I wouldn’t know about this because I’ve only ever been to Helsinki in December.
My memories regarding the cold are (1) the snow didn’t melt at all, so instead of the streets being filled with horrible slush as they would be in London, there was just this dry, sand-like dirty snow everywhere, which made it actually less unpleasant; and (2) all the dogs were wearing little shoes, which was monstrously adorbs.
Then I took the overnight “party boat” to Stockholm, and when I went up on the deserted top deck in the middle of the Baltic, there was nothing all around but blackness and a lot of very cold wind which pushed you around the icy metal deck like an air hockey puck. Was pretty cool.
