Editing Greenhouse effect (section)

=== Effect of pressure ===
The relative sizes of the greenhouse effects on different bodies cannot be predicted simply by comparing the amount of greenhouse gases in their atmospheres. This is because factors other than the quantity of these gases also play a role in determining the size of the greenhouse effect.

Overall atmospheric pressure affects how much thermal radiation each molecule of a greenhouse gas can absorb. High pressure leads to more absorption and low pressure leads to less.<ref name="Pierrehumbert2011" />

This is due to "pressure broadening" of [[spectral lines]]. When the total atmospheric pressure is higher, collisions between molecules occur at a higher rate. Collisions broaden the width of absorption lines, allowing a greenhouse gas to absorb thermal radiation over a broader range of wavelengths.<ref name="PierrehumbertTextbook" />{{rp|226}}

Each molecule in the air near Earth's surface experiences about 7 billion collisions per second. This rate is lower at higher altitudes, where the pressure and temperature are both lower.<ref name="eiuatmo">{{cite web |title=The Atmosphere |url=https://www.ux1.eiu.edu/~cfjps/1400/atmos_struct.html#:~:text=Near%20sea%20level%2C%20an%20atom,is%20about%201%20each%20minute. |access-date=30 May 2023 |website=Eastern Illinois University}}</ref> This means that greenhouse gases are able to absorb more wavelengths in the lower atmosphere than they can in the upper atmosphere.<ref name="Plass1950" /><ref name="Wallace2006" />

On other planets, pressure broadening means that each molecule of a greenhouse gas is more effective at trapping thermal radiation if the total atmospheric pressure is high (as on Venus), and less effective at trapping thermal radiation if the atmospheric pressure is low (as on Mars).<ref name="Pierrehumbert2011" />