Tundra Climate and Polar Desert

Tundra climate (Köppen ET) is defined by having its warmest month average between 0°C and 10°C — too cold to sustain tree growth but warm enough to allow at least a brief thaw and low-growing vegetation — and occurs in two distinct settings: Arctic tundra, forming a circumpolar belt across northern Canada, Alaska, Greenland's coastal fringe, Scandinavia, and Siberia beyond the northern limit of the taiga, and alpine tundra, occurring at high elevation on mountains at almost any latitude, wherever elevation substitutes for latitude in pushing temperatures below the threshold that trees require. The boundary between taiga and tundra, called the tree line, is controlled less by absolute cold than by the length and warmth of the growing season, since trees require a sustained period of soil-warming and active photosynthesis that the shortest, coolest Arctic summers cannot provide.