地理--气候带.doc

1、Arctic tundra（北极苔原气候） History of a name: The word tundra is believed to come from the Finnish word tunturia which refers to a treeless plain. This feature helps define the tundra. In transition zones from other biomes, such as the taiga or coniferous forests, stunted trees and shrubs are all that grows. Another factor that helps define the vegetation in the tundra, and thus the tundra itself is the presence of permafrost, a layer of permanently frozen ground just below the surface. Where is it found? Arctic tundra is found almost entirely in the Northern Hemisphere. In the southern hemisphere it is found on isolated islands off the coast of Antarctica as well as the Antarctic peninsula. The tundra falls between two other major biomes, the taiga and ice caps. In essence the zone is controlled by the extremes in the weather. If its too cold, the ice caps dont melt enough to allow vegetation. If its a bit warmer, trees are able to send to send roots deep enough to root and grow. Climate In the arctic tundra there are two seasons: winter and summer. In the summer, the sun is present almost 24 hours a day. This sun however, only warms the tundra up to a range of about 3°C to 12°C. In the winter the opposite light conditions are present. There are several weeks where the sun never rises. This causes the temperatures to drop to extremely cold levels. The average temperature of the tundra is around -28°C while extremes can dip to -70°C. Precipitation levels in the tundra are from 6 to 10 inches a year. Most of this falls as snow. In many ways this biome is a cold desert, lacking significant precipitation. However, something that many people might find unusual given the lack of rain or snow is the presence of a lot of standing water. This is the result of the permafrost. Each summer, the upper layer melts just enough to create small bogs and pools. The water will not soak into the ground however, because the permafrost blocks it. In the winter these pools freeze and the cy