Look up Beehive Ventilation by V Shaparew, part 1 & 2, Gleenings September & October 1986.
Even though I'm in New Zealand, we get damp hives during winter without top ventilation. I played with the size of opening over a few winters until I got a size that suits me, about 1 inch by 3/8 which I cut into the split board I have on top of all my hives.
As already stated, too big an entrance causes the bees to use more stores to keeping warm.
Frank LindsayWellington, New Zealand  		 	   		  
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