AOS 100: Weather and Climate

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AOS 100 Weather and Climate

• Instructor Nick Bassill
• Class TA Courtney Obergfell

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Miscellaneous

• Homework Reminder
• Exam Reminder

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Review of September 17th Radiation

• Radiation is the one form of heat transfer that
  does not require a medium (air, water, etc.)
• The energy of radiation is carried in the form of
  electromagnetic waves
• All objects emit/receive radiation
• As objects emit radiation, they lose energy and
  cool (unless they are gaining more radiation from
  absorption or some other process)

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Review Continued

• Wiens Law the wavelength of radiation an object
  emits depends on its temperature
• The peak wavelength (?) of emission is
  approximately 2897/ T
• Stefan-Boltzmann Law warmer objects emit more
  energetic radiation than do colder objects, by
  EsT4
• Shorter wavelengths carry more energy
• The albedo of an object is a measure of the
  amount of sunlight it reflects
• The Earths average albedo is .3, meaning it
  reflects 30 of incoming solar radiation on
  average

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A Comparison of Emittances
Fromwww.csulb.edu/rodrigue/geog140/sunwavelength
.gif
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• The atmosphere absorbs some radiation
• However, this doesnt happen uniformly for all
  wavelengths
• Since the Suns radiation and the Earths
  radiation are almost completely separate
  wavelengths, these variations are important!
• Much more of the Earths radiation is absorbed
  by the atmosphere than the Suns

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Kirchoffs Law

• Kirchoffs Law The efficiency of absorption at a
  certain wavelength of energy is exactly equal to
  the efficiency of emission in the same wavelength
• For example, snow is very good at absorbing IR
  radiation, which means it is also very good at
  emitting IR radiation
• If something is perfect at absorbing all
  wavelengths of radiation (and thus perfect at
  emitting all wavelengths), then we call it a
  blackbody
• The Earth is a close approximation to a blackbody
  (for a given temperature, it will emit as much
  radiation as possible for that temperature)
• This means Wiens Law and Stefan-Boltzmanns Law
  can be used with good accuracy for the Earth

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Stefan-Boltzmann Law Revisited

• EnergysT4
• Lets compare the Earth and Sun

The Earths temperature is 288
K E5.6710-8(288)4 E390 Watts/m2
This means the Suns surface emits roughly
190,000 times more energy from every square meter
than the Earth does
The Suns temperature is 6000 K E5.6710-8(6000
)4 E73,483,200 Watts/m2
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The Earths Energy Balance

• Of the radiation emitted from the Sun, the amount
  that makes it to the Earth is approximately 342
  W/m2 at any given spot at the top of the
  atmosphere
• But remember we need to factor in the albedo,
  which is .3, so 30 of this is reflected to space

342.7 239.2 W/m2 Using EsT4, we get 239.2
5.67x10-8 T4 Solving for T gets about 255 K
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Energy Balance Continued

• This calculation gives us an average surface
  temperature of 255 K
• 255 K is equal to about -18 ºC, or about 0 ºF
• But we know our Earths temperature is much
  warmer!
• (more than 50 ºF warmer!)
• Whats going on then?

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We have an atmosphere!

• Our atmosphere absorbs more longwave radiation
  (the kind that the Earth emits) than it does
  shortwave radiation (the kind the Sun emits)

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Energy Balance Continued

• Our atmosphere emits the absorbed radiation in
  all directions
• Therefore, half of it goes back towards the
  surface and half goes out to space
• So the extra bit that the atmosphere is
  continuously sending the surface keeps it warmer
• The actual average temperature is about 288 K
  (about 15 ºC or 59 ºF)
• Radiative equilibrium the state where the rate
  an object emits radiation is equal to the rate
  the object absorbs radiation

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• Due to the Earths tilt, different locations
  receive different intensities of sunlight
• This leads to both differing daytime
  temperatures and the different seasons

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The Seasons (Intro)
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Diurnal Temperature Change

• Obviously, the Earth is only receiving
  (shortwave) solar radiation when the Sun is above
  
• However, the Earth is always transmitting
  (longwave) radiation
• This means that the surface is often losing
  energy at nighttime, and gaining it during the
  daytime
• This is largely what causes temperature changes
  from day to night, in addition to other things
  such as warm or cold air advection

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What can modify this?