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 10 °C (50 °F) summer isotherm; i.e., places where the average temperature in the warmest calendar month of the year is below 10 °C (50 °F) cannot support forests. See Köppen climate classification for more information.
Otto Nordenskiöld theorized that winter conditions also play a role: His formula is 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 (this would mean, for example, that if a particular location had an average temperature of −20 °C (−4 °F) in its coldest month, the warmest month would need to average 11 °C (52 °F) or higher for trees to be able to survive there). 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 improved on these schemes, by defining biotemperature: the mean annual temperature, where all temperatures below 0 °C (32 °F) are treated as 0 °C (32 °F) (because it makes no difference to plant life, being dormant). If the mean biotemperature is between 1.5 and 3 °C (35 and 37 °F), Holdridge quantifies the climate as subpolar (or alpine, if the low temperature is caused by altitude).
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