- Defined by W. Köppen (1931) as the zone having a definite winter with snow, and a short summer, generally hot. It includes a large part of North America between the Arctic Zone and about 40°N, extending to 35°N in the interior. In Central Europe and in Asia the boreal zone extends southward from the tundra to 40°–50°N.
- A biogeographical zone or region characterized by a northern type of fauna or flora.
The term boreal region is used mainly by American biologists, and includes the area between the mean summer isotherm of 18°C or 64.4°F (roughly 45°N latitude) and the Arctic Zone.
Köppen, W. P. 1931. Grundriss der Klimakunde. 2nd ed., Berlin: Walter de Gruyter.
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