The emergence of drop-type and thin-layer-type quasi-liquid layers on ice crystal surfaces and their thermodynamic origin

•Two kinds of quasi-liquid layers (QLLs) with different morphologies (droplets and thin layers) are observed in-situ on basal and prism faces of ice crystals by laser confocal microscopy combined with differential interference contrast microscopy (LCM-DIM).•The drop-type and thin-layer-type QLLs are 20- and 200-times less fluidic than bulk water, respectively.•The drop-type and thin-layer-type morphologies are formed by the wetting transition of QLLs on ice crystal surfaces.•The drop-type and thin-layer-type QLLs are a metastable phase that is formed kinetically only in supersaturated or undersaturated water vapor.