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

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

New Business Selective Absorption and Emission Earth-Atmo Energy Balance

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

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

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!

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

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

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

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

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

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

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

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

Absorption Visible (0.4-0.7 m) is absorbed very little IR Visible (0.4-0.7 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

Total Atmospheric Absorption Ahrens, Fig. 2.9 Visible radiation (0.4-0.7 m) is not absorbed Infrared radiation (5-20 m) is selectively absorbed, but there is an emission window at 10 m

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.

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

Atmosphere Heated from Below Ahrens, Fig. 2.11 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

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

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

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.

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