I would say a minimum 20cm of frozen snow as a base for fresh snow to lie on and build up. The base acts as a freezer to keep the new snow below freezing and prevents it from thawing. That's what you'll see the pieste bashing team creating on the cams now. A solid compact base to hopefully see us through the season.
I know - its all this snow makes me go a bit giddy! - That and the 3 kids I have off school poorly.
Sorry. Not very helpful was it, but I knew someone sensible (who knew) would reply with a proper answer.
Thanks Pamporovo Pete & Snow White. New poster, I've been reading the forum for a while, got a week booked for 15 Jan for the family of first timers and we've got answers to virtually all of our questions plus lots of good information without having to do anything other than browse on here.