Distinguishing between rain and snow

If the air temperature becomes lower enough, rainfall will change to snowfall.
Two scenario were provided for distinguishing between rain and snow.

1. Using 'Critical Temperature'
In general, the switching often assumes to be distinguished by the criterion temperature 0 (℃).

2. Using 'Snow ratio' function
According to Yamazaki (1998), the snow ratio s(Tw) can be estimated by using wet bulb temperature Tw.

where T is the air temperature and e the water vapor pressure at each 1x1 deg grid.
Snow ratio depends on Tw like the figure below.