Isothermal snow is snow that is at the same temperature throughout a given depth range. In practice, the term isothermal is most often used to describe a snowpack that has reached its melting point, 0 Celsius, throughout its depth. As a snowpack reaches its melting point, the bonds between snow grains transform from ice to water, resulting in a loss of cohesion throughout the snowpack.
Loose wet avalanches and wet slab avalanches often result from surface layers reaching 0 degrees. As deeper layers of the snowpack begin to melt, larger avalanches become possible.