Yet Another Reason to Love Balloons
This is another one that will delight the littler kids, and possibly even surprise your bigger ones. Or you. We won't judge.
While it's not strictly a snowy experiment, this is still a great way to introduce kids (and not-so-science-savvy adults) to contraction and expansion. Simply blow up a balloon (helium isn't necessary), and tie it up outside. If it's really cold -- far below freezing -- you'll notice that it will quickly deflate. If it's a little warmer, it might take a few minutes.
A little primer if you've forgotten your science lessons: As air gets colder, the molecules crowd closer, which means the balloon loses volume. That's contraction. Now bring the balloon inside and impress the kids by explaining that as the air heats, all of its molecules spread out again et voilá: a reinflated balloon, exhibited with expansion.