For these reasons, air temperature is used as the index to snow cover energy exchange.
To be more exact, the snow model uses 6-hourly mean areal air temperature estimates as the index.
In the snow model, air temperature is used as an index to energy exchange across the snow-air interface.

This is different from the degree-day method, which uses air temperature as the index to snow cover outflow.
The degree-day method does not explicitly account for those processes (the freezing of melt water due to a heat deficit and the retention and transmission of liquid-water) which cause snow cover outflow to differ from snowmelt.
An empirical relationship which has given good results in many investigations indicates that snowmelt is proportional to the difference between the mean air temperature and a base temperature.

This relationship can be expressed as:

M = Mf * (T - MBASE)
   where
      M = melt
      Mf = a proportionality factor, commonly referred to as the melt factor

      MBASE = the base temperature (usually 32^oF)