1. NATS 101 Lecture 6 Greenhouse Effect and Earth-Atmo Energy Balance

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

3. New Business Selective Absorption and Emission
Earth-Atmo Energy Balance

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

5. Takes energy from environment Emits energy to environment
Latent Heat Take 2
Williams, p 63
Takes energy from environment
Emits energy to environment

6. 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
Objects that are good absorbers of radiation are also good emitters…today’s lecture!

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

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

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

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

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

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

13. 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
H2O molecule H2O Bands

14. 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!
H2O molecule
Williams, p 63

15. Absorption Visible (0.4-0.7 m) is absorbed very littleIR
Visible ( m) is absorbed very little
O2 an O3 absorb UV (shorter than 0.3 m)
Infrared (5-20 m) is selectively absorbed
H2O & CO2 are strong absorbers of IR
Little absorption of IR around 10 m – atmospheric window
Ahrens, Fig. 2.9

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

17. 1 Unit Outgoing IR to Space 2 Units IR Emitted by Ground
Simple Example of the Greenhouse Effect (0% Solar absorbed, 100% IR absorbed)
Radiative Equilibrium
1 Unit Outgoing IR to Space
1 Unit Incoming Solar
1/2
1/4
1/8
1/16
½ emitted to space
½ emitted to ground 1
1/2
1/4
1/8
1/16
2 Units IR Emitted by Ground
Take Home Point: Surface is warmer with selectively absorbing atmosphere than it would be without it.

18. Global Solar Radiation Balance (Not all Solar Radiation SR reaches the surface)
30% SR reflects back to space
Albedo: percent of total SR reflected
~20% absorbed by atmosphere
70% SR absorbed by earth-atmosphere
Ahrens, Fig. 2.13
~50% SR absorbed by surface

19. Atmosphere Heated from Below
Ahrens, Fig old ed.
Air above ground heats by convection and absorption of some IR from ground
Net Effect: Atmosphere is Heated From Below
Air contacting ground heats by conduction
Ground heats further through absorption of IR from atmosphere
Solar radiation heats the ground

20. Global Atmo Energy Balance
Ahrens, Fig. 2.14
Solar
Atmosphere
Ground

21. Summary Greenhouse Effect (A Misnomer) Energy Balance
SFC Warmer than Rad. Equil. Temp
Reason: selective absorption of air
H2O and CO2 most absorbent of IR
Energy Balance
Complex system has a delicate balance
All modes of Heat Transfer are important

22. Assignment Next Class - Quiz #1 Review Session the Prior Day?
If successful, time & location will be sent over listserv no later AM of day before quiz, probably by late PM today.

23. Assignments for Next Lectures
Quiz #1
All material through today’s lecture
Ahrens (next lecture)
Pages 42-50
Problems 2.15, 2.16, 2.18