Editing Greenhouse effect (section)

==== Infrared active gases ====
Gases which can absorb and emit longwave radiation are said to be ''infrared active''<ref name="Archer2011Ch4">{{cite book |last1=Archer |first1=David |title=Global Warming: Understanding the Forecast, Chapter 4: Greenhouse Gases |date=2011 |publisher=Wiley |isbn=978-0470943410 |edition=2 |url=http://forecast.uchicago.edu/chapter4.pdf |access-date=14 June 2023}}</ref> and act as greenhouse gases.

Most gases whose molecules have two different atoms (such as carbon monoxide, {{chem|CO}}), and all gases with three or more atoms (including {{H2O-nl}} and {{CO2}}), are infrared active and act as greenhouse gases. (Technically, this is because when these molecules [[molecular vibration|vibrate]], those vibrations modify the molecular [[Bond dipole moment|dipole moment]], or asymmetry in the distribution of electrical charge.   See [[Infrared spectroscopy#Number of vibrational modes|Infrared spectroscopy]].)<ref name="IPCC" />

Gases with only one atom (such as argon, Ar) or with two identical atoms (such as nitrogen, {{chem|N|2}}, and oxygen, {{chem|O|2}}) are not infrared active. They are transparent to longwave radiation, and, for practical purposes, do not absorb or emit longwave radiation. (This is because their molecules are symmetrical and so do not have a dipole moment.) Such gases make up more than 99% of the dry atmosphere.<ref name="IPCC" />