Editing Greenhouse effect (section)

=== Emission temperature and altitude ===

[[File:Spectral OLR.png|thumb|upright=1.35|The temperature at which thermal radiation was emitted can be determined by comparing the intensity at a particular wavenumber to the intensity of a [[Planck's law|black-body emission curve]]. In the chart, emission temperatures range between T<sub>min</sub> and T<sub>s</sub>. "Wavenumber" is frequency divided by the speed of light).]]
Greenhouse gases make the atmosphere near Earth's surface mostly opaque to longwave radiation. The atmosphere only becomes transparent to longwave radiation at higher altitudes, where the air is less dense, there is less water vapor, and reduced [[Spectral line|pressure broadening]] of absorption lines limits the wavelengths that gas molecules can absorb.<ref name="Plass1950">{{cite journal |last1=Strong |first1=J. |last2=Plass |first2=G. N. |date=1950 |title=The Effect of Pressure Broadening of Spectral Lines on Atmospheric Temperature |url=https://ui.adsabs.harvard.edu/abs/1950ApJ...112..365S/abstract#:~:text=Pressure%20broadening%20causes%20lines%20in,absorbed%20by%20the%20upper%20layers. |journal=Astrophysical Journal |volume=112 |page=365 |bibcode=1950ApJ...112..365S |doi=10.1086/145352}}</ref><ref name="Wallace2006" />

For any given wavelength, the longwave radiation that reaches space is emitted by a particular ''radiating layer'' of the atmosphere. The intensity of the emitted radiation is determined by the weighted average air temperature within that layer. So, for any given wavelength of radiation emitted to space, there is an associated ''effective emission temperature'' (or [[brightness temperature]]).<ref name="Pierrehumbert2011">{{cite web |last1=Pierrehumbert |first1=R. T. |date=January 2011 |title=Infrared radiation and planetary temperature |url=https://geosci.uchicago.edu/~rtp1/papers/PhysTodayRT2011.pdf |website=Physics Today |publisher=American Institute of Physics |pages=33–38}}</ref><ref name="Wallace2006" />

A given wavelength of radiation may also be said to have an ''effective emission altitude'', which is a weighted average of the altitudes within the radiating layer.

The effective emission temperature and altitude vary by wavelength (or frequency). This phenomenon may be seen by examining plots of radiation emitted to space.<ref name="Pierrehumbert2011" />