nats 101 lecture 5 greenhouse effect and earth-atmo energy balance and the seasons
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NATS 101
Lecture 5
Greenhouse Effect and Earth-Atmo Energy Balance
and the Seasons
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Review Items
• Heat Transfer• Latent Heat • Wien’s Displacement Law Ramifications
• Stefan-Boltzman Law Ramifications
max2900 mKm
Tμμλ ⎛ ⎞
⎜ ⎟⎝ ⎠
≈
8-2 -2 -4 4(W m ) (5.67 10 Wm K )E T−= ×
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New Business
• Selective Absorption and Emission
• Earth-Atmo Energy Balance
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Modes of Heat Transfer
Conduction Convection Radiation
Williams, p. 19
Latent Heat
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
Wien’s Displacement Law• Objects that are good absorbers of radiation are
also good emitters…today’s lecture!
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Sun’s Radiation Spectrum
Ahrens, Fig. 2.7Planck’s Law
Key concept: Radiation is spread unevenly across all wavelengths
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Sun - Earth Radiation SpectraAhrens, Fig. 2.8
Planck’s Law
Key concepts: Wien’s Law and Stefan-Boltzman Law
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What is Radiative Temperature of Sun if Max Emission Occurs at 0.5
m?• Apply Wien’s Displacement Law
max
max
2900
2900
29000.5
5800
mKT
mK
mKm
T K
T
T
μ
μλ
μμ
λ
≈
≈
≈
≈
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How Much More Energy is Emitted by the Sun than the Earth?
• Apply Stefan-Boltzman Law
2
2
-2 -2 -4 48
8 4 4 54448
25 43
(W m ) W m K(5.67 10 )
(5.67 10 ) 5800 5800 1.6 10202905.67( 10 ) 290
4 7.0 10 1.2 10 (12,000 )4 6.4 10
2.0 1
Sun
Earth
Sun Sun
Earth Earth
Sun Sun
EarthEarth
E T
EE
A r times largerA r
A EA E
ππ
⎛ ⎞⎜ ⎟⎜ ⎟⎜ ⎟⎝ ⎠
−
−
−
= ×
× = ×≈ ==×
×= ×≈ ≈×
×≈ 90 (2 )billion times more
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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 LineEach 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 (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
Visible
IR
Ahrens, Fig. 2.9
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Total Atmospheric Absorption
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
Ahrens, Fig. 2.9
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Simple Example of the Greenhouse Effect
(0% Solar absorbed, 100% IR absorbed)
1 Unit Incoming Solar
1
1/2 1/4 1/8 1/16
1/2 1/4 1/8 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
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Global Solar Radiation Balance (Not all Solar Radiation SR reaches the surface)
Ahrens, Fig. 2.13
70% SR absorbed by earth-atmosphere
~50% SR absorbed by surface~50% SR absorbed by surface
30% SR reflects back to space30% SR reflects back to space
Albedo: percent of total SR reflected
~20% absorbed by atmosphere
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Atmosphere Heated from Below
Ahrens, Fig. 2.11 old ed.
Solar radiation heats the ground
Air contacting ground heats by conductionAir 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: Net Effect: Atmosphere is Heated Atmosphere is Heated
From BelowFrom Below
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Global Atmo Energy BalanceAhrens, Fig. 2.14
SolarSolar
GroundGround
AtmosphereAtmosphere
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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 Climate
Next subject:The Seasons
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Supplemental References Supplemental References for Today’s Lecturefor Today’s 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 Earth’s Axis - Obliquity
Angle between the Equatorial Plane and the Orbital Plane
• Eccentricity of Earth’s Orbit
Elongation of Orbital Axis
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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
AphelionPerihelion
Ahrens (2nd Ed.), akin to Fig. 2.15
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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
Ahrens, Fig. 2.19L
arge
Large
Area
Area
Small Small AreaArea
Short Path
Long Path
Equal Energy 23.523.5
oo
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Beam Spreading
Low Zenith - Large Area, Much Spreading
High Zenith - Small Area, Little Spreading
Ahrens, Fig. 2.16
Large Zenith Angle Zero
Zenith Angle Large
Zenith Angle
Small Zenith Angle
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Beam Spreading
Zenith Angle Equivalent Area 0o 1.00
10o 1.02 30o 1.15 50o 1.56 70o 2.92 80o 5.76
Horizon Infinite
Schematic Ignores Earth’s Curvature
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Atmospheric Path Length
Zenith Angle Equivalent Atmospheres 0o 1.00
10o 1.02 30o 1.15 50o 1.56 70o 2.92 80o 5.70
Horizon 45.0
Schematic Ignores Earth’s Curvature
Cloud
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Length of Day
Lutgens & Tarbuck, p33
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Day Hours at Solstices - US Sites
Summer-WinterTucson (32o 13’ N)
14:15 - 10:03
Seattle (47o 38’ N) 16:00 - 8:25
Anchorage (61o 13’ N) 19:22 - 5:28
Fairbanks (64o 49’ N) 21:47 - 3:42
Hilo (19o 43’ N)
13:19 - 10:46 Gedzelman, p67
Arctic Circle
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Path of SunHours 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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Noon Zenith Angle at Solstices
Summer-WinterTucson 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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Is Longest Day the Hottest Day?
USA Today WWW Site
Consider Average Daily Temperature for Chicago IL:
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Annual Energy Balance
Heat transfer done by winds and ocean currentsDifferential heating drives winds and currents
We will examine later in course
NH SH
Radiative WarmingRadiative
CoolingRadiative Cooling
Ahrens, Fig. 2.21
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Summary
• Tilt (23.5o) 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
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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