Editing Cloud physics (section)

=== Supersaturation ===
The amount of water that can exist as vapor in a given volume increases with the temperature.  When the amount of water vapor is in equilibrium above a flat surface of water the level of [[vapor pressure]] is called saturation and the [[relative humidity]] is 100%. At this equilibrium there are equal numbers of molecules evaporating from the water as there are condensing back into the water. If the relative humidity becomes greater than 100%, it is called supersaturated. Supersaturation occurs in the absence of condensation nuclei.{{fact|date=January 2018}}

Since the saturation vapor pressure is proportional to temperature, cold air has a lower saturation point than warm air.  The difference between these values is the basis for the formation of clouds.  When saturated air cools, it can no longer contain the same amount of water vapor.  If the conditions are right, the excess water will condense out of the air until the lower saturation point is reached.  Another possibility is that the water stays in vapor form, even though it is beyond the saturation point, resulting in [[supersaturation]].{{fact|date=January 2018}}

Supersaturation of more than 1–2% relative to water is rarely seen in the atmosphere, since cloud condensation nuclei are usually present.<ref>{{cite book |first1=R.R. |last1=Rogers |first2=M.K. |last2=Yau |title=A Short Course in Cloud Physics |publisher=Elsevier Science |year=1989 |isbn=978-0750632157 |url=https://books.google.com/books?id=dZkRAQAAIAAJ |edition=3rd |volume=113 |series=International Series in Natural Philosophy}}{{pn|date=January 2018}}</ref> Much higher degrees of supersaturation are possible in clean air, and are the basis of the [[cloud chamber]].

There are no instruments to take measurements of supersaturation in clouds.<ref>{{cite journal |doi= 10.1175/1520-0469(2003)060<2957:sowvic>2.0.co;2|year=2003 |volume=60 |issue=24 |pages=2957–74 |title=Supersaturation of Water Vapor in Clouds |journal=[[Journal of the Atmospheric Sciences]] |last1=Korolev |first1=Alexei V |last2=Mazin |first2=Ilia P |bibcode=2003JAtS...60.2957K |doi-access=free }}</ref>