Editing Permafrost (section)

==== Discontinuous permafrost ====
[[File:Digging in permafrost.jpg|thumb|left|Excavating ice-rich permafrost with a [[jackhammer]] in [[Alaska]].]]
The below-ground temperature varies less from season to season than the air temperature, with mean annual temperatures tending to increase with depth due to the geothermal crustal gradient. Thus, if the mean annual air temperature is only slightly below {{convert|0|°C|°F|abbr=on}}, permafrost will form only in spots that are sheltered (usually with a northern or southern [[aspect (geography)|aspect]], in the north and south hemispheres respectively) creating discontinuous permafrost. Usually, permafrost will remain discontinuous in a climate where the mean annual soil surface temperature is between {{convert|-5|and|0|C|F}}. In the moist-wintered areas mentioned before, there may not even be discontinuous permafrost down to {{convert|-2|°C|°F}}. Discontinuous permafrost is often further divided into extensive discontinuous permafrost, where permafrost covers between 50 and 90 percent of the landscape and is usually found in areas with mean annual temperatures between {{convert|-2|and|-4|C|F}}, and sporadic permafrost, where permafrost cover is less than 50 percent of the landscape and typically occurs at mean annual temperatures between {{convert|0|and|-2|C|F}}.<ref name="BrownPéwé">{{Cite journal |last1=Brown |first1=Roger J. E. |last2=Péwé |first2=Troy L. |title=Distribution of permafrost in North America and its relationship to the environment: A review, 1963–1973 |journal=Permafrost: North American Contribution – Second International Conference |volume=2 |pages=71–100 |year=1973 |isbn=978-0-309-02115-9 |url=https://books.google.com/books?id=SjErAAAAYAAJ&pg=PA72}}</ref>

In soil science, the sporadic permafrost zone is abbreviated '''SPZ''' and the extensive discontinuous permafrost zone '''DPZ'''.<ref>{{Cite report |first= S. D. |last= Robinson |editor-last=Phillips |contribution= Permafrost and peatland [[carbon sink]] capacity with increasing latitude |title= Permafrost |year=2003 |pages=965–970 |publisher=Swets & Zeitlinger |url=http://www.arlis.org/docs/vol1/ICOP/55700698/Pdf/Chapter_169.pdf |isbn=90-5809-582-7 |display-authors=etal |display-editors=etal |access-date=18 August 2023 |archive-url=https://web.archive.org/web/20140302190815/http://www.arlis.org/docs/vol1/ICOP/55700698/Pdf/Chapter_169.pdf |archive-date=2 March 2014 |url-status=live }}</ref> Exceptions occur in un-glaciated [[Siberia]] and [[Alaska]] where the present depth of permafrost is a [[Relict (geology)|relic]] of climatic conditions during glacial ages where winters were up to {{convert|11|C-change}} colder than those of today.