When added heat liquefies or thaws snow.

Melt occurs when heat is added to the snowpack through some combination of solar radiation, warming temperatures, airflow, or rain on snow.   Small amounts of melt can strengthen snow layers by adding suction between certain types of snow or after wet grains refreeze into crusts.  Conversely, excess meltwater can break snowpack bonds, decrease the strength of the snowpack, and work as a catalyst for avalanche activity.

Melt introduces liquid water into the pore spaces between snow grains, shown in this magnified view. Credit: The International Classification for Seasonal Snow on the Ground / Sturm