Tundra, also known as permafrost, tundra or tundra, comes from the Sami language T ū ndra (tundra), which means "treeless plain". In physical geography, it refers to the environment where trees can't grow because of low temperature and short growing season; Geologically, it refers to all kinds of rocks and soils with ice below 0℃. Generally, it can be divided into short-term frozen soil (hours, days or even half a month), seasonal frozen soil (half a month to several months) and permafrost (more than a few years). Frozen soil is a state in which the frozen soil layer is below the freezing point of water for thousands of years or even tens of thousands of years. Permafrost, seasonal permafrost and short-term permafrost account for about 50% of the land area, of which permafrost accounts for 25% of the land area.