NATS 101 Lecture 5 Greenhouse Effect and Earth-Atmo Energy Balance and the Seasons

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

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 Ahrens, Fig. 2.7 Planck’s Law 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

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).

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

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 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

Energy States for Molecules Molecules can rotate, vibrate 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

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

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

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

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

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

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

Global Atmo Energy Balance Ahrens, Fig Solar Ground Atmosphere

Summary Greenhouse Effect (A Misnomer) Surface 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

NATS 101 Intro to Weather and Climate Next subject: The Seasons

Supplemental References for Today’s Lecture Aguado, E. and J. E. Burt, 2001: Understanding Weather & Climate, 2 nd Ed. 505 pp. Prentice Hall. (ISBN ) Danielson, E. W., J. Levin and E. Abrams, 1998: Meteorology. 462 pp. McGraw-Hill. (ISBN ) Gedzelman, S. D., 1980: The Science and Wonders of the Atmosphere. 535 pp. John-Wiley & Sons. (ISBN ) Lutgens, F. K. and E. J. Tarbuck, 2001: The Atmosphere, An Intro- duction to the Atmosphere, 8 th Ed. 484 pp. Prentice Hall. (ISBN ) Wallace, J. M. and P. V. Hobbs, 1977: Atmospheric Science, An Introductory Survey. 467 pp. Academic Press. (ISBN )

Reasons for Seasons Tilt of Earth’s Axis - Obliquity Angle between the Equatorial Plane and the Orbital Plane Eccentricity of Earth’s Orbit Elongation of Orbital Axis

Earth is 5 million km closer to sun in January than in July. Solar radiation is 7% more intense in January than in July. Why is July warmer than January in Northern Hemisphere? Eccentricity of Orbit Aphelion Perihelion Ahrens (2nd Ed.), akin to Fig. 2.15

147 million km152 million km Ahrens, Fig. 2.17

Solar Zenith Angle Depends on latitude, time of day & season Has two effects on an incoming solar beam Surface area covered or Spreading of beam Path length through atmosphere or Attenuation of beam Ahrens, Fig Large Area Small Area Short Path Long Path Equal Energy 23.5 o

Beam Spreading Low Zenith - Large Area, Much Spreading High Zenith - Small Area, Little Spreading Ahrens, Fig Large Zenith Angle Zero Zenith Angle Large Zenith Angle Small Zenith Angle

Beam Spreading Schematic Ignores Earth’s Curvature

Atmospheric Path Length Schematic Ignores Earth’s Curvature Cloud

Length of Day Lutgens & Tarbuck, p33

Day Hours at Solstices - US Sites Summer-Winter Tucson (32 o 13’ N) 14: :03 Seattle (47 o 38’ N) 16:00 - 8:25 Anchorage (61 o 13’ N) 19:22 - 5:28 Fairbanks (64 o 49’ N) 21:47 - 3:42 Hilo (19 o 43’ N) 13: :46 Gedzelman, p67 Arctic Circle

Path of Sun Hours of daylight increase from winter to summer pole Equator always has 12 hours of daylight Summer pole has 24 hours of daylight Winter pole has 24 hours of darkness Note different Zeniths Danielson et al., p75

Solar Declination Aguado & Burt, p46 Solstice Equinox

Noon Zenith Angle at Solstices Summer-Winter Tucson AZ (32 o 13’ N) 08 o 43’ - 55 o 43’ Seattle WA (47 o 38’ N) 24 o 08’ - 71 o 08’ Anchorage AK (61 o 13’ N) 37 o 43’ - 84 o 43’ Fairbanks AK (64 o 49’ N) 41 o 19’ - 88 o 19’ Hilo HI (19 o 43’ N) 3 o 47’ (north) - 43 o 13’ Aguado & Burt, p46

Incoming Solar Radiation (Insolation) at the Top of the Atmosphere Wallace and Hobbs, p346 C C W W

Is Longest Day the Hottest Day? USA Today WWW Site Consider Average Daily Temperature for Chicago IL:

Radiation Budget Summer hemisphere shows a surplus, warms Winter hemisphere shows a deficit, cools Equator/S. Pole always shows a surplus/deficit Why doesn’t the equator warm and S. Pole cool? Lutgens & Tarbuck, p51 NH SH

Annual Energy Balance Heat transfer done by winds and ocean currents Differential heating drives winds and currents We will examine later in course NHSH Radiative Warming Radiative Cooling Ahrens, Fig. 2.21

Summary Tilt (23.5 o ) is primary reason for seasons Tilt changes two important factors 1.Angle at which solar rays strike the earth 2.Number of hours of daylight each day Warmest and Coldest Days of Year Occur after solstices, typically around a month Requirement for Heat Transport Done by Atmosphere-Ocean System

Assignments for Next Lectures Ahrens (next lecture) Pages 42-52, Problems: 2.15, 2.16, , 3.2, 3.5, 3.6, 3.14