Earth, is 240 Wm-2.
Calculate the temperature of Earth.
Assume Earth behaves like a black body
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Solar and terrestrial radiation
Solution: The greenhouse effect I
Assume energy balance: energy input into earth = energy loss from Earth
Input and loss are by radiation as the energy transfer is through space.
Now one can use a law describing energy loss by radiation (since
input=output and input and output are via radiation):
Use Stefan Boltzmann law: E=σT4
T = (E/σ)1/4
= (240Wm-2/(5.67 x 10-8 Wm-2K-4))1/4
= 240/(5.67 x 10-8)1/4 K = (240/5.67)1/4 x (1/10-8)1/4 K = 42.330.25 x 102 K
= 2.55 x 100 oK = 255oK
Much too cold! The observed temperature is about 288oK! What’s wrong?
Atmosphere makes the difference!
How? Interaction of radiation with matter depends on the wavelength - or
“colour ” - of the radiation
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Solar and terrestrial radiation
The electromagnetic spectrum
Spectrum
visible light
long-wave radiation
short-wave radiation
Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere
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Solar and terrestrial radiation
Wave properties
Wavelength
Amplitude
Crest
Trough
Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere
Frequency = 1/wavelength
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Solar and terrestrial radiation
Sky of Mars
Viking 2 Lander image, NASA
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Solar spectrum at top of atmosphere
Intensity
Solar and terrestrial radiation
5785 deg. K blackbody
Wave length (μm)
Source: Houghton, H.G., 1985. Physical Meteorology. After data from Air Force Cambridge Research Laboratories, 1965. Handbook of Geophysics and Space.
GY
Solar and terrestrial radiation
Solar spectrum at top of atmosphere and Earth’s surface
Gases are
selective
absorbers
Source: Peixoto, J.P. and A.H. Oort, 1992. Physics of Climate. After: Gast (1965)
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Solar and terrestrial radiation
Black Body curves of Sun and Earth - Atmospheric Radiation absorption
6000oK
255oK
solar,
short-wave
terrestrial,
long-wave
Wave length (μm)
Source: Peixoto, J.P. and A.H. Oort, 1992. Physics of Climate. After: Goody (1964), Howard et al. (1955), Fels und Scharzkopf (1988)
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Solar and terrestrial radiation
Radiation laws II
Wien's displacement law
For a black body is the product of the wavelength of maximal emission
and temperature constant:
λmaxT = A = const = 2898 μm K
Planck’s law - determines the “black body curves”
The intensity of radiation of wavelength λ emitted by a black body is:
2
2hc
B λ ( T ) = -------------------------------------hv⎞
⎛
-----exp - – 1
⎝ kT⎠
c speed of light
h Planck constant, h=6.63x10-34 J s,
k Boltzman constant, k=1.38x10-23 J K-1
The energy of light of frequency ν is given by: E=hν
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Solar and terrestrial radiation
Venus and Mars
Some physical properties of Earth and its neighbours
Property
Venus
Earth
Mars
Rel. mass of atmosphere
100
1
0.06
Distance from Sun (106km)
108
150
228
Solar constant (Wm-2)
2613
1370
589
Albedo (%)
75
30
15
Cloud cover (%)
100
50
variable
Radiative temperature (oC)
-39
-18
-56
Surface temperature (oC)
427
15
-53
Greenhouse effect (oC)
466
33
3
N2 (%)
<2
78
<2.5
O2 (%)
<1 ppmv
21
<0.25
CO2 (%)
>98
0.036
>96
H2O (%)
1x10-4-0.3
4x10-4-3
<0.0001
After: Graedel, T.E. and P.J. Crutzen, 1993. Atmospheric change. An Earth System Perspective
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Solar and terrestrial radiation
Summary
• Conduction, convection and radiation are energy transfers
• Radiation travels through the vacuum
• Interaction of radiation with matter depends on the matter
• Energy can only be transformed, never destroyed; absorbed energy is
either used to do external work or to increase the internal energy (1. Law)
• All objects with positive temperature emit radiation - the hotter the more
• Black bodies emit energy according to the Stefan Boltzman Law
• Wavelength, frequency, colour, Wien’s law
• Interaction of radiation with matter depends on wavelength of the radiation
• Gases are selective absorbers
• Atmosphere absorbs some solar but most terrestrial radiation -> warming of the atmosphere - Greenhouse effect
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