Snow cover controls seasonally frozen ground regime on the southern edge of Altai Mountains

•The thermal effect of snow cover will reach maximum when snow depth is at its optimal thickness varying from 40 to 70 cm.•Snow cover caused an average 44.5-cm decline of maximum freezing depth of SFG during 1961-2015 on the SEAM.•SFG regime was dominated by snow cover whereas air temperature was the primary factor controlling SFG degradation.•A new empirical method was established to evaluate the thermal effect of snow cover.