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Chapter 2 temperature, radiation

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Chapter 2 temperature, radiation & energy Temperature vs. Heat Temperature: A measure of internal energy (in this case, 1575oF). Heat: Thermal energy transferred ... – PowerPoint PPT presentation

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Title: Chapter 2 temperature, radiation


1
Chapter 2 temperature, radiation energy
2
Temperature vs. Heat
Why is this man unharmed?
  • Temperature A measure of internal energy (in
    this case, 1575oF).
  • Heat Thermal energy transferred between systems
    at different temperatures.

3
Energy Transfer
  • Conduction, convection, and advection require
    molecules
  • Radiation is an electromagnetic phenomenon, and
    is able to pass through the vacuum of space.

4
conduction molecular vibration
5
(b) convection eddy transfer
1
6
1
(b) convection
2
7
(b) convection
1
2
3
8
advection mass transfer
icecold
cool
9
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!

10
radiation the solar spectrum
l
Blue has a shorter wavelength than red
11
colors in the sky
  • Why is the clear sky blue?
  • Why are sunsets red?

12
Scattering of Visible Light
K scattering efficiency
K ?-4 K(blue) / K(red)
(lred / lblue)4
(0.64m / 0.47m)4
3.5 ? blue is scattered more than red
Rayleigh scattering molecules of size r ltlt ?
l wavelength
13
Scattering of Visible Light
Mie scattering haze, dust r ? ?
little color variation
14
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15
Three forms of light scattering Rayleigh r
ltlt ? Mie r ? geometric r gtgt ?
Geometric scattering r gtgt ? (water droplets,
ice crystals)
light is reflected or refracted
16
Question
  • How much of the solar radiation reaching the
    earth, is reflected into space?

30
17
Planet Earths Albedo 30
Albedo the fraction of solar radiation that is
reflected or scattered back into space
How bright is the moon?
18
Mars
Albedo 17
Moon
Albedo 6
19
Venus
visible view
radar view
.below a thick CO2 atmosphere with sulphuric
acid clouds
volcanoes and dark lava rocks
Albedo 78
20
the Earths albedo is far from constant
3-10
10-30
5-20
75-95
15-45
21
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22
now
23
The albedo of the ocean is very low
Zenith angle, ?
?
7
24
Interpret the global mean albedo
25
The solar radiation budget on earth
30
100
4
20
6
19
51
26
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
27
Answer the Earth emits radiation as well!
micrometer
micrometer
28
We all emit IR radiation!
29
radiation
  • solar radiation (0.5 mm) terrestrial
    radiation (10 mm)

solar
terrestrial
Now we can connect to the concept of greenhouse
gases
30
Terrestrial radiation emitted
  • Each surface emits radiation, at capacity
    (blackbody)
  • The most likely type of radiation emitted depends
    on temperature T (K)
  • Wiens displacement law (b 2900)
  • lmax is the wavelength at
    which the radiation peaks (mm)
  • The amount of radiation emitted (W) increases
    with the 4th power of T
  • Stefan Boltzmans equation
    s 5.67 10-8 W/(m2 K)
  • The atmosphere will absorb some of the radiation
    emitted by the Earth surface.

We are closer to the concept of greenhouse gases
31
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32
Absorption of radiation by the atmosphere
big window
small window
33
If we had no atmosphere
the global mean temperature would be 0F
34
Our atmosphere acts as a greenhouse, and causes
the air temperature to be 33 K (59F) above the
Earths radiative equilibrium temperature
with an atmosphere
without atmosphere
T 59F (15C)
T 0F (-18C)
35
Pop quiz
  • Is the greenhouse effect of the Earths
    atmosphere
  • manmade (mainly due to the burning of fossil
    fuels)
  • or mostly natural and existed before human
    history ?
  • What is the ratio of the manmade to the natural
    greenhouse warming?
  • Answer about 133, but rising

(Source Climate Research Unit, Univ. of East
Anglia, UK)
36
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 Earths radiative
    balance.

37
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 !

38
reminder the solar radiation budget
30
100
4
20
6
19
51
The Earth surface is emitting IR radiation, but
then some of it is absorbed by the atmosphere.
39
The Earths energy budget
70
130
energy gained by the atmosphere
NET infrared radiation lost at the earth surface
-11796-21
gt There is net deficit of 30 units in the
atmosphere, and a net excess of 30 units at the
surface
40
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?

41
Non-radiative atmospheric heating Conduction
convection sensible heating Condensation,
freezing latent heating
The lower atmosphere is heated from below.
42
Evaporation takes energy
43
Oceans continuously heat up by net radiation
uptake. They are air-conditioned by evaporation
at the surface.
evaporation
trade winds
evaporation over the ocean
44
condensation (latent energy release)
evaporation (cooling)
45
Satellite IR image shows cold anvils on top of
thunderstorms
evaporation
Thunderstorms!
Inter-tropical convergence zone
evaporation
46
The Earths energy budget
-30 net radiation
30 net radiation
-30
47
100
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
Solar constant 1380 W/m2
48
Why are the tropics warmer than polar regions?
Global mean surface energy balance
net rad net SW rad net LW rad
R Sn Ln
R ? H LE
and
R 7 23 30
R 51 21 30
49
net outgoing terrestrial radiation
net incoming solar radiation
50
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

51
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?
52
Why is the sun stronger when it is higher in the
sky?
normal
oblique
(2) Because oblique insolation is more
attenuated than is direct insolation.
(1) Because normal insolation provides more
energy, per unit area, than does oblique
insolation.
Air Mass traversed is double at 60º
Atmospheric attenuation scattering absorbance
53
Seasonal variation of the net radiation R at the
surface
W/m2
What explains the seasons?
54
What explains the seasons?
Sun above equator
Sun above 23½ºN
Sun above 23½ºS
try this animation!
Sun above equator
55
Fig. 2.17
56
total insolation, all day long, at various
latitudes
June 21 summer solstice
December 21 winter solstice
Attenuation removes a great amount of solar
energy at the pole.
Axial tilt has plunged the North Pole into
24-hour darkness.
57
Axial Tilt of Earth, 21 June
Tilted by 23.5? from the perpendicular
41?N
58
Solar angle v season
Length of day as function of time of year and
latitude
40N
Fig. 2.16 in textbook
Fraction of solar constant
59
Energy Balance at the Earths Surface
Net radiation R Sn Ln
  • R H LE
  • R warms the surface causing convective currents
    (H), and R evaporates water (LE)

60
Pop quiz
Energy Balance at the Earths Surface
  • Sensible heat flux H versus latent heat flux LE.
    Which one is true?
  • a over the ocean LE gt H
  • b over a dry desert surface, at noon, H gt LE
  • c as a global average, LE gt H
  • d all of the above.

61
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.

62
These bars respresent different continents
Energy flux
Which bar represents Australia South
America Antarctica
63
Energy flux
64
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.

65
results
  • 9 Sept 2003, Prexy lawn, 115 pm. Sunny day. Air
    temperature 81F
  • a a white styrofoam cooler, lid closed 78F
  • b a white styrofoam cooler, lid off 88F
  • c a styrofoam cooler painted black on the
    inside, lid off 103F
  • d a styrofoam cooler, painted black on the
    inside, lid off, but covered by a glass pane
    189F
  • e a metal toolbox, painted black on the inside,
    lid off, but covered by a glass pane 124F
  • f a metal toolbox, painted black on the inside,
    lid off, but covered by a glass pane,
    half-buried 115F

66
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

67
End of Chapter 2
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