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树苗冻拔一般多发生在季节交替的早春或深秋,当气温下降时,表层土壤的冷却速度比下层的土壤快,因此表层土壤中的水汽开始凝结起来,下层土壤中的水汽从下层上升到上层水汽凝结的地方,这样使表层积蓄了大量的水分。如果地温下降到0℃以下时,土壤表层含有的大量水分就冻结成针状冰晶,久之,表层土壤上形成柱状冰晶体。次日气温升高,地温也随之上升到0℃以上时,表层土壤中的柱状冰晶体融化成水,密度增大,体积减小。由于没有冰柱支撑土壤,冻结的土壤下沉恢复原
Sapling generally occurs mostly in the early spring or late autumn when the seasons alternate. When the temperature drops, the surface soil cools faster than the underlying soil, so the water vapor in the surface soil begins to condense and the moisture in the lower soil rises from the lower to the upper Condensation of water, so that the accumulation of a lot of surface water. If the ground temperature drops below 0 ℃, the soil surface contains a lot of water to freeze into a needle-like ice crystals, a long time, the formation of surface soil columnar ice crystals. The next day the temperature rises, the ground temperature also will rise above 0 ℃, the columnar ice crystals in the surface soil melt into water, the density increases, the volume decreases. Since there is no icicle to support the soil, the frozen soil sinks back to its original state