The rate of snow melt is dependent on energy availability, which is mostly in the form of radiation . Cold snowpacks have a negative energy balance, but warming causes the snowpack to become isothermal (0oC) and additional energy results in positive energy balance and melt. Daily snow melt in forested areas is considerably less than melt in open areas, as forests protect the snow cover from solar radiation and wind. Canopy warming can increase longwave radiation, but the net effect of forest is reduction in melt. Rain falling on snowpack may accelerate its melt rate, but intense sunshine of late spring and summer is the principal melting energy source.
Most operational procedures for snow melt prediction rely on
ambient air temperature as the index of the energy available for
melt. The temperature index is usually used to characterize
the level of the energy balance
because it is superior to other simple methods for the full energy balance at
the snow surface .
The most common expression relating snow melt to the temperature index is: