As always, Allen Dick's comments on the effects of cold on bees were
accurate and worthwhile.  Nonetheless, Allen and I approach this subject
from very different perspectives.  As a personal goal of mine is to get
beekeepers to open up their hives with ventilation (winter and summer), I
think it is worthwhile to provide a word or two on the difference in
perspectives, and why I think (despite Allen's worthwhile experience and
thoughts) that beekeepers should adopt the position that 'cold does not hurt
bees'.

Our perspectives are different because:
1.  Allen is in very cold Alberta, Canada, where the winds blow hard all
winter.  I am near Albany, New York.  Here the winters are much colder than
most of the US, but not nearly as cold as in Alberta or in Northern Maine
(we are colder than southern Maine), Minnesota, or similar places.  While we
have a fair degree of cold (often enduring a week or more of nightly
temperatures of -10 degrees F), and think our westerly winds are bad, we do
not nearly have the near-prairie conditions that lead to the kind of winds
that Allen gets.
2.  I am trying to give advice to the thousands of beekeepers who keep fewer
than 25 or so hives and think it is best to speak to 'normal' conditions
(whatever those are) rather than the extreme cold conditions faced by a few.

From studies by Mark Winston (now at Simon Fraser University in British
Columbia, Canada) we know that once bees form a cluster they 'stay put' and
do not, as once thought, rotate so that the bees on the outside (where it is
very cold) move to the center (where it is 90 degrees F) and bees in the
center move to the outside.  The only warmth the bees on the outside of the
cluster get is that provided by those on the inside...sometimes for several
months at a stretch.

Marla Spivak at the University of Minnesota, and others, have used probes to
measure the interior of beehives, as well as the temperature of clusters.
In the dead of winter, they found conditions where bees were surviving with
an interior cluster temperature of 90 degrees, an exterior cluster
temperature of 50 degrees, and a temperature of -30 degrees just 4 inches
from the cluster!  Moreover, the bees survived as strong clusters until
spring.

This is why I make the generalization that 'cold does not hurt bees'.

Without exception, every year literally dozens of beekeepers tell me their
bees died during the winter, despite ample honey stores and appropriate and
timely mite treatments.  They want to know why and how to prevent similar
deaths in the future.  After I have asked all the questions to eliminate
disease and lack of honey stores as possibilities, I ask 'when you opened
the hive in the spring, what did the interior look like?'  By then, I pretty
much know the reply I will get 'there was a black soggy mess, mold or fungus
was everywhere'.  Yuck!

Then we go through the drill, with me telling them to open up their hives,
particularly with top entrances and them telling me that they are afraid to
do so 'because the draft (or cold) will kill their bees'.  I have been
through this so many times that if I were more intelligent I'd avoid the
discussion.  But I patiently explain to them that a by-product of the bees
maintaining  a 90 degree temperature in the center of the cluster is that
air is heated.  Warm air will gain moisture as it rises upward in the hive.
If it hits the top of the hive, where it is cold, that moisture will
precipitate (as cold air will not hold as much moisture as warm air),
literally causing 'cold rain' to fall onto the bees below.  Dry and cold the
bees can handle, wet and cold they cannot handle.  They die, mold and fungus
can survive just fine in 50 degree temperatures, and they feed on the
carcasses...yuck!

So, if your conditions are at all like those in Alberta, Northern Maine,
Minnesota, etc., protect your bees from cold...and still provide a means for
the warm moisture-laden air to escape the hive.  If your conditions are more
like Chicago, Boston, Kansas City, Detroit, or Albany, NY, don't worry about
the cold, and be generous about upper ventilation.

Hope I can help,

Lloyd
Lloyd Spear, Owner of Ross Rounds, manufacturer of comb honey equipment
for beekeepers and Sundance pollen traps.
http://www.rossrounds.com
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