Wow, you are talking my language when you mention UPPER ENTRANCE.  I think
every colony in the world should have an upper entrance 365 days of the year.
I am in my 65 year of beekeeping, spending much of the last 20 years teaching
beekeeping all over the world FREE OF CHARGE; and still do it in the U> S> in
spite of being disabled by strokes.
I hate and despise drilling holes in my supers for upper entrances as a lot of
people have done.  I cut a piece of wood out of the front edge of the inner
cover that leaves a hole 5/16" high x 1 1/2" long into the top body, be it a
super or brood body. Someone might say there is a shallow and a thick side to
an inner cover and the shallow side should always be down.  That is baloney!
I put the thick side down so there is about 3/8" of space between the inner
cover surface and the tops of frames.  Bees will only build burr comb there if
they are short on super space - no other reason.  So they stay in that
position 365 days of the year.
What are the advantages: Primarily TWO, and both quite simple.  Talk about
summertime first.  Why make FORAGING bees enter the front door, CLIMB UP
through the CONGESTED BROOD nest to put nectar in the supers.  A FORAGING BEE
only work is nectar gathering - it does nothing else!  When it has an upper
entrance (particularly if you are not using Imirie Shims between supers), the
FORAGER learns to LEAVE and RETURN through that upper entrance and NOT ADD
FURTHER CONGESTION TO THE BROOD NEST.  This results in more flights per day
per bee and certainly tends to reduce the MAIN CAUSE OF SWARMING - congestion
of the brood nest!
How would you react on a cold winter night if someone poured freezing water on
your bare back?  When air is breathed into the lungs and than discharged out,
one of the products is water vapor (clean your eye glasses by blowing your
breath on them).  This warm vapor goes UP (everybody knows heat rises), and in
the winter this warm vapor rises and contacts the cold inner cover where it
liquifies into a drop of cold water.  In time more drops of cold water
accumulate and finally it RAINS of the cluster of warm bees below, and many
freeze to death.  If there is an upper entrance in place, that warm rising
vapor from the bees breathing has a way to escape from the hive by going out
of the entrance to outside.  Lastly, sometimes (but not very often) the front
entrance of a colony is covered by snow or sleet and the weather suddenly
warms to flight weather for an hour or so.  But the bees are penned inside by
several inches of snow and sleet which hasn't melted yet.  If there was an
upper entrance, the bees can fly on a cleansing flight.
 
I have used Upper entrances on my 100+ colonies for at least 50 years, and
just think everybody would be wiser if they used upper entrances too.
                                                                                        George Imirie