Finman,
As usual we have different snow to you.
Often it can be quite warm compared to your hive temperatures. The cluster is not so tight. Some on here have found bees spattering the snow under certain conditions. I am sure -20 with a clear sky is not unusual where you live. Often here it is +5, or more, in the sun with melting snow.
The average temperatures in the UK (some parts?) stayed under zero for about 4 weeks last year, but that was simply due to the low night temps of around -12 and lower. Day temperatures were not that cold. Most colonies will not be subjected to strong reflected rays and many will be sheltered (shaded) from direct afternoon sunlight.
The losses can be significant if the colony is marginal, but not for a strong colony, as far as survival is concerned. Slightly slower spring build-up will be an obvious outcome (but not easily quantified) and may not be that important.
I don't have a problem with the colonies in my garden. The sun is too low in the sky to bother them - unless we get snow late in the winter, maybe.
So, overall not a big problem for most, when they think about it. It is just a problem for those that worry about any snow falling in the UK and a very few with marginal colonies in direct sunlight at the wrong time.
Remember the whole road system collapses at the first sign of snow over here. Whole comunities are sometimes cut off for a couple of days and will be in panic mode (well the press reports make that to be the case).
Me? I couldn't care a jot what happens! Nothing really long lasting since 1963.
RAB