Earths Radiation Balance and the Seasons

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Earths Radiation Balance and the Seasons

• Geog 1, Week 3, March 12
• Chapters 2 and 4 of Christopherson
• Need to Know
• Energy flow through the atmosphere
• Energy changes with latitude and season

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-270oC
6000oC
15oC
EQUILBRIUM
What is the incoming and outgoing radiation?
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Wavelength in micro-metres (microns or ?m)
0.01
103
10-8
0.4-0.7
106
?-rays
X-rays
UV
Infrared
Microwave
Radio
10?m infrared
Peak energy output at 20oC
Visible spectrum Peak energy output of sun
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24 absorbed in atmosphere
10,000 times more energy than we use
31 reflected
45 absorbed at surface
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Incoming Solar Radiation

• Top of atmosphere, solar constant 1.37kW/m2
• 5.5x1024 J/year (humans use 4x1020 J/year)
• Full sun at surface, facing sun, 1kW/m2
• But due to clouds, absorption, scattering only
  about 25 of sunlight, on average, reaches
  surface
• At one location only daylight ½ the time

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From Christopherson p89
Hobart 135W/m2
Some trivia Australians each use about 250GJ
fossil fuel per year, about 6m2 in Hobart
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All the energy absorbed by the air and surface is
radiated back out into space
Radiation
Convection
Evaporation/ Condensation (latent heat)
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Outgoing Radiation

• Infrared with a peak intensity at about 10?m
• Gases such as CO2 absorb some of this infrared
  radiation, hence concern about global warming
• Cloudy night is warmer because radiation cannot
  escape to space

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From Christopherson p95
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Part 2

• Having discussed radiation in and out
• Now it is important to consider the effect of
  latitude on heating of the surface
• And the fact that the tilt of the Earths axis
  means different latitudes will get more or less
  radiation at different times of the year.

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The Seasons

• Solar System
• Earth about 150,000km from sun

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Above Arctic Circle no sun
23.5o
71o
43oS
Hobart 21 Dec. Day 15 h long
South Pole no night
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24o
Hobart 21 June Day 9 h long
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71o
24o
E
W
N
Summer and Winter Sun Paths for Hobart Very
important for solar house design
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Summary

• Radiation from sun
• Radiation from Earth
• More incoming energy near equator
• Earths tilt- seasons

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Next Week

• Global Circulation (Synoptic Winds)
• Chapter 6 of Christopherson
• Especially pages 140 to 155