CLASS 4: 7 September 2006

REVIEW: Read this website on the basics of blackbody radiation.

Today's dialogue considered brightness temperature, and infrared thermometry.

Brightness temperature is only a 'real' temperature when the object is a blackbody.
Otherwise it is the temperature that a perfect blackbody would have to produce the amount of radiance observed by some calibrated sensor that might be looking
at a cloud, or other object. It is generally a function of wavelength since the emissivity of an object is a function of wavelength.

The IR radiance emitted by the atmosphere varies in time and space because the emissivity of the atmosphere is a strong function of composition, especially of the
variable water vapor amount. Also, the temperature structure of the atmosphere changes rapidly as well.

Examples of atmospheric emission as measured from space and from the ground, looking up, were given.

A specific example was taken from previous years FTIR measurements in class. An outline of the
websites visited is given here:

A photo of the instrument being used for measurements.

A radiance measurement of the downwelling IR energy in the atmosphere, spectrally resolved.
(Be sure to review for your self the relationship of wavenumber and wavelength. Wavenumber is the 1 / wavelength.)
(Note especially that at the saturated IR lines, the emission is coming from the atmosphere near the instrument.)
(It may be useful to compare the radiance measurement with the theoretical spectra for absorption by IR active gases.
Peaks for particular gases are a guide for where to look for peaks in the emission spectra.)

Then measured brightness temperatures for two very different days were given, along with their soundings.
The soundings are from the Reno National Weather service. Note that the brightness temperatures cross
at several key points --- remember that emission is due both to source temperature, and to emissivity. Emissivity
is especially affected by the concentration of the absorber, and water vapor is the most variable infrared active gas
in the atmosphere that also has a large effect on the IR spectrum.
Note that the IR atmospheric window region is where the brightness temperatures are lowest, and the radiance is still appreciable.
Low brightness temperature corresponds to low emissivity, and low emissivity corresponds to low absorptivity.
In turn, low absorptivity corresponds to high transmissivity.