Announcements I will go over homework assignments for the semester during the next two classes. TODAY: Writing assignment options + assignment lists WEDNESDAY: Lab experiment demonstrations by TAs FRIDAY: Lab experiment kits distributed I am in need of three notetakers for students in the course. If you would be willing to do this, please see me after class or during office hours. You will receive recognition for doing this from the DRC.

Summary of Lecture 5 The three modes of heat transfer are conduction, convection, and radiation. Radiation is energy propagated by electromagnetic waves and is emitted by all objects so long as they have a temperature. The type of radiation an object emits is dependent on its temperature. The smaller the wavelength of radiation, the greater the energy and the higher the temperature. The entire range of radiation types is given by the EM spectrum. The total radiant energy emitted by an object is described by the Stefan- Boltzmann law. The wavelength of maximum radiation emission is described by Wien’s law. Solar radiation, or shortwave radiation, is that which comes from the sun and is most intense in the visible part of the spectrum. Terrestrial radiation, or longwave radiation, is that which comes from the Earth and is most intense in the infrared part of the spectrum.

1 st Review question from last lecture Given that the sun’s temperature is around 6000 K, approximately what is the wavelength of maximum radiation emission? A) 5 × m B) 5 × m C) 5 × m D) 5 × m E) 5 × m

2 nd Review question from last lecture How much more energy per unit area is emitted from an object at 200 K vs. an object at 400 K? A) About 2 times more B) About 4 times more C) About 8 times more D) About 16 times more E) About 32 times more

NATS 101 Section 6: Lecture 6 The Greenhouse Effect and Earth-Atmosphere Energy Balance

Survey question Thinking about the issue of global warming, sometimes called the 'greenhouse effect,' how well do you feel you understand this issue? Would you say very well, fairly well, not very well, or not at all? A) Very Well B) Fairly Well C) Not Very Well D) Not at All E) I don’t understand the question.

Gallup Poll March 13-16, 2006 Thinking about the issue of global warming, sometimes called the 'greenhouse effect,' how well do you feel you understand this issue? Would you say very well, fairly well, not very well, or not at all? Very Well 21% Fairly Well 53% Not Very Well 20% Not at All 6% My comments as an atmospheric scientist: Are the greenhouse effect and global warming really the same thing? If not, then NOBODY understands much of anything “well”—including whoever wrote this question!!

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

Transmitted: Radiation passes through object SUNLIGHT GLASS WINDOW TRANSMITTED SUNLIGHT

Reflected: Radiation turned back MIRROR SUNLIGHT REFLECTED SUNLIGHT

Scattered: Path of radiation deflected SUNLIGHT FROSTED GLASS SCATTERED SUNLIGHT

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

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

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

Internal energy increases by changes on the molecular and atomic levels ENERGY TRANSITIONS: Translational Rotational Vibrational Electronic: molecular Electronic: atomic Less Energy required Changes on molecular level Longer wavelength of radiation More energy required Changes on the atomic level Shorter wavelength of radiation What Happens When Radiation is “Absorbed”? ENERGY REQUIRED

Translational Energy Gross movement of atoms and molecules through space. The translational energy reflects the kinetic energy—and thus the temperature.

Rotational Energy (Gedzelman 1980, p 105) Energy associated the rotation of the molecule. Takes on discrete values (or quanta) dependent on the type of molecule. Corresponds to energy changes shorter than 1 cm (far infrared). Rotation of water molecules Infrared photon

(Hartmann 1994) Molecules have rotational energy only if they have a permanent dipole moment, or asymmetric charge distribution. Rotational Energy of Common Gases in the Atmosphere

Vibrational Energy Molecular energy stored in the vibrations (or stretching and bending) of atomic bonds. Takes on discrete values (or quanta) dependent on the type of molecule. Corresponds to energy changes in the infrared spectrum. Vibration of water molecules (Gedzelman 1980, p 105) Infrared photon

Vibrational Energy for Common Gases in the Atmosphere Most effective absorbers are molecules that have a dipole moment and/or are bent. Carbon dioxide creates a dipole moment as a result of its vibrational transitions, so has rotational energy as well. (Hartmann 1994)

Electronic energy: Photodissociation (Molecular level) Energy associated with breaking of atomic bonds of molecules. Takes on discrete values (or quanta) dependent on the type of molecule. Corresponds mainly to energy changes in the ultraviolet spectrum. O O UV PHOTON O O Oxygen molecule (O 2 )Oxygen atoms (O)

Electronic energy: Excitation (Atomic Level) Energy associated with excitation of electrons in the outer shell of an atom. Takes on discrete values (or quanta) dependent on the type of molecule. Corresponds to energy changes mainly in the ultraviolet. p+ e- UV Photon p+ e- Hydrogen atom Excited hydrogen atom

Electronic energy: Ionization (Atomic level) Energy associated with removal of electrons from an atom. Takes on discrete values (or quanta) dependent on the type of molecule. Corresponds to energy changes mainly higher than ultraviolet. p+ e- Short wavelength UV or X-Ray photon p+ e- Hydrogen atomIonized hydrogen atom

inversion isothermal 6.5 o C/km THERMOSPHERE: Warming because of ionization of gases. STRATOSPHERE: Warming because of photodissociation of ozone and oxygen. Flashback: Why is there warming in Thermosphere and Stratosphere?

Aurora Borealis or Aurora Australis Caused by ionization of gases in the Earth’s atmosphere from the solar wind, which appears as visible light. Occur only at the poles because of the properties Earth’s magnetic field

Absorption of Radiation by Common Atmospheric “Greenhouse” Gases SOLARTERRESTRIAL λ max Sunλ max Earth The “greenhouse gases” are called such because they absorb and emit very effectively in the infrared band (5-20 µm) at selective wavelengths. Water and CO 2 are good greenhouse gases Oxygen (O 2 ) and ozone (O 3 ) are absorbed in the UV. Little or no absorption in some places

Total Radiation Absorption Spectrum by all Atmospheric Gases SOLARTERRESTRIAL Little or no absorption. A lot of absorption because of greenhouse gases, except in an atmospheric window of 8-11 µm. Absorption %

Since there is little or no absorption of radiation in the most intense part of the solar radiation band, the heat supplied comes from the Earth itself. SOLAR RADIATION TERRESTRIAL RADIATION LATENT HEATING Atmospheric Heating From Below

Is the presence of the greenhouse gases in our atmosphere good or bad?

The Importance of the Greenhouse Effect The presence of the gases in our atmosphere that absorb and emit infrared radiation help maintain the Earth’s average temperature at about 59 °F. Without these gases, ALL infrared radiation from the Earth’s surface would be lost to space and the Earth would be a frozen ice ball!

The Greenhouse Effect DOES NOT EQUAL Global Warming or Climate Change! Global warming: The increase in Earth’s mean temperature that would result because of the increase in greenhouse gases due to human activities. This would enhance the greenhouse effect. Climate change: Long-term change in global, regional, or local climate resulting from both enhanced greenhouse gases and/or other human activities.

Greenhouse Effect: Venus, Earth, and Mars VENUS (Same size as Earth) EARTH MARS (Half size of Earth) Pressure = 90 Atm. Atmosphere composed of 96% CO 2 Temperature = 482 °C Pressure = 1 Atm. Atmosphere composed of less than 1% CO 2 Temperature = 15 °C Pressure = 0.01 Atm. Atmosphere composed of 95% CO 2 Temperature = -63 °C GREENHOUSE EFFECT ON STERIODS! GREENHOUSE EFFECT JUST RIGHT VIRTUALLY NO ATMOSPHERE TO HAVE A GREENHOUSE EFFECT

We’ve talked so far about absorption and emission of radiation in the atmosphere. This process is most important for terrestrial radiation. Reflection and scattering are more important for solar radiation.

Scattering: Radiation deflected Atmospheric gases: Molecules preferentially scatter the smaller wavelengths of visible light. Cloud drops: Scatter all wavelengths of visible light equally. Rayleigh Scattering Mie Scattering

Why is the sky blue? Air molecules preferentially scatter smaller wavelengths of visible light, which are blue!

Reflection: Radiation turned back In meteorology the reflectivity is called the albedo. Albedo = 1  All radiation reflected Albedo = 0  All radiation absorbed Snow : 0.7 Water: 0.1 Vegetation: 0.1 to 0.3 Sand: 0.15 to 0.45 Clouds: Depends on type and location NET ALBEDO OF EARTH = 0.3

Important Aside: Clouds are a big part of the radiation budget too! ABSORPTION AND REFLECTION OF SOLAR RADIATION VERY EFFECTIVE ABSORBERS AND EMITTERS OF TERRESTRIAL RADIATION

Now that we know what happens to radiation in the atmosphere, we’re ready to look at the Earth’s average energy budget

Energy Budget: Shortwave Radiation EARTH’S ALBEDO ATMOSPHERICABSORPTION SURFACE SOLAR ABSORPTION (ABOUT HALF OF TOTAL INCOMING) INCOMING SOLAR AT TOA

Energy Budget: Longwave radiation and Sensible and Latent Heating SURFACE SOLAR ABSORPTION SURFACE LONGWAVE ABSORPTION: GREENHOUSE EFFECT SUM OF SHORTWAVE + LONGWAVE ABSORPTION ( > 100) SURFACE LONGWAVE EMISSION SENSIBLE AND LATENT HEATING ATMOSPHERIC ENERGY GAINED FROM SURFACE ATMOSPHERIC LONGWAVE EMISSION TO SURFACE ATMOSPHERIC LONGWAVE EMISSION TO SPACE ( = 1- ALBEDO)

Summary of Lecture 6 Radiation in the atmosphere has four possible fates: transmitted, reflected, scattered, or absorbed. A perfect absorber and emitter of radiation is called a blackbody. The atmosphere selectively reflects, scatters and absorbs radiation at certain wavelengths, which depend on the specific gas constituents. Absorbed radiation increases the internal energy by changes on the molecular and atomic level. Terrestrial radiation is associated with translational, rotational, and vibrational energy transitions on the molecular level. Solar radiation is associated with electronic energy transitions on the atomic level. Greenhouse gases are those which absorb and emit very effectively in the infrared, like water and CO 2. Because of them the atmosphere is very opaque to terrestrial radiation and the Earth’s surface temperature is maintained. Reviewed the atmospheric energy budget to prove the point. Though the atmosphere is fairly transparent to solar radiation, scattering and reflection of solar radiation is important. Scattering of visible light is why the sky is blue!

Reading Assignment and Review Questions Ahrens, Chapter 3, pp Chapter 2 Questions Questions for Review: 11,12,13,14,17,18,19,21 Questions for Thought: 7,9,10,11,12,13