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AOS 100: Weather and Climate

AOS 100: Weather and Climate. Instructor: Nick Bassill Class TA: Courtney Obergfell. Miscellaneous. Homework Reminder Exam Reminder. Review of September 17 th : Radiation. Radiation is the one form of heat transfer that does not require a medium (air, water, etc.)

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AOS 100: Weather and Climate

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  1. AOS 100: Weather and Climate Instructor: Nick Bassill Class TA: Courtney Obergfell

  2. Miscellaneous • Homework Reminder • Exam Reminder

  3. 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)

  4. Review Continued • Wien’s Law: the wavelength of radiation an object emits depends on its temperature • The peak wavelength (λ) of emission is approximately ~ 2897/ T • Stefan-Boltzmann Law: warmer objects emit more energetic radiation than do colder objects, by E=σ*T4 • Shorter wavelengths carry more energy • The albedo of an object is a measure of the amount of sunlight it reflects • The Earth’s average albedo is .3, meaning it reflects 30% of incoming solar radiation on average

  5. A Comparison of Emittances From:www.csulb.edu/~rodrigue/geog140/sunwavelength.gif

  6. The atmosphere absorbs some radiation • However, this doesn’t happen uniformly for all wavelengths • Since the Sun’s radiation and the Earth’s radiation are almost completely separate wavelengths, these variations are important! • Much more of the Earth’s radiation is absorbed by the atmosphere than the Sun’s

  7. Kirchoff’s Law • Kirchoff’s Law: The efficiencyof absorptionat a certain wavelength of energy is exactly equalto the efficiency of emissionin 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 Wien’s Law and Stefan-Boltzmann’s Law can be used with good accuracy for the Earth

  8. Stefan-Boltzmann Law Revisited • Energy=σT4 • Let’s compare the Earth and Sun: The Earth’s temperature is ~288 K E=5.67*10-8*(288)4 E=390 Watts/m2 This means the Sun’s surface emits roughly 190,000 times more energy from every square meter than the Earth does The Sun’s temperature is ~6000 K E=5.67*10-8*(6000)4 E=73,483,200 Watts/m2

  9. The Earth’s 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 E=σ*T4, we get 239.2 = 5.67x10-8 * T4 Solving for T gets about 255 K

  10. 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 Earth’s temperature is much warmer! • (more than 50 ºF warmer!) • What’s going on then?

  11. 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)

  12. 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 emitsradiation is equalto the rate the object absorbsradiation

  13. Due to the Earth’s tilt, different locations receive different intensities of sunlight • This leads to both differing daytime temperatures and the different seasons

  14. The Seasons (Intro)

  15. 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

  16. What can modify this?

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