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

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

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Review Items

• Heat Transfer
• Latent Heat
• Wiens Displacement Law Ramifications
• Stefan-Boltzman Law Ramifications

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New Business

• Selective Absorption and Emission
• Earth-Atmo Energy Balance

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Modes of Heat Transfer
Williams, p. 19
Latent Heat
Convection
Conduction
Radiation
Remember this thought experiment and the
incandescent light bulb thru the prism
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Latent Heat Take 2
Williams, p 63
Takes energy from environment Emits energy to
environment
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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
  Wiens Displacement Law
• Objects that are good absorbers of radiation are
  also good emitterstodays lecture!

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Suns Radiation Spectrum
Plancks Law
Ahrens, Fig. 2.7
Key concept Radiation is spread unevenly across
all wavelengths
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Sun - Earth Radiation Spectra
Ahrens, Fig. 2.8
Plancks Law
Key concepts Wiens Law and Stefan-Boltzman Law
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What is Radiative Temperature of Sun if Max
Emission Occurs at 0.5 ?m?

• Apply Wiens Displacement Law

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How Much More Energy is Emitted by the Sun than
the Earth?

• Apply Stefan-Boltzman Law

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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).

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Radiative Equilibrium (cont.)

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

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Why Selective, Discrete Absorption/Emission?

• Life as we perceive it A continuous world!
• Atomic perspective A quantum world!

Gedzelman 1980, p 103
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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
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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
H2O molecule H2O Bands
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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
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Absorption
Visible

• 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

IR
Ahrens, Fig. 2.9
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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

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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
1/16
1
1/2
1/4
1/8
2 Units IR Emitted by Ground
Take Home Point Surface is warmer with
selectively absorbing atmosphere than it would be
without it.
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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
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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
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Global Atmo Energy Balance
Ahrens, Fig. 2.14
Solar
Atmosphere
Ground
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Summary

• Greenhouse Effect (A Misnomer)
• Surface 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

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NATS 101Intro to Weather and ClimateNext
subjectThe Seasons

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Supplemental References for Todays Lecture

• Aguado, E. and J. E. Burt, 2001 Understanding
  Weather Climate, 2nd Ed. 505 pp. Prentice Hall.
  (ISBN 0-13-027394-5)
• Danielson, E. W., J. Levin and E. Abrams, 1998
  Meteorology. 462 pp. McGraw-Hill. (ISBN
  0-697-21711-6)
• Gedzelman, S. D., 1980 The Science and Wonders
  of the Atmosphere. 535 pp. John-Wiley Sons.
  (ISBN 0-471-02972-6)
• Lutgens, F. K. and E. J. Tarbuck, 2001 The
  Atmosphere, An Intro-duction to the Atmosphere,
  8th Ed. 484 pp. Prentice Hall. (ISBN
  0-13-087957-6)
• Wallace, J. M. and P. V. Hobbs, 1977 Atmospheric
  Science, An Introductory Survey. 467 pp. Academic
  Press. (ISBN 0-12-732950-1)

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Reasons for Seasons

• Tilt of Earths Axis - Obliquity
• Angle between the Equatorial Plane and
  the Orbital Plane
• Eccentricity of Earths Orbit
• Elongation of Orbital Axis

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Eccentricity of Orbit
Perihelion
Aphelion
Ahrens (2nd Ed.), akin to Fig. 2.15
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?
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147 million km
152 million km
Ahrens, Fig. 2.17
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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

Long Path
Large Area
Equal Energy
23.5o
Small Area
Short Path
Ahrens, Fig. 2.19
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Beam Spreading

• Low Zenith - Large Area, Much Spreading
• High Zenith - Small Area, Little Spreading

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Beam Spreading
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Atmospheric Path Length
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Length of Day
Lutgens Tarbuck, p33
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Day Hours at Solstices - US Sites

• Summer-Winter
• Tucson (32o 13 N) 1415 - 1003
• Seattle (47o 38 N) 1600 - 825
• Anchorage (61o 13 N) 1922 - 528
• Fairbanks (64o 49 N) 2147 - 342
• Hilo (19o 43 N) 1319 - 1046
  

Arctic Circle
Gedzelman, p67
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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
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Solar Declination
Solstice
Equinox
Solstice
Aguado Burt, p46
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Noon Zenith Angle at Solstices

• Summer-Winter
• Tucson AZ (32o 13 N) 08o 43 - 55o 43
• Seattle WA (47o 38 N) 24o 08 - 71o 08
• Anchorage AK (61o 13 N) 37o 43 - 84o 43
• Fairbanks AK (64o 49 N) 41o 19 - 88o 19
• Hilo HI (19o 43 N) 3o 47 (north) - 43o
  13

Aguado Burt, p46
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Incoming Solar Radiation (Insolation) at the Top
of the Atmosphere
W
C
C
W
Wallace and Hobbs, p346
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Is Longest Day the Hottest Day?
Consider Average Daily Temperature for Chicago IL
USA Today WWW Site
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Radiation Budget

• Summer hemisphere shows a surplus, warms
• Winter hemisphere shows a deficit, cools
• Equator/S. Pole always shows a
  surplus/deficit
• Why doesnt the equator warm and S. Pole cool?

NH
SH
NH
SH
Lutgens Tarbuck, p51
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Annual Energy Balance
Radiative Warming
Radiative Cooling
Radiative Cooling
NH
SH
Ahrens, Fig. 2.21

• Heat transfer done by winds and ocean currents
• Differential heating drives winds and currents
• We will examine later in course

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Summary

• Tilt (23.5o) is primary reason for seasons
• Tilt changes two important factors
• Angle at which solar rays strike the earth
• 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

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Assignments for Next Lectures

• Ahrens (next lecture)
• Pages 42-52, 55-64
• Problems
• 2.15, 2.16, 2.18
• 3.1, 3.2, 3.5, 3.6, 3.14