In reply to john arran:
It's very arbitrary, but then again so is the whole concept of "rounds".
It would be nice if a winter round had to exceed some kind of wintry conditions criteria, but I can't think of how you'd easily set that criteria.
Going off simple things like temperature or snow/ice does rather allow some not that freakish conditions to be classified as winter when it definitely isn't winter.
Also, the criteria would have to be so that you would know at the start whether it was going to be a winter round - hardly fair if you got most of the way round to discover that it'd unexpectedly warmed up or whatever.