I've somehow managed to lead a charmed existence with respect to Berlin weather -- my first winter here (2006-07) was very mild and the next winter I spent 5 weeks in India. I was here four months of the third (2008-09), missing only December, but don't remember it as particularly cold. Last year I know January was tough but again I was in Madrid. This year, though, I've hit the nail on the head; December has been very, very cold. For the first time I can remember since grad school in upstate New York, I'm dealing with temperatures of -11 to -13°C (well into the teens on the Fahrenheit scale).
There's been a lot of snow and I'm happy as can be to be back cat-sitting for the compelling neurotic Emile and Betty near Flughafen Tempelhof. I've already investigated and yes, the rollerblade paths of summer are now cross-country ski paths. Der Berg Ruft in Kreuzberg rents weekend ski packages at 20 euros and since Christmas Eve falls on a Friday, that means Thursday through Monday this weekend, instead of the usual Saturday through Monday. That's where I'll be heading soon, and I'm praying for more snow as the temperatures look set to climb back to normal.