I would not open a hive at low temperatures except in an emergency.  To find
out what is happening in your hives in the winter look at the debris that has
fallen onto your varroa floor.  You will soon gain experience at reading and
interpreting what you find there.
There is an excellent little book "At the Hive Entrance" by H.Storch which
tells you how to know what happens inside the hive by observation on the
outside.
Chris Slade