Using four reanalysis data sets and ground-based observations, this paper uncovers that on average, 30% of the time, Northern Hemisphere snow cover experiences unfrozen bottom soil. It is demonstrated that the probability of occurrence of unfrozen soils is correlated with the snow types and is maximum over the ephemeral followed by the maritime and prairie snow. The results based on reanalysis data unveil that the seasonal evolution of the unfrozen soil areas is not synchronous with the snow cover extent and exhibits sub-annual bi-modality with two annual maxima in April and October. Interannual trend analyses indicate that shrinkage of spring snow in the past few decades has been accompanied by an increase in the proportion of unfrozen bottom soils, more significantly over polar climate regimes dominated by the tundra and taiga snow. The findings imply that the snowpack basal melting could have increased due to global warming.Plain Language Summary Unfrozen bottom soils below snow layers play an important role in the persistence and stability of snowpack, yet less knowledge is known about its spatial variability and seasonal evolution on a global scale. This study uncovers that, on average, around 30% of annual Northern Hemisphere's snow cover extent is over unfrozen soils with a spatial variability that is highly correlated with snow types. Specifically, unfrozen soils appear more frequently below the ephemeral followed by the maritime and prairie snow. In addition, it is demonstrated that the unfrozen soil areas exhibit different seasonal evolution from snow cover extent with two annual peaks in April and October. The results show that the areas of unfrozen soils are expanding in spring as the snow cover extent is shrinking.< Réduire