Re: What are the best temps for snow to form? Is it ever too cold to snow?

Precipitation is funny business - depending on the temperature gradient 
between where the precipitation originates and ground level, rain or snow 
will fall unchanged, or one will transform itself into the other. And then 
there's freezing rain...

The simplest answer is that precipitation is rain above 0 degrees C and snow 
below 0 degrees C. If you allow a few degrees either way to compensate for 
marginally warmer or marginally cooler air between you and the altitude 
where the precipitation is formed, this is a good rule of thumb. It is also 
possible for snow to fall through a particularly warm layer of atmosphere to 
become rain or graupel (snow pellets), and conversely, for rain to fall 
through a cooler layer of atmosphere to become freezing rain or ice pellets.

There is a common belief that snow will not fall below a certain 
temperature. While there is some element of fact to this myth, it is 
generally untrue. If the temperature is very low, and falls off at a normal 
rate with altitude, it is not likely to snow; since the phase change of 
water (ie, the point[s] where ice becomes water or water becomes vapor) is 
dependent on temperature), even minute amounts of water will condense out as 
ice fog at low temperatures. (You kind of have to be Canadian to appreciate 
ice fog. If you're from Florida, the only place you'll ever see ice fog is 
in a movie.)

Snow can, however, fall at temperatures considerably (-20 to -30 degrees C) 
below zero, if the temperatures at the upper levels are relatively warm. An 
upper cold low, for instance, can kick humidity high into the atmosphere, 
and can also usher in very cold temperatures behind its cold front. Since 
snow will not get any snowier as it falls through cold air, in Canada, we 
often see snow accompanying bitterly cold temperatures. And, of course, 
should strong winds also be present, we have classical blizzard conditions.

Thank goodness it's a dry cold...