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Blog - the color of night (Rev #18, changes)

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Or: how big is the "greenhouse effect" really?

We will need to calculate thermodynamic properties of the atmosphere, at least approximately, to determine the molecular emission spectra.

What about the dependency of emission and absorption on the concentration of infrared active gases?

Calculating the DLR

Calculating the DLR on a sheet of paper, even with the use of a pocket calculator, would quickly turn out to be quite a task.

We could start by making some assumptions about the different layers of the atmosphere. We would also need to look up molecular spectra. Thankfully, for this task there is some help: The HITRAN database. HITRAN was founded by the US air force. Why? I don’t know, but I guess that they needed the data for air craft design, for example. Look out for the interview with Dr. Laurence Rothman for some background information about HITRAN; there is a link to it on the home page.

But anyway: We see that this task is complicated enough to justify the effort to write computer code to handle it. But we are lucky: Not only have others done this already for us. In fact you can find a survey of some of the existing software programs on Wikipedia:

• page atmospheric radiative transfer codes, Wikipedia

A kind soul has provided a web interface to one of the most prominent software programs, MODTRAN, for us to play around with:

• MODTRAN: Webinterface

Now that we have some confidence in the theory behind DLR, we are ready to look into measurements.

Measuring DLR

To measure DLR and check that it is really the energy flux coming from infrared active components of the atmosphere and not some strange artifact, we have to

• point some measurement device to the sky, to measure what goes down, not what goes up and

• check that the spectrum we measure consists of the characteristic molecular spectra of $CO_2$, $H_20$ etc.

The kind of measurement device we could use for this is called pyrgeometer, for pyr = fire and geo = earth.

For starters we should look for conditions where there is minimum radiation from other sources, no clouds and only a small amount of water vapor. What would be a good place and time on Earth to go to? A dedicated team of scientists decided to weather the grim conditions of the antarctic during polar night for this purpose:

• Michael S. Town, P. Walden, Stephen G. Warren: Spectral and Broadband Longwave Downwelling Radiative Fluxes, Cloud Radiative Forcing, and Fractional Cloud Cover over the South Pole online here.

Just to have a number, the flux of DLR (downwards longwave radiation) is about 300 $W m^{-2}$.

Further Reading

• Vardavas, Taylor: “Radiation and Climate”, Oxford University Press, International Series of Monographs on Physics, 2007

• Grant W. Petty: “A First Course in Atmospheric Radiation”, Sundog Publishing, 2nd ed. 2006