| I'm not sure, but I got this rule of thumb from Will some time ago, which is | one of many rough guide criteria for snow (or not).
| Add together the Dew point and the temperature = A then if:-
| If A 7 prob. snow is very small
| If A=7 prob. snow = 10%
| A=6? 20%
| A=5 30%
| A=4 40%
| A=3 50%
| A=2 60%
| A=1 70%
| A=0 80% or more

The dew point is the much more important of these two figures and the formula should be weighted in some way to reflect this. *On most occasions
where I live, a temperature of 2C and dewpoint of 1C will result in rain, perhaps a trace of snow in it if you look very hard. *Temperature 3C,
dewpoint 0C will be snow most times. Combining these may produce the 50% shown above for A=3, but does not really tell the story.
The temperature structure of the atmosphere above is also important. *Last week we had rain with A= -9!
- Yokel - Yokel @ Ashurst New Forest SU 336 107 * * 17m *a.s.l.

Only a layman's inexpert curiosity but:
At this late date, Yokel, do you remember what were the temp and
dewpt that gave A= - 9 and rain, not snow ?
The formula has worked brilliantly so far here ( E.Anglian coast) .

From the forum's FAQ: Dew Point (strictly dew-point temperature) The
temperature (of an air sample that contains water vapour), to which
that sample must be cooled (Pressure and humidity content being held
constant) to achieve saturation with respect to a water
surface...Frost point (strictly frost-point temperature) The
temperature (of an air sample that contains water vapour), to which
that sample must be cooled (Pressure and humidity content being held
constant) to achieve saturation with respect to an ice surface. (see
also dew point).

Why would the inventor imagine that adding these two temperatures
would give an indication of snow; it seems to be an attempt to balance
two conflicting parameters ?
And why would dewpoint of a surface be more important than air temp
through which the precipitation is falling, as you observed
empirically?
--
Giles