Following my recent post on 10 Ways to Keep Warm While Camping, a friend asked me about the best clothing to wear when it’s cold. The answer to that question is technical, and you should see my article on Layering for more detailed information. We ended up talking about down jackets. I told her that the best down jackets are thick, have a hood, fall low on the hips, and have a high fill power. But what is fill power?