Editing Polar climate (section)

==Quantifying polar climate==
There have been several attempts at quantifying what constitutes a polar climate.

Climatologist [[Wladimir Köppen]] demonstrated a relationship between the Arctic and Antarctic tree lines and the {{convert|10|C|F}} summer isotherm; i.e., places where the average temperature in the warmest calendar month of the year is below the fixed threshold of {{convert|10|C|F}} cannot support forests. See [[Köppen climate classification]] for more information.

[[Otto Nordenskjöld]] theorized that winter conditions also play a role: His formula is {{nowrap|1=''W'' = 9 − 0.1 ''C''}}, where ''W'' is the average temperature in the warmest month and ''C'' the average of the coldest month, both in degrees Celsius. For example, if a particular location had an average temperature of {{convert|−20|C|F}} in its coldest month, the warmest month would need to average {{convert|11|C|F}} or higher for trees to be able to survive there as {{nowrap|1=9 − 0.1(−20) = 11}}. Nordenskiöld's line tends to run to the north of Köppen's near the west coasts of the Northern Hemisphere continents, south of it in the interior sections, and at about the same latitude along the east coasts of both Asia and North America. In the Southern Hemisphere, all of [[Tierra del Fuego]] lies outside the polar region in Nordenskiöld's system, but part of the island (including [[Ushuaia]], [[Argentina]]) is reckoned as being within the Antarctic under Köppen's.

In 1947, [[Holdridge life zone|Holdridge improved on these schemes]], by defining [[biotemperature]]: the mean annual temperature, where all temperatures below {{convert|0|C|F|disp=or}} (and above {{convert|30|C|F|disp=or}}) are treated as 0&nbsp;°C (because it makes no difference to plant life, being dormant). If the mean biotemperature is between {{convert|1.5|and|3|C|F}},<ref>{{cite web|url=http://www.personal.dundee.ac.uk/~amjones/dundee/36biomes.htm|title=Biodiversity lectures and practicals|first=Allan|last=Jones|publisher=University of Dundee|archive-url=https://web.archive.org/web/20070929131915/http://www.personal.dundee.ac.uk/~amjones/dundee/36biomes.htm |archive-date=2007-09-29 }}</ref> Holdridge quantifies the climate as [[subarctic climate|subpolar]] (or alpine, if the low temperature is caused by elevation).