Earth: A Dynamic Planet A 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 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 Energy and radiation Outline 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
Bullets Introduction 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 gradient The electromagnetic spectrum electromagnetic spectrum wavelength, frequency UV, visible light, PAR, infrared, Colors Some 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 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 Earth: A Dynamic Planet
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 ~6000 o K selective absorbers - absorb only at certain wavelengths O 2, O 3, H 2 O and CO 2 glass - hot dashboard - green house UV absorption within the atmosphere - O 2 and O 3 - 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 CO 2 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 Earth: A Dynamic Planet
GY1003 - Earth: A Dynamic Planet A, Lecture 12, Jörg Kaduk
Energy and radiation Potential energy energy due to location in a force field Kinetic energy energy due to movement arth: A Dynamic Planet
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 1 o K 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
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 GY1k
Reflection Interaction of radiation with matter 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? GY1
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. GYk
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 objects need to emit energy - otherwise they would heat up indefinitely Gk
Blackbody Radiation laws I Stefan-Boltzman law For a Black body E=σT 4 All objects emit energy - the hotter the more radiative equilibrium temperature absorbs all incident radiation a perfect absorber and emitter E energy flux (radiation) emitted from the object T temperature σ Stefan-Boltzmann Constant σ=5.67 x 10-8 Wm -2 K -4 temperature at which radiative energy loss equals absorbed radiation Gk
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! GY1
Earth s surface temperature? Get together in groups of four 5 min activity Assume: Radiation heating Earth, is 240 Wm -2. Calculate the temperature of Earth. Assume Earth behaves like a black body k
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=σT 4 T = (E/σ) 1/4 = (240Wm -2 /(5.67 x 10-8 Wm -2 K -4 )) 1/4 = 240/(5.67 x 10-8 ) 1/4 K = (240/5.67) 1/4 x (1/10-8 ) 1/4 K = 42.33 0.25 x 10 2 K = 2.55 x 100 o K = 255 o K Much too cold! The observed temperature is about 288 o K! What s wrong? Atmosphere makes the difference! How? Interaction of radiation with matter depends on the wavelength - or colour - of the radiation GY1
Spectrum The electromagnetic spectrum visible light long-wave radiation short-wave radiation Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere GY10k
Wave properties Wavelength Amplitude Crest Trough Frequency = 1/wavelength Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere GY10k
Sky of Mars Viking 2 Lander image, NASA GY1
Solar spectrum at top of atmosphere Intensity 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 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) Gk
Black Body curves of Sun and Earth - Atmospheric Radiation absorption 6000 o K solar, short-wave 255 o K 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) GYk
Radiation laws II Wien's displacement law For a black body is the product of the wavelength of maximal emission and temperature constant: λ max T = 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: Bλ( T) = 2hc 2 ------------------------------------- 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 GY1k
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 (10 6 km) 108 150 228 Solar constant (Wm -2 ) 2613 1370 589 Albedo (%) 75 30 15 Cloud cover (%) 100 50 variable Radiative temperature ( o C) -39-18 -56 Surface temperature ( o C) 427 15-53 Greenhouse effect ( o C) 466 33 3 N 2 (%) <2 78 <2.5 O 2 (%) <1 ppmv 21 <0.25 CO 2 (%) >98 0.036 >96 H 2 O (%) 1x10-4 -0.3 4x10-4 -3 <0.0001 After: Graedel, T.E. and P.J. Crutzen, 1993. Atmospheric change. An Earth System Perspective GYk
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 GY1k