The estimation of the long wave radiation exchange between building outside surfaces and the sky requires the knowledge of the sky temperature.
Several models are presented in the literature for the prediction of the sky temperature. Which one should we use ?
Nearly all of these models are empirical, which means they perform best for areas whose radiative climate is similar to the one for which they were originally obtained. Moreover, they contain errors due to experimental uncertainty. The best is to test them for your area...
Physical models do not have these limitations, but are typically much more complex...
Thank you M. Chris.
When searching in the literature, I found several correlations that vary from simple formulations, where The sky temperature depends only on the dry air temperature, to more complex formulations, where the sky temperature depends also on the air water vapor tension and the sky cloud cover.
How to test these correlations in order to choose the most suitable for my area?
You would need to find a nearby station where these variables are measured along with the longwave irradiance. This is typically done at research-class stations, such as of those of BSRN, http://bsrn.awi.de/
Thank you M. Chris for this interesting link.
There is no running station in Tunisia, but I will try with a nearby station.
Hi - sorry to answer a question with a question(!), but I'd be interested in finding out more about the models you describe, for the purpose of a quantitative conceptual grasp of how sky temperature varies with atmospheric water vapour and cloud profile, if possible. Thanks!
@Naouel: There are 2 other networks monitoring longwave radiation (IR): FLUXNET and CEOP-GEWEX.
@Peter: There are many publications on this topic. Among the recent and open literature, see e.g.
http://www.tdx.cat/bitstream/handle/10803/31841/tavm.pdf?sequence=3
http://www.hindawi.com/journals/amete/2014/525148/
http://www.mdpi.com/2072-4292/5/10/4735
http://www.the-cryosphere.net/3/75/2009/tc-3-75-2009.pdf
http://journals.ametsoc.org/doi/pdf/10.1175/1520-0426%282002%29019%3C1500%3ADOANSM%3E2.0.CO%3B2
Hi - many thanks both, suspect these'll be really useful references that I'm pretty sure I wouldn't have stumbled on otherwise. For info, my initial interest is in the extent to which cloud cover can be used as a proxy for the nocturnal longwave cooling responsible for formation of nocturnal stable atmospheric boundary layers - i.e. it's not perfect as the fraction of cloud cover is not the only governing factor.
Peter, This paper might be useful:
https://www.researchgate.net/publication/251429725_Automatic_cloud_amount_detection_by_surface_longwave_downward_radiation_measurements
Article Automatic amount detection by surface longwave downward radi...
Another recent useful paper on this:
http://onlinelibrary.wiley.com/doi/10.1002/2014JD022310/abstract
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