Katie O’Reilly, writing for Sierra Magazine:

First things first: Make sure your base-layer game is strong.

“What most people don’t realize is that when it’s 30 to 40 degrees below zero, the concern isn’t about getting too cold, but rather getting too hot,” Larsen cautions. “When you sweat, moisture builds up and can replace the layer of air next to your skin.” Hence, the importance of a close-fitting base layer made from moisture-wicking fabric that allows for ventilation. Larsen prefers the wide range of base layers available through Norwegian technical outfitter Helly Hansen. “They make a variety, and I layer up and down depending on the temperature and the activity. For polar expeditions, they have a great line of mountain apparel.”

Get down with down.

Larsen describes a big down jacket as his “last defense against really cold weather—whether you’re in a sub-zero-degree tent or just walking around town.” He explains, “It’s the lightest-weight and warmest of insulators,” and says he relies on Helly Hansen’s shell jackets. “They use Allied Feather & Down, which is sustainably harvested.”