1. 1 NATS 101 Lecture 4 TR Radiation Selective Absorption

2. 2 Modes of Heat Transfer Energy is only converted from one form to another or transferred from one place to another. Energy is transferred from hot to cold. Conduction - Molecules colliding; most efficient at interface. Convection - Requires movement of a fluid or gas.

3. 3 Modes of Heat Transfer Heat-Energy transfer due to temperature differences Three modes of heat transfer Conduction – molecule to molecule Convection – transport of fluid Radiation – electromagnetic waves (This lecture) Latent Heat – energy of phase changes

4. 4 Latent Heat Energy associated with phase of matter. Must be either added to or taken from a substance when it changes its phase. –To turn liquid water into solid ice, must remove energy from the liquid water. –To turn liquid water into vapor, must add a lot of energy to the liquid water.

5. 5 Latent Heat Take 2 Polarity Williams, p 63 Ice =>Liquid =>Vapor Takes energy from environment Vapor =>Liquid =>Ice Emits energy to environment 540 cal/gm 80 cal/gm

6. 6 Modes of Heat Transfer Williams, p. 19 Latent Heat

7. 7 New Business Radiation Selective Absorption and Emission

8. 8 Radiation Any object that has a temperature greater than 0 K, emits radiation. This radiation is in the form of electromagnetic waves, produced by the acceleration of electric charges. These waves do not need matter in order to propagate; they move at the “speed of light” (3x10 5 km/sec) in a vacuum.

9. Electromagnetic Waves Two important aspects of waves are: –What kind: Wavelength or distance between adjacent peaks. –How much: Amplitude or distance between crests and valleys. 9 Wavelength Distance btw peaks Amplitude Height of crests/troughs Frequency # crests per unit time

10. 10 Why Electromagnetic Waves? Radiation has an Electric Field Component and a Magnetic Field Component –Electric Field is Perpendicular to Magnetic Field

11. 11 Photons Can also think of radiation as individual packets of energy or PHOTONS. Electron volts In simplistic terms, radiation with shorter wavelengths corresponds to photons with more energy (i.e. more BB’s per second) and with higher wave amplitude (i.e. bigger BB’s)

12. 12 Emitted Spectrum White Light from Flash Light PurpleGreen Red Emitted radiation has many wavelengths. Prism (Danielson, Fig. 3.14)

13. 13 Electromagnetic Spectrum Danielson, Fig. 3.18 WAVELENGTH Wavelengths of Meteorological Significance

14. 14 Plank’s Law: Emitted Spectrum Energy from Sun is spread unevenly over all wavelengths. Wavelength Energy Emitted Emission spectrum of Sun Ahrens, Fig. 2.7 Planck’s Law

15. 15 Planck’s Law and Wien’s Law The hotter the object, the shorter the brightest wavelength. Danielson, Fig. 3.19

16. 16 Wien’s Law Relates the wavelength of maximum emission to the temperature of mass λ MAX = (0.29×10 4 μm K) × T -1 Warmer Objects => Shorter Wavelengths Sun-visible light λ MAX = (0.29×10 4 μm K)×(5800 K) -1 ≈ 0.5 μm Earth-infrared radiation λ MAX = (0.29×10 4 μm K)×(290 K) -1 ≈ 10 μm

17. 17 Wien’s Law What is the radiative temperature of an incandescent bulb whose wavelength of maximum emission is near 1.0 μm ? Apply Wien’s Law: λ MAX = (0.29 × 10 4 μm K) × T -1 Temperature of glowing tungsten filament T= (0.29 × 10 4 μm K) × (λ MAX ) -1 T= (0.29 × 10 4 μm K) × (1.0 μm) -1 ≈ 2900K

18. 18 What is Radiative Temperature of Sun if Max Emission Occurs at 0.5 μm? Apply Wien’s Displacement Law

19. 19 Stefan-Boltzmann’s (SB) Law The hotter the object, the more radiation emitted. Double the temperature: Total emitted radiation increases by a factor of 16! Stefan-Boltzmann’s Law E= (5.67×10 -8 Wm -2 K -4 )×T 4 E=2×2×2×2=16 2 4 Sun Temp: 6000K Earth Temp: 300K Aguado, Fig. 2-7

20. 20 How Much More Energy is Emitted by the Sun per m 2 than the Earth? Apply Stefan-Boltzman Law The Sun is 160,000 Times More Energetic per m 2 Than the Earth, Plus Its Area is Much Bigger!

21. 21 How Much More Energy is Emitted by the Sun than the Earth? Apply Stefan-Boltzman Law

22. 22 Radiative Equilibrium Radiation absorbed by an object increases the energy of the object. –Increased energy causes temperature to increase (warming). Radiation emitted by an object decreases the energy of the object. –Decreased energy causes temperature to decrease (cooling).

23. 23 Radiative Equilibrium (cont.) When the energy absorbed equals energy emitted, this is called Radiative Equilibrium. The corresponding temperature is the Radiative Equilibrium Temperature. Concept is analogous to a bathtub with the faucet running and the drain open. If water in exceeds water out, level rises. If water in is less than water out, level falls. If water in equals water out, level is constant, or it is said to be at an equilibrium.

24. Clicker Instructions 1.Turn on clicker 2.Make sure you’ve entered in your UA NetID correctly 3.Join the class: MULLEN 4.When a question comes up, enter the answer on your key pad. 5.You will have a certain number of seconds to enter an answer, as indicated on the presentation 6.When you successfully send your answer, you will see a “received” message on the device. 7.Can change your answer as long as time has not run out.

25. Participation Scoring + Attendance WARNING: IF YOU DON’T ENTER ANY ANSWER THEN YOU WILL BE RECORDED AS ABSENT, SO ALWAYS ENTER SOMETHING. SEE SYLLABUS POLICY FOR ADMINSTRATIVE DROP POLICY. Always get one point for entering an answer, whether right or wrong An additional point if you get the right answer

26. FOUR POSSIBLE FATES OF RADIATION: 1.Transmitted 2. Reflected 3. Scattered 4. Absorbed The atmosphere does ALL of these…

27. Transmitted: Radiation passes through object SUNLIGHT GLASS WINDOW TRANSMITTED SUNLIGHT

28. Reflected: Radiation turned back MIRROR SUNLIGHT REFLECTED SUNLIGHT

29. Scattered: Path of radiation deflected SUNLIGHT FROSTED GLASS SCATTERED SUNLIGHT

30. Absorbed: Radiation transferred to object Blackbody: a perfect absorber and emitter of radiation in equilibrium, with no reflection or scattering. BLACK BOX SUNLIGHT

31. Radiative equilibrium: Absorption = Emission (Kirchoff’s Law) SUNLIGHT (SHORTWAVE) INFRARED (LONGWAVE) EMISSION BLACK BOX

32. A “Grey Body” = Not all radiation absorbed How the atmosphere behaves SUNLIGHT (SHORTWAVE) INFRARED (LONGWAVE) EMISSION GREY BOX SOME TRANSMISSION OF SUNLIGHT THROUGH BOX

33. 33 Why Selective, Discrete Absorption/Emission? Life as we perceive it: A continuous world! Atomic perspective: A quantum world! Gedzelman 1980, p 103

34. 34 Energy States for Atoms Electrons can orbit in only permitted states A state corresponds to specific energy level Only quantum jumps between states can occur Intervals correspond to specific wavelengths of radiation Hydrogen Atom Probability States Gedzelman 1980, p 104 Hydrogen Atom

35. 35 Energy States for Molecules Molecules can also rotate, vibrate, librate But only at specific energy levels or frequencies Quantum intervals between modes correspond to specific wavelengths Gedzelman 1980, p 105 H 2 O molecule H2O Bands H2O Bands

36. 36 Selective Absorption The Bottom Line Each molecule has a unique distribution of quantum states! Each molecule has a unique spectrum of absorption and emission frequencies of radiation! H 2 O molecule Williams, p 63

37. 37 Humans are Selective Absorbers Danielson, Fig. 3.18 http://hyperphysics.phy-astr.gsu.edu/hbase/mod4.html#c1

38. 38 Absorption Visible (0.4-0.7 μm) is absorbed very little by atmosphere UV (shorter than 0.3 μm) is absorbed by O 2 an O 3 Infrared (5-20 μm) is selectively absorbed by atmosphere H 2 O & CO 2 are strong absorbers of IR Little absorption of IR around 10 μm – “atmospheric window” MODTRAN 5 (D. Archer) Visible Akin to Ahrens, Fig. 2.9 IR UV

39. 39 Total Atmospheric Absorption Visible radiation (0.4-0.7 μm) is not absorbed Ultraviolet radiation (<0.3 μm) is blocked Infrared radiation (5-20 μm) is selectively absorbed, but there is an emission window at 10 μm Akin to Ahrens, Fig. 2.9

40. 40 Take Home Points Three modes of heat transfer Conduction: molecule-to-molecule Convection: fluid motion Radiation: electromagnetic waves Heat transfer works to equilibrate temperature differences

41. 41 Review Items All objects above 0K emit radiation Hotter the object, shorter the wavelength of maximum emission: Wien’s Law Hotter objects radiate more energy than colder objects: Stefan-Boltzman Law Objects that are good absorbers of radiation are also good emitters…today’s lecture!

42. 42 Take Home Points Radiative equilibrium and temperature Energy In = Energy Out (Eq. Temp.) Each molecule has a unique distribution of permitted, quantum energy states Unique spectrum of absorption and emission frequencies of radiation Air is transparent to incoming solar opaque to outgoing infrared

43. 43 Next Class Assignment Seasons Reading - Ahrens 3rd: Pg. 42-51 4th: Pg. 42-51 5th: Pg. 42-51 Homework02: D2L - Due Monday Feb 1st 3rd-Pg. 52: 2.15, 16, 18 4th-Pg. 52: 2.15, 16, 18 5th-Pg. 52: 2.15, 16, 18