chapter 2 temperature, radiation & energy
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Chapter 2 temperature, radiation & energy. Temperature vs. Heat. Why is this man unharmed?. Temperature: A measure of internal energy (in this case, 1575 o F). Heat: Thermal energy transferred between systems at different temperatures. - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 2
temperature, radiation & energy
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Temperature vs. Heat
• Temperature: A measure of internal energy (in this case, 1575oF).
• Heat: Thermal energy transferred between systems at different temperatures.
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Energy Transfer
• Conduction, convection, and advection require molecules
• Radiation is an electromagnetic phenomenon, and is able to pass through the vacuum of space.
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(a) conduction: molecular vibration
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(b) convection: eddy transfer
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(b) convection
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22
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(b) convection
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advection: mass transfer
icecold
cool
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Pop quiz
• It is a balmy winter day in Chicago. This is because of warm air …………. by winds from the Gulf of Mexico. – conduction;
– advection;
– convection;
– radiation.
• You can burn your hand holding it above a candlelight because of …– Convection!
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radiationthe solar spectrum
Blue has a shorter wavelength than red
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colors in the sky …
• Why is the clear sky blue?
• Why are sunsets red?
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Scattering of Visible Light
Rayleigh scattering: molecules of size r <<
K ~ -4
K(blue) / K(red) = (red / blue)4
= (0.64/ 0.47)4
= 3.5 blue is scattered more than red
K : scattering efficiency
: wavelength
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Mie scattering: haze, dust r little color variation
Scattering of Visible Light
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Geometric scattering: r >>
(water droplets, ice crystals)
Three forms of light scattering:
Rayleigh : r << Mie : r ~ geometric : r >>
light is reflected or refracted
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Question:
• How much of the solar radiation reaching the earth, is reflected into space?
30%
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Planet Earth’s Albedo: 30%
Albedo: the fraction of solar radiation that is reflected or
scattered back into space
How bright is the moon?
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Mars
Moon
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Venus
volcanoes and dark lava rocks
radar view
visible view
….below a thick CO2 atmosphere with sulphuric acid clouds
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the Earth’s albedo is far from constant
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The albedo of the ocean is very low
Zenith angle,
7%
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Interpret the global mean albedo
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The solar radiation budget on earth
30%100%
4% 20% 6%
19%
51%
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the sun shines every day
every day, the earth cumulates more solar radiation
radiation = energy = heat
so the earth should become warmer every day
puzzle
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Answer: the Earth emits radiation as
well!
micrometer micrometer
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We all emit IR radiation!
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radiation
• solar radiation (0.5 m) terrestrial radiation (10 m)
solar
terrestrial
Now we can connect to the concept of greenhouse gases
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Terrestrial radiation emitted
• Each surface emits radiation, at capacity (‘blackbody’)
• The most likely type of radiation emitted depends on temperature T (K):– Wien’s displacement law (b= 2900)
– max is the wavelength at which the radiation peaks (m)
• The amount of radiation emitted (W) increases with the 4th power of T:– Stefan Boltzman’s equation [= 5.67 10-8 W/(m2 K) ]
• The atmosphere will absorb some of the radiation emitted by the Earth surface.
T
bmax
4TW
We are closer to the concept of greenhouse gases
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Wavelength (micrometer)
Absorp
tion
0.01 0.05 10.50.1 10 1005 50
100%
0%
Wavelength (micrometer)
Radia
tion Inte
nsi
ty
0.01 0.05 10.50.1 10 1005 50
Ultra Violet InfraredVisible
Absorption of radiation by the atmosphere
big window
smal
l win
dow
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If we had no atmosphere …
… the global mean temperature would be 0°F
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Our atmosphere acts as a greenhouse, and causes the air temperature to be 33 K (59°F) above the
Earth’s ‘radiative equilibrium’ temperature
with an atmosphere without atmosphere
T = 59°F (15°C) T = 0°F (-18°C)
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Pop quiz1. Is the greenhouse effect of the Earth’s atmosphere:
– manmade (mainly due to the burning of fossil fuels); – or mostly natural and existed before human history ?
2. What is the ratio of the manmade to the natural greenhouse warming?1. Answer: about 1:33, but rising
(Source: Climate Research Unit, Univ. of East Anglia, UK)
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A petroleum geologist told me this …
• In the last 100 years or so, we have been burning a lot of coal and oil and gas, fossil fuels. That produces heat. That heat adds up and spreads globally. That causes the global warming.
• 3. Is his argument right or false? Why?
• 4. What (else) does cause global warming?
• Answer (3): False. The heat generated by burning of fossil fuels is insignificant compared to other terms in the global energy balance. The heat that was generated by cars and industry years ago has long been dissipated into space as terrestrial radiation.
• Global warming is largely due to the greenhouse gases contained in the burnt fossil fuels (mainly CO2). These gases alter the Earth’s radiative balance.
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How long does it take for the Earth to cool, if the Sun suddenly went out?
• Without the oceans, the Earth would cool from the current average (59ºF) to freezing (32ºF) in 7 days.
• The oceans store a lot of heat. Depending on the rate at which this is released, the cooling down to freezing would probably take some 59 days.
• The heat associated with the burning of all fossil fuels in the past century corresponds with all the solar radiation received by the Earth in just 4 days !
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30%100%
4% 20% 6%
19%
51%
reminder: the solar radiation budget
The Earth surface is emitting IR radiation, but then some of it is absorbed by the atmosphere.
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The Earth’s energy budget
130
NET infrared radiation lost at the earth surface
energy gained by the atmosphere
-117+96=-21
=> There is net deficit of 30 units in the atmosphere, and a net excess of 30 units at the surface
+70
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Global energy balance
• At the top of the atmosphere, outgoing terrestrial radiation is balanced by incoming solar radiation.
• At the earth surface, the net longwave radiation emitted (21%) is insufficient to offset the net solar radiation (51%) received.
• The atmosphere continuously cools by radiation: the net longwave radiation lost (49%) exceeds the net solar radiation (19%) received
• So what prevents the earth surface from heating up & the atmosphere from cooling down?
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Non-radiative atmospheric heating:Conduction + convection = sensible heating
Condensation, freezing = latent heating
The lower atmosphere is heated from below….
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Evaporation takes energy
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Oceans continuously heat up by net radiation uptake. They are ‘air-conditioned’ by evaporation at the surface.
evaporation over the ocean
evaporation
trade winds
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Satellite IR image shows cold anvils on top of thunderstorms
Inter-tropical convergence zone
evaporation
evaporation
Thunderstorms!
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The Earth’s energy budget
-30 +30 net radiation
-30 net radiation
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Fig 2.20 in the textbook. The units are NOT % of the incoming radiation at the top of the atmosphere, but rather in W/m2
=100%
Solar constant = 1380 W/m2
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Global mean surface energy balance:
R = Sn+ Ln
and R H + LE
R = 51 –21 = 30
R = 7 + 23 = 30
Why are the tropics warmer than polar regions?
net rad = net SW rad + net LW rad
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net incoming solar radiationnet outgoing terrestrial radiation
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Why are the tropics warmer than polar regions?
• net radiation R is positive in the tropics, negative at poles.
heat transfer:– atmospheric currents (especially near
fronts)– ocean currents
• in winter, the high-latitude radiation deficit is even larger,
• therefore the pole-to-equator temperature difference is larger,
• therefore the currents need to transport more heat poleward
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There are two reasons why the solar radiation at the surface is weaker
when the Sun is lower in the sky
What are these reasons?
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(1) Because normal insolation provides more energy, per unit area, than does oblique insolation.
Atmospheric attenuation: {scattering + absorbance}
Why is the sun stronger when it is higher in the sky?
(2) Because oblique insolation is more attenuated than is direct insolation.
Air Mass traversed is double at 60º
normal
oblique
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Seasonal variation of the net radiation R at the surface
What explains the seasons?
W/m2
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What explains the seasons?
Sun above equator
Sun above equator
Sun above 23½ºNSun above 23½ºS
try this animation!
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Fig. 2.17
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total insolation, all day long, at various latitudes
June 21: summer solstice December 21: winter solsticeAttenuation removes a great amount of solar energy at the pole.
Axial tilt has plunged the NorthPole into 24-hour darkness.
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Axial Tilt of Earth, 21 June
41N
Equator
Tilted by 23.5 from the perpendicular
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Solar angle v season
Length of day as function of time of year and latitude
40°N
Fraction of solar constant
Fig. 2.16 in textbook
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Energy Balance at the Earth’s Surface
R = H + LE
R warms the surface causing convective currents (H), and R evaporates water (LE)
Net radiation: R = Sn+ Ln
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Pop quiz
• Sensible heat flux H versus latent heat flux LE.
Which one is true?– a: over the ocean LE > H;
– b: over a dry desert surface, at noon, H > LE;
– c: as a global average, LE > H;
– d: all of the above.
Energy Balance at the Earth’s Surface
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H vs. LE Globally
• Over oceans, 90% of R is used to evaporate water (LE), only 10% used to warm the air (H) by conduction or convection.
• On land, H LE.
• Globally, LE = 23 units (77%), H = 7 units.
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Energy
flux
Which bar represents:Australia
South AmericaAntarctica
These bars respresent different continents
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Energy
flux
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Local energy balance
Inside which one is it warmer on a sunny day? Why?
– a white styrofoam cooler, lid closed;
– a white styrofoam cooler, lid off;
– a styrofoam cooler painted black on the inside, lid off;
– a styrofoam cooler, painted black on the inside, lid off, but covered by a glass pane;
– a metal toolbox, painted black on the inside, covered by a glass pane.
– a metal toolbox, painted black on the inside, covered by a glass pane, and buried in the ground so that the top is level with the surface.
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results
• 9 Sept 2003, Prexy lawn, 1:15 pm. Sunny day. Air temperature: 81°F
– a: a white styrofoam cooler, lid closed: 78°F
– b: a white styrofoam cooler, lid off: 88°F
– c: a styrofoam cooler painted black on the inside, lid off: 103°F
– d: a styrofoam cooler, painted black on the inside, lid off, but covered by a glass pane: 189°F
– e: a metal toolbox, painted black on the inside, lid off, but covered by a glass pane: 124°F
– f: a metal toolbox, painted black on the inside, lid off, but covered by a glass pane, half-buried: 115°F
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Summary of chapter 2
• Electromagnetic radiation• Heat transfer (convection, conduction, advection)• Scattering and absorption of radiation by the atmosphere• Shortwave (solar) and longwave (terrestrial) radiation• The natural greenhouse effect• Global energy balance (solar radiation, terrestrial radiation,
and heat transfer)• Seasonal/regional variations of the surface energy balance
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End of Chapter 2