Down is a very fine, fluffy feather that is a natural insulator. Ever wondered how ducks and geese manage daily polar bear swims without blinking an eye? Part of the reason is the thick down feathers that line their undercarriage in winter months.
Nothing beats down for warmth and comfort in a jacket, but not all down jackets are created equal. When you read about the “fill power” of a down coat, the measurement refers to the loft or fluffiness of the down used. That number is loosely related to the insulating value – the higher the fill power, the warmer the jacket. A rating of 400-power will warm you, 600 is better, and 800-plus is downright toasty.
Like all forms of insulation, down works by trapping layers of air between your warm body and the cold air around you – the thicker the layer of trapped air, the better the insulation. Fine down has more spaces to trap air. Greater loft – or space – means higher insulating value.
Because it has more loft, it takes less 800-power down to create a warm jacket that it does 600-power down. This cuts down on weight of the coat. Add in the selection of an ultra-light, water-resistant, rip-stop nylon shell and you’ve got a superior down jacket.
I’ve gone through several down coats over the years – each one a little better than the one before. I thought my last down was close to perfect, but when I tried on the Brooks-Range Alpini Mountain Anorak, I was duly impressed by its lightness. I liked the fact that it compressed easily and came back to full loft with a couple quick shakes. And then I put it on and headed outside. I’ll be hard-pressed to find another down coat any warmer and lighter than this one!