1 ATMO 336 Weather, Climate and Society Heat Transfer

2 What is Heat? Heat-Energy in the process of being transferred from a warmer object to a cooler object Consider a pot of water on a hot burner. Consider the following questions: Williams, p. 19

3 Heat Transfer Questions What causes the… Pan bottom and handle to get warmer? Top of the water to become warmer? Water temperature to not exceed 100 o C? √ Region away from side of pan to feel warm? Williams, p. 19

4 Conduction Heat transfer due to collision of molecules. Conduction warms the bottom of the pan! Conductivity - rate of heat transfer across a 1 cm thick slab of material if one side is kept 1 o C warmer than the other Do a Cheap Experiment: Touch metal on your chair! 1 cm Metal WaterAir Heat Transfer 1oC1oC 0oC0oC

5 Heat Conductivity

6 Specific Heat Capacity Heat required to raise temperature of 1 gm of substance 1 o C. Metal has lower heat capacity than water!

7 Convection Heat transfer due to vertical exchange of mass Occurs in fluids (liquids, gases) because of gravity Warm, buoyant air rises - Cool, dense air sinks Convection warms top of liquid! Warm Cool Warm Cool Warm Cool heat below - convectionheat side - convectionheat top - no convection gravity

8 Convection Movies 2D Convection Tank Animation2D Convection Tank Animation  2D Convection Model Ra=10**6 2D Convection Model Ra=10**7 IC1 2D Convection Model Ra=10**7 IC2 3D Rayleigh-Benard Convection Model3D Rayleigh-Benard Convection Model 

9 Energy States and Water Phases water molecules are tightly packed in a crystal alignment that prevents them from changing shape LOW ENERGY STATE attractive forces btw molecules weaken and individual molecules can move around each other, but they can not break away SLIGHTLY HIGHER ENERGY STATE water molecules move very rapidly and are not bound together EXTEMELY HIGH ENERGY STATE Small Energy Change Large Energy Change

10 Latent Heat Ahrens, p 28 Weak attraction between molecules +540 cal/gm +80 cal/gm -540 cal/gm -80 cal/gm -620 cal/gm +620 cal/gm Strong attraction between molecules Some attraction More Ordered Phase Less Ordered Phase Surrounding air warms Surrounding air cools

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

12 Take Home Points Heat-Energy transfer due to temperature differences Three modes of heat transfer Conduction – molecule to molecule Convection – transport of fluid Radiation – electromagnetic waves (On Deck) Latent Heat – energy of phase changes

13 ATMO 336 Weather, Climate and Society Radiation Selective Absorption

14 Modes of Heat Transfer Energy is only transferred from one place to another or converted from one form to another (phase change, E=mc 2, …). Energy is transferred from hot to cold. Conduction - Molecules colliding; most efficient at interface. Convection - Requires movement of a fluid or gas.

15 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.

16 Modes of Heat Transfer Conduction Convection Radiation Williams, p. 19 Latent Heat Remember this thought experiment and the incandescent light bulb demo

17 Energy States and Water Phases water molecules are tightly packed in a crystal alignment that prevents them from changing shape LOW ENERGY STATE attractive forces btw molecules weaken and individual molecules can move around each other, but they can not break away SLIGHTLY HIGHER ENERGY STATE water molecules move very rapidly and are not bound together EXTEMELY HIGH ENERGY STATE Small Energy Change Large Energy Change

18 Latent Heat Take 2 Williams, p 63 Ice  Liquid  Vapor Takes energy from environment Vapor  Liquid  Ice Emits energy to environment

19 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 don’t need matter in order to propagate; they move at the “speed of light” (3x10 5 km/sec) in a vacuum.

20 Electromagnetic Waves Two important aspects of waves are: –What kind: Wavelength or distance between peaks. –How much: Amplitude or distance between peaks and valleys. Wavelength AmplitudeFrequency

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

22 Photons NOT TO CONFUSE YOU, but… Can also think of radiation as individual packets of energy or PHOTONS. 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)

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

24 Electromagnetic Spectrum WAVELENGTH Danielson, Fig Wavelengths of Meteorological Significance

25 Radiation Effects on Humans Danielson, Fig

26 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

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

28 Wien’s Law Relates the wavelength of maximum emission to the temperature of mass MAX = (0.29x10 4  m K) x T -1 Warmer Objects => Shorter Wavelengths Sun-visible light MAX = (0.29x10 4  m K) x (5800 K) -1 = 0.5  m Earth-infrared radiation MAX = (0.29x10 4  m K) x (290 K) -1 = 10  m

29 Wien’s Law What is the radiative temperature of an incandescent bulb whose wavelength of maximum emission is near 1.0  m ? Apply Wien’s Law: MAX = (0.29 x 10 4  m K) x T -1 Temperature of glowing tungsten filament T= (0.29 x 10 4  m K) x ( MAX ) -1 T= (0.29 x 10 4  m K) x (1.0  m) -1 = 2900K

30 What is Radiative Temperature of Sun if Max Emission Occurs at 0.5  m? Apply Wien’s Displacement Law

31 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= (5.67x10 -8 Wm -2 K -4 ) x T 4 E=2 x 2 x 2 x 2=16 4 times Sun Temp: 6000K Earth Temp: 300K Aguado, Fig. 2-7

32 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 Mucho Bigger!

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

34 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).

35 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 unplugged. 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 at an equilibrium level.

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

37 Review Items Heat Transfer Latent Heat Wien’s Displacement Law Stefan-Boltzman Law Ramifications

38 General Laws of Radiation All objects above 0 K emit radiant energy Hotter objects radiate more energy per unit area than colder objects, result of Stefan-Boltzman Law The hotter the radiating body, the shorter the wavelength of maximum radiation, result of Wien’s Displacement Law Final point: objects that are good absorbers of radiation are also good emitters!

39 Sun’s Radiation Spectrum Ahrens, Fig. 2.7 Planck’s Law Key concept: Radiation is spread unevenly across all wavelengths

40 Sun - Earth Radiation Spectra Ahrens, Fig. 2.8 Planck’s Law Key concepts: Wien’s Law and Stefan-Boltzman Law

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

42 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 Applet Probability StatesHydrogen Applet Probability States Gedzelman 1980, p 104 Hydrogen Atom

43 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 H2O Bands

44 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

45 Humans are Selective Absorbers Danielson, Fig

46 Ahrens, Fig. 2.9

47 Absorption Visible (  m) is absorbed very little O 2 an O 3 absorb UV (shorter than 0.3  m) Infrared (5-20  m) is selectively absorbed H 2 O & CO 2 are strong absorbers of IR Little absorption of IR around 10  m – atmospheric window MODTRAN3 (D. Archer) Full Spectrum (D. Archer) Visible IR Ahrens, Fig. 2.9 UV

48 Total Atmospheric Absorption Visible radiation (  m) is not absorbed Infrared radiation (5-20  m) is selectively absorbed, but there is an emission window at 10  m Ahrens, Fig. 2.9

49 Key Points Radiation is emitted from all objects that have temperatures warmer than absolute zero (0 K). Wien’s Law: wavelength of maximum emission MAX = (0.29x10 4  m K) x T -1 Stefan-Boltzmann Law: total energy emission E= (5.67x10 -8 W/m 2 ) x T 4

50 Key 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

51 Key Points Three modes of heat transfer Conduction: molecule-to-molecule Convection: fluid motion Radiation: electromagnetic waves Heat transfer works to equilibrate temperature differences