We call the isothermal period of gr

We call the isothermal period of ground temperature during the seasonal freezing and thawing period as the zero-curtain period, as a precursor to delimiting the boundary of permafrost from remotely sensed thermal infrared data. During the thaw season, a high thermal gradient between the warming surface and the cold permafrosteffectively conducts heat through the soil without forming a zero curtain; during freeze-up, the coldground surface and the cold permafrost create a low thermal gradient, ensuring a condition at somedepth in the soil where the temperature is at or near 0 ◦C for a prolonged period . In otherwords, seasonally frozen ground not overlying permafrost may exhibit a zero curtain during the thaw, whereas soil underlain by permafrost may exhibit a zero curtain only during freeze-up.