单词	permafrost
释义	permafrost /ˈpəːməfrɒst / noun [mass noun] A thick subsurface layer of soil that remains below freezing point throughout the year, occurring chiefly in polar regions: hilly terrain underlain by permafrost... The permafrost below the topsoil is frozen all year around, and this prevents roots from penetrating deeply into the ground. In regions influenced by permafrost, water migrates along the thermal gradient from warm to cold, thereby feeding ice in the frozen core. In many high mountains regions, permafrost may be present. Origin 1940s: from permanent + frost.
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