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

5 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 Ahrens, Fig. 2.7 Planck’s Law 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 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).

12 Radiative Equilibrium (cont.) When the energy absorbed equals energy emitted, this is called Radiative Equilibrium. The corresponding temperature is the Radiative Equilibrium Temperature.

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

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

15 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

16 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

17 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 Visible IR Ahrens, Fig. 2.9

18 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

19 Simple Example of the Greenhouse Effect (0% Solar absorbed, 100% IR absorbed) 1 Unit Incoming Solar 1 1/21/41/81/16 1/21/41/8 1/16 1 Unit Outgoing IR to Space 2 Units IR Emitted by Ground ½ emitted to space ½ emitted to ground Take Home Point: Surface is warmer with selectively absorbing atmosphere than it would be without it. Radiative Equilibrium

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

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

22 Global Atmo Energy Balance Ahrens, Fig Solar Ground Atmosphere

23 Summary Greenhouse Effect (A Misnomer) SFC Warmer than Rad. Equil. Temp Reason: selective absorption of air H 2 O and CO 2 most absorbent of IR Energy Balance Complex system has a delicate balance All modes of Heat Transfer are important

24 Assignments for Next Lectures Ahrens (next lecture) Pages Problems 2.15, 2.16, 2.18