lecture 6 - uci sitessites.uci.edu/climatechange101/files/2016/09/2016... · •controls energy...
TRANSCRIPT
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Lecture 6Solar vs. terrestrial radiation and the
“bare rock” climate model.
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• Controls energy balance of Earth
• Is all around us all the time.
• Can be labeled
• by its source (solar, terrestrial)
• or its name (ultra violet, visible, near infrared, infrared, microwave, etc….)
• or by its wavelength (e.g. < 3 micrometers)
Radiation
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• Which electromagnetic radiation waves have the shortest wavelength and highest frequency?
• a) gamma rays b) radiowaves
• c) x rays d) UV rays
i-clicker quiz:
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• All matter with a temperature glows radiation energy.
• The idea of “Blackbody” radiation yields a powerful law of nature:
Review from last time:
FBB =σT4
• How much radiation energy is glowing based on T.
• The amount is really sensitive to temperature!
• T4 = T x T x T x T
Blackbody energy flux (W/m2)
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Hotter things glow radiation at shorter wavelengths!
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I-clicker quiz: In the following movie, the process of allows us to see energy move from a person to a chair through the process of .
• A: radiation, convection
• B: radiation, conduction
• C: convection, radiation
• D:conduction, radiation
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• The Sun emits energy at a lot of wavelengths, some we feel warms us, most we see as visible light
Solar emission
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Solar radiation has peak intensities in the shorter wavelengths, dominant in the region we know as visible,
but extends at low intensity into longwave regions.
Solar emission
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• Images taken in thermal infrared wavelengths produce accurate measurements of temperature
Thermal Imaging
Thermal EmissionSome we can’t see!
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SUN
EARTH
Terrestrial emission: The Earth emits radiation too. But at much lower temperatures, so therefore at longer wavelengths.
• Both sun & earth are almost perfect blackbodies!
• The hot sun radiates at shorter (visible) wavelengths that carry more energy
• Energy absorbed by the cooler earth is then re-radiated at longer (thermal infrared) wavelengths
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i-clickersurvey
TheStephan-BoltzmanlawisF=σT4IfFhasunitsofW/m2andTistemperatureinK,whataretheunitsofσ?
A)W/m2KB)W/m2K4C)m2K/WD)m2K4/W
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Long Waves = small photons
Short Waves = BIG PHOTONS
Waves and photons• Is light a wave?
• YES!
• Is light a particle?
• YES!
• All light travels at the same speed
• Think of short waves as BIG HEAVY particles
• Think of longer waves as small, lightweight particles
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Most everything that happens on our planet…
(Recall from Lecture 3)
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… Is a link on the chain of energy flowing out from the hot sun and dissipating into outer space.
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Fine, but what actually happens to solar radiation energy once it enters the Earth’s atmosphere?
Energy from solar rays
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Remember:Conservation of Energy
I = R + A + T
What happens when radiation meets matter.
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I-clicker question: Does the Earth reflect solar radiation?
A: Yes
B: No
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• Albedo: the fraction of incoming radiation that gets reflected
• Surface albedo varies according to the material • Spatially • Temporally
Reflection of radiation - jargon alert: “albedo”
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- By Prof. Dargan Frierson - University of Washington
“Albedo”
https://www.youtube.com/watch?v=aj25vm8eN2M
Music to help you remember the unfamiliar word:
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Diagram of the solar radiation “budget”
30% reflected by clouds, air, dust, and surface 19% absorbed by the atmosphere (mostly clouds)
51% absorbed at the surface
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Get ready to nerd out!
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We now have enough building blocks to do our first legit climate calculation…
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We now have enough building blocks to do our first legit climate calculation…
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Energy in = Energy outThe first law of thermodynamics requires that:
Watts in from solar radiation = Watts from terrestrial radiation
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How many Watts come into the Earth from solar radiation?
• At the distance of the Earth’s orbit from the sun, a constant solar energy flux shines towards the Earth.
• We give this a special name:
• S = 1360 W/m2 = “the solar constant”
• S can be calculated for other planets too.
• Gets smaller the farther they are from the sun.
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How many Watts come into the Earth from solar radiation?
So to find the Watts, we multiply S by the area of this disc in m2, over which the solar energy flux is absorbed.
S = Watts per square meter, constant
Wm2
x ? = W
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• Formula for area of a circle?
• Area = πR2
= π x R x R
• R is radius of Earth.
• π ~ 3.14152
So far, Watts in = S x π x R x R
How many Watts come into the Earth from solar radiation?
… almost correct, but not quite…
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Energy in = S x pi x R2
How many Watts come into the Earth from solar radiation?
Does the Earth absorb all the solar energy that strikes it?
No. 30% is reflected back to outer space. Only remaining 70% is
available for absorption. Need to multiply S by 0.7 = (1 - albedo)
What’s missing?
Energy in = (1 - α) x S x π x R2 (complete)
Wm2
solar flux
absorption areax m2
Wm2
non-reflected solar flux
So far,
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Energy in = Energy outThe first law of thermodynamics requires that:
Watts in from solar radiation = Watts out from thermal radiation
= FBB x emission area
Blackbody energy
flux
Surface area of the Earth
(the whole Earth glows)
=This is the “Bare rock” climate model
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=
The power of math:
..solar constant,
…and the albedo
We can solve for a planet’s temperature!This is the “bare rock” model: A climate prediction from laws of
energy balance, black body radiation and geometry!
If we know…
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What temperature does the “bare rock” model predict?
• Solving for T predicts an equilibrium temperatue that is really cold: Minus eighteen degrees Celsius. -18C
• If Earth were this cold it would have:
• frozen oceans, miles of ice
• So something must be missing from the model…
• The atmosphere! The Earth is not a bare rock.
• If it were it would be real cold here on the surface.
• But the atmosphere blankets our rocky surface. This makes a big difference to the temperature.
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Next time: The Greenhouse effect.
Why the atmosphere keeps us warmer than we should be.