Our heat vents out through the chimney...?

my mom told me today that our heat vents our of our house via the chimney. This concerned me because today it snowed a lot (I'd say 10 inches or so). How does the heat vent if the chimney is packed with snow? Of course I have no idea why I'm worried about this, considering how we've had lots more snow in the past (as much as two feet in a day) and we've never gotten carbon monoxide poisoning or anything. So how does the heat vent if it's snowed a lot? Thanks.