Tires: winter tires, winter tires, season tires

Question
I live in Canada, in a part that gets serious winter time - we can expect to hit -40 at least a couple of times per winter (that's the point where F and C are the same - too bloody cold!!).
Do all season radials work as well as winter tires in that range?  If not, to what point do the all season radials work well enough not to make it worth the bother of switching?  At what point should the winter tires be taken off?

Answer
Tom all season tires by their nature have to be able to operate in very hot places.  Winter tires are not designed for those condition - so needless to say, winter tires will always be better than all season tires when only temperature is considered.

However, all season tires are also expected to provide a level of wear resistance that winter tires are not.  As a result, snow traction derived by the tread pattern for all season tires is compromised.

There is a general statement being floated around that says the 40 degrees F is the crossover point for temperature.  But don't forget, there is more to this than just temperature.