Hi Bryan:

Eva Crane says this in her book: ‘Bees and Beekeeping’:

“Colonies that are buried in snow seem to suffer remarkable little damage,
but if snow covers the ground without blocking the flight entrances, and
the sun shines brightly, bees are stimulated to fly out; absorption of
sunlight by the hive walls increases the temperature inside the hive, and
reflected light from the snow at the flight entrance has a smaller effect.
A flying bee normally receives most light from the sky above, but because
the snow is much brighter than the sky, the angular light distribution
(ALD) above snow-covered ground is reversed (Velthuis & Verheijen, 1963).
The bee therefore flies upside down, in a disoriented way, and crashes on
the snow, where she dies of cold. While snow is on the ground, a board
placed so that it darkens the full width of the entrance can prevent most
of these deaths. In certain circumstances, not well understood, honeybees
and other insects can adjust to the reversed ALD, and fly without
difficulty over sunlit snow. But flying bees have been observed to crash
land on white coral and sand beaches in Wake Island during bright sunshine
(Hitchcock, 1986), probably also because the ALD is reversed.”

Velthius, H. H. W.; Verheijen, F.J. (1963) Why the combination of sun and
snow can be fatal to honeybees. Bee Wld 44(4): 158-162

Although we don't seem to pay that much attention to them on bare ground,
dead bees and feces are always around the hives.  I seem to notice more
dead bees in front of my hives after a fresh snowfall, too. Like most
beekeepers, I’m generally not too concerned about it. If dead bees are seen
in the snow, it must mean there are still live ones in the hive.

In the ‘Bee Tidings’ newsletter, Marion Ellis had some thoughts similar to
those already mentioned by others here on dead bees in the snow:

http://entomology.unl.edu/beekpg/tidings/btid2001/btdapr01.htm#Article2

Regards,
Dick Allen