Physics of Planetary Climate Cors221: Physics in Everyday Life Fall 2009 Module 3 Lecture 2:...
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Transcript of Physics of Planetary Climate Cors221: Physics in Everyday Life Fall 2009 Module 3 Lecture 2:...
Physics of Planetary ClimateCors221: Physics in Everyday Life
Fall 2009 Module 3
Lecture 2: Equilibrium Temperature and The Greenhouse Effect
From Last Time✔ Science can only disprove, not prove✔ Climate is what you expect; weather is what you get✔ Light, all light from radio to gamma rays, is
electromagnetic radiation✔ Shorter, bluer wavelengths have more energy and are
more dangerous than longer, redder wavelengths.✔ All bodies emit blackbody radiation; hotter bodies emit
more ( T4) light and bluer (shorter wavelength) light.✔ Earth cools by radiating infrared light to space by
blackbody radiation
Radiative Equilibrium
In equilibrium, the Earth is neither heating up nor coolingoff; its average temperature stays pretty much constant.
Therefore the same amount of total energy that Earthreceives each second must be being radiated to keepEarth in long-term equilibrium.
Radiative Equilibrium
How much power does Earth absorb in total?R
E2 AF/a2 = total power radiated
Planet cross-sectional areaSolar irradiance at a AU from the Sun
Radiative Equilibrium
How much power does Earth radiate in total?4R
E2 T4 = total power radiated
Planetary surface areaBlackbody power radiated per square meter
Radiative Equilibrium
Setting power in = power out:
AF/a2 = 4RE
2 T4
AF/a2 = 4 T4
AF / (4 a2) = T4
T = (AF / (4 s a2) ).25
F = 1370 W / m2
= 5.67 x 10-8 W / (m2 K4)a = distance from Sun in AUA = fraction of light absorbed
Equilibrium TemperaturesT = (AF / (4 a2) ).25 F = 1370 W / m2
= 5.67 x 10-8 W / (m2 K4)a = distance from Sun in AUA = fraction of light absorbed
Earth: a=1 A=0.7
Mercury: a=0.39 A=0.82
Venus: a=0.72 A=0.25
Moon: a=1.00 A=0.89
Mars: a=1.52 A=0.75
Equilibrium TemperaturesT = (AF / (4 a2) ).25 F = 1370 W / m2
= 5.67 x 10-8 W / (m2 K4)a = distance from Sun in AUA = fraction of light absorbed
Earth: a=1 A=0.7 Actual T = 288K Teq
= 254.3K
Mercury: a=0.39 A=0.82 Actual T = 440K Teq
= 442.5K
Venus: a=0.72 A=0.25 Actual T = 737K Teq
= 231.7K
Moon: a=1.00 A=0.89 Actual T = 273K Teq
= 274.5K
Mars: a=1.52 A=0.75 Actual T = 210K Teq
= 210.1K
Interaction of Light & Matter
T = fraction of light transmitted
R = fraction of light reflected
A = fraction of light absorbed
T + R + A = 1
All the light goessomeplace; it doesn'tdisappear
Leading Greenhouse GasesFamily Feud style:
#1: Water Vapor (~55% or 19K)
#2: Carbon Dioxide (~18% or 6K)
#3: Methane (~7% or ~2K)
Key Points•In equilibrium, a body in space emits the same amount of energy as it absorbs• Equilibrium Temperature•Earth's equilibrium T = 255K (-18oC!). Teq is good for airless worlds, not for ones with air•Transmission + Reflection + Absorption =1•Gases in Earth's atmosphere absorb at different wavelengths•The greenhouse effect comes about because atmospheric gases transmit sunlight at visible wavelengths, but absorb Earth's blackbody emission in the infrared•#1: H
2O #2: CO
2 #3: CH
4
T = (AF / (4 a2)).25
This has been true throughout the past 500 million years