ERTH 535:
Planetary Climate Change
(Spring 2018)
Summary of Notes
on Basic Laws of Radiation
Dr. Dave Dempsey
Dept. of Earth & Climate Sci.,

  1. Electromagnetic Spectrum

    1. The electromagnetic spectrum consists of a range of wavelengths of electromagnetic radiation. We assign names to groups of wavelengths based on the interactions between radiation of those wavelengths and matter (atoms and molecules) (e.g., "visible light" is so named because humans can see those wavelengths).

    2. Infrared, visible (0.4 to 0.7 microns), and ultraviolet radiation are the most important for understanding climate and climate change

  2. Basic Laws of Radiation

    1. Most matter emits radiative energy, all the time. Because radiative emission is process in which heat energy in matter is transformed into radiative energy (which then propagates away), it constitutes a mechanism of heat loss. Because most matter emits radiative energy continuously, this means that most matter loses heat continuously via radiative emission.

      Much (though not all) of the matter that emits radiative energy emits radiation at all wavelengths (though not in equal amounts of each wavelength).

    2. The warmer an object is, the more intensely it emits radiative energy.
      • The Stefan-Boltzmann Law captures this relationship quantitatively. (It relates radiative emission flux, or intensity of radiative emission, to absolute temperature of a blackbody.)
      • Recall that radiative intensity is another term for flux of radiative energy. Flux is defined generally as the rate at which some quantity—energy, mass, momentum, etc.—encounters, passes through, is absorbed by, or reflects from a unit of area of some surface. A rate is the change in some quantity per a unit of time.
      • A blackbody is an object or substance that absorbs all wavelengths of radiation that strike them, and emit radiative energy at a theoretically maximum possible rate that depends on the blackbody's absolute temperature, as described by the Stefan-Boltzmann Law.
      • The sun, and the earth as a whole (at least, in the longwave infrared portion of the spectrum), behave nearly as blackbodies, and the Stefan-Boltzmann Law applies to a good approximation.
      • Given Earth's long-term, global-average surface temperature of 59°F, calculate global-average radiative emission intensity at Earth's surface.
      • Given the answer to the previous question, calculate the total rate of radiative energy emission by Earth's surface.

    3. Wien's Law (relating wavelength of maximum emission intensity to absolute temperature of a blackbody)
      • A blackbody emits all wavelengths of radiation, but not all with equal intensity.
      • There is a wavelength of peak emission, which is inversely proportional to the absolute temperature of the blackbody.
      • Calculate wavelengths of maximum emission intensity of Earth's surface and of the sun.
      • In what parts of the electromagnetic spectrum do the wavelengths of peak emission intensity for Earth's surface and for the sun lie?

    4. Emission spectra (graph with plots illustrating both the Stefan-Boltzmann Law and Wien's Law
      • Example: earth vs. sun
        • Basis for distinguishing between solar radiation (or shortwave radiation) and terrestrial radiation (longwave infrared [LWIR] radiation )
        • Solar (shortwave) radiation consists mostly of visible, shorter wavelengths of infrared radiation (near infrared radiation), and ultraviolet (UV) radiation.
      • Example: burner on electric stove

    5. Not all substances are blackbodies.
      • Some objects/substances absorb some wavelengths but not others (selective absorbers).
      • Such objects/substances emit well only wavelengths that they absorb well.
        • Examples of selective absorbers: Every gas in the atmosphere; snow/ice and clouds.
        • Nitrogen, oxygen, and argon absorb little or no radiative energy of any wavelength; hence, they emit little or no radiative energy as well.
      • Some substances act as blackbodies in certain portions of the electromagnetic spectrum but not others
        • Examples: greenhouse gases (absorb at least some wavelengths of longwave infrared radiation but most absorb little or no solar radiation)

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