Will Hand's snow probability rule of thumb

Note to Will - Google Groups: I have a rule of thumb which has worked for over 30 years!
Just add the temperature and dewpoint together in the rain and call the sum
S.
If S is <=7 prob turning to snow in the next half-hour is 10%
S <=6 prob is 20%
S <=5 prob is 30%
S <=4 prob is 40%
S <=3 prob is 50%
S <=2 prob is 60%
S <=1 prob is 70%
S <=0 prob is >80%

(S > 7 prob is NIL)

E.g. T 5.0 D -2.0 gives S=3 and a snow prob of 50%
T 3.0 D 3.0 gives S=6 and a prob. of 20%
T 7.0 D -6.0 gives S=1 and a prob. of 70%