Earth: A Dynamic Planet A Solar and terrestrial...

Solar and terrestrial radiation

Earth: A Dynamic Planet A


AimsTo understand the basic energy forms and principles of energy transferTo 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• 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


Outline

Introduction• Solar radiation - the main energy supply to Earth

Energy and radiation• Energy, energy forms• Heat, temperature• Radiation• Reflection, albedo, absorption, transmission, emission

Laws of thermodynamics

Radiation laws I• Stefan-Boltzman law

The electromagnetic spectrum• Electromagnetic spectrum

Some properties of light• Diffuse light, scattering, refraction

Radiation laws II• Planck’s law• Wien's displacement law

Incoming solar radiation and its absorption• Spectrum• Albedo, Clouds, Planetary albedo• Solar radiation in the atmosphere

Emission and absorption of terrestrial radiation• Long wave radiation emission and absorption• The atmospheric window of longwave radiation• Venus and Mars

Earth: A Dynamic Planet

Earth: A Dynamic Planet A


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



BulletsIntroduction• 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?Laws of thermodynamics• 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 gradientThe electromagnetic spectrum• electromagnetic spectrum• wavelength, frequency• UV, visible light, PAR, infrared, ColorsSome properties of light• 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 wave-

lengths 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 scat-tered away, too, and the sunsets are even more red



• 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• 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• 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• 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• 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



GY1003 - Earth: A Dynamic Planet A, Lecture 12, Jörg Kaduk

n a force field

t

Solar and terrestrial radiation E


Energy and radiation

• Potential energy

• Kinetic energy

energy due to location i

energy due to movemen

p that matter by 1oK

atter

warm air/water.

due to potential and les of a body


Gk

• Hot body?

• Heat - internal kinetic energy

• Temperature - average internal kinetic energy

Heat capacity of matter: energy needed to heat u

Heat transfer• Conduction: Heat flux due to direct contact of m

• Convection: Heat transport by water vapor and

Energy Sun -> Earth ?

internal energy: energy kinetic energy of molecu


GY1k

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

matter

n

ic energy of molecules


GY1

Interaction of radiation withReflection

albedo, the reflectivity, is the fraction of radiation which is reflected back

Transmission Radiation passes through the body without interaction

Absorption matter can absorb radiatio

heat up due do absorptionabsorption increases kinetEveryday example?

d to do external work

is conserved.

ed, only transformed.

. In a body of uniform

hange in temperature.


GYk

Laws of thermodynamics

First lawEnergy absorbed by a body at rest is either use

or to increase the internal energy - total energy

Energy cannot be created nor can it be destroy

Second lawHeat never passes from a cooler to a hotter body

temperature there will never be a spontaneous c

o:

less it loses energy

on

y - otherwise itely


Gk

Consequence

Earth receives Energy from the sun all the time, s

First law: Earth would heat up all the time un

Loss must be to space - only possible via radiati

Emissionobjects need to emit energthey would heat up indefin

the more

ls absorbed radiation

tion

mitter

d from the object

=5.67 x 10-8 Wm-2K-4


Gk

Radiation laws I• Blackbody

Stefan-Boltzman law

All objects emit energy - the hotter

• radiative equilibrium temperature

temperature at which radiative energy loss equa

• absorbs all incident radia

• a perfect absorber and e

For a Black body

E=σT4

E energy flux (radiation) emitteT temperatureσ Stefan-Boltzmann Constant σ

transfers

tion

l energyd (1. Law)ork or to increase the

eating (1. Law)on - the hotter the morergy according to the


GY1

Sum up I

• Conduction, convection and radiation are energy• Radiation travels through the vacuum• Reflection, transmission and absorption of radia• Heat is an expression of total internal energy• Temperature is an expression of average interna• Energy can only be transformed, never destroye• Absorbed energy is either used to do external w

internal energy (1. Law)• For a body at rest radiation absorption leads to h• All objects with positive temperature emit radiati• Black bodies - idealized absorbers - radiate ene

Stefan Boltzman Law• Do not confuse radiation, temperature and heat!

e?
k

Earth’s surface temperatur

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

ect I energy loss from Earthfer is through space.

radiation (since ):

σT4

1/4 K = 42.330.25 x 102 K

288oK! What’s wrong?nce!

on the wavelength - or n


GY1

Solution: The greenhouse effAssume energy balance: energy input into earth =Input and loss are by radiation as the energy transNow one can use a law describing energy loss byinput=output and input and output are via radiation

Use Stefan Boltzmann law: E=

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)

= 2.55 x 100 oK = 255oK

Much too cold! The observed temperature is aboutAtmosphere makes the differe

How? Interaction of radiation with matter depends“colour ” - of the radiatio

um

, F.K. and E.J. Tarbuck, 1998. The Atmosphere

hort-wave radiation

e light


GY10k

The electromagnetic spectr

Source: Lutgens

long-wave radiation s

visibl

Spectrum

, F.K. and E.J. Tarbuck, 1998. The Atmosphere

Crest


GY10k

Wave properties

Source: Lutgens

Frequency = 1/wavelength

WavelengthAmplitude

Trough

Viking 2 Lander image, NASA


GY1

Sky of Mars

phere

ratories, 1965. Handbook of Geophysics and Space.


GY

Solar spectrum at top of atmos

Source: Houghton, H.G., 1985. Physical Meteorology. After data from Air Force Cambridge Research Labo

5785 deg. K blackbody

Inte

nsity

Wave length (μm)

Earth’s surface

Gases are

selective

absorbers


Gk

Solar spectrum at top of atmosphere and

Source: Peixoto, J.P. and A.H. Oort, 1992. Physics of Climate. After: Gast (1965)

c Radiation absorption

ward et al. (1955), Fels und Scharzkopf (1988)

errestrial,ong-wave

255oK


GYk

Black Body curves of Sun and Earth - Atmospheri

Source: Peixoto, J.P. and A.H. Oort, 1992. Physics of Climate. After: Goody (1964), Ho

Wave length (μm)

solar,short-wave

tl

6000oK

of maximal emission

K

es”by a black body is:

, h=6.63x10-34 J s, nt, k=1.38x10-23 J K-1

ν


GY1k

Radiation laws II

Wien's displacement law For a black body is the product of the wavelengthand temperature constant:

λmaxT = A = const = 2898 μm

Planck’s law - determines the “black body curvThe intensity of radiation of wavelength λ emitted

c speed of lighth Planck constantk Boltzman consta

The energy of light of frequency ν is given by: E=h

Bλ T( ) 2hc2

hvkT-------⎝ ⎠

⎛ ⎞ 1–exp--------------------------------------=

nge. An Earth System Perspective

its neighboursrth Mars

0.060 228

70 589

0 150 variable8 -56

5 -53

3 3

8 <2.51 <0.2536 >96-4-3 <0.0001


GYk

Venus and Mars

After: Graedel, T.E. and P.J. Crutzen, 1993. Atmospheric cha

Some physical properties of Earth and Property Venus Ea

Rel. mass of atmosphere 100 1

Distance from Sun (106km) 108 15

Solar constant (Wm-2) 2613 13

Albedo (%) 75 3Cloud cover (%) 100 5

Radiative temperature (oC) -39 -1

Surface temperature (oC) 427 1

Greenhouse effect (oC) 466 3

N2 (%) <2 7O2 (%) <1 ppmv 2CO2 (%) >98 0.0H2O (%) 1x10-4-0.3 4x10

transfers

he matter

; absorbed energy is internal energy (1. Law)

on - the hotter the more

n Boltzman Law

elength of the radiation

trial radiation -> warm-


GY1k

Summary • Conduction, convection and radiation are energy

• Radiation travels through the vacuum

• Interaction of radiation with matter depends on t

• Energy can only be transformed, never destroyedeither used to do external work or to increase the

• All objects with positive temperature emit radiati

• Black bodies emit energy according to the Stefa

• Wavelength, frequency, colour, Wien’s law

• Interaction of radiation with matter depends on wav

• Gases are selective absorbers

• Atmosphere absorbs some solar but most terresing of the atmosphere - Greenhouse effect

Earth: A Dynamic Planet A Solar and terrestrial...

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