Sleeping Bag Rating - What's In A Number?
"It was barely 30 that night, but I'd thought I was going to freeze to death in my 20-degree bag!"
That was my takeaway after my first, and only, winter backpacking trip. Most seasoned campers have learned that a 20-degree bag, at best, will only keep you alive when the temp plunges to 20. Sleep? Not a chance.
Sleeping bag temperature ratings seem vague at best, and at times darn fictitious; pick any sleeping bag and check out the reviews. Customer reactions range from comfortable to crippling. For the same bag. At the same temperature. Why is that?
That number on the bag is supplied from tests by two organizations; the European Norm (EN) 13537, and International Organization for Standardization (ISO) 23537. They provide two rather interesting measures; a Comfort rating, and a Lower rating. REI does a nice job explaining these numbers (plus a whole lot more) in their article How to Choose the Best Backpacking Sleeping Bag. Factors include whether you're male or female, and a "cold" or "warm" sleeper.
But here's an oft-missed point about these ratings. It assumes you're in a tent, on a 1-inch insulated sleeping pad, wearing a hat, and long underwear. I hadn't in the past, but I do now. Understanding what these temperature ratings mean, recognizing how you sleep (warm or cold), and bringing along proper sleep gear will go a long way to a comfortable night's sleep.
So remember all you campers...it's going to be cold out there. Bundle up.