Wool dominated the outdoor world in the early 20th century because of its exceptional warmth. The secret is in the fibers, which create millions of air pockets that trap the warm air coming off your body. Unlike cotton fibers, which collapse when they're wet and squish those air pockets, wool remains rigid so it stays warm even when it's raining or snowing.

Wool fell out of favor, however, when synthetic insulations cropped up in the 1970s. At the time wool was a pain in the ass to wash—it would bunch up and essentially shrink—plus it was itchy. Synthetics solved both those problems and soon everyone was ditching their old sweaters.

The turnaround for wool came in the 1990s and early 2000s when the textile industry hunkered down and figured out how to make wool more washable. They also started using more Merino wool, which is much softer and nicer on the skin. Nowadays dozens of outdoor brands are back to relying on this natural fiber, and below are five of my favorite pieces I've been using this winter to stay warm.