Gerald Bybee

Not to offend our athletes-turned-smoldering fashion models—pro freeskier/surfer Chuck Patterson and former Telluride ski patrol Kit DesLauriers—but long underwear isn't just sexy. Base layers are crucial outdoor gear. Aerobic activities like cross-country skiing demand thin layers that rapidly disperse sweat and body heat—keeping you cool, not warm. When you're winter camping, though, you'll want heavyweight layers to trap that warm air. And for back-and-forth endeavors like ski touring—you're cold, then hot, then cold again—midweights split the difference. Variations of temperature, humidity, and terrain make it tricky to choose the right skivvies, but our base-layer breakdown will help you nail it more often than not.

LIGHTWEIGHT
Skate Skiing, Running, Biking
We tested Nike's quick-drying polyester Dri-FIT top ($36; Kit) by itself mountain biking, and under a shell spring randonnée skiing. The top has a silky feel and is styled like a bike jersey, making it outer-underwear you can use all year.
Terramar's Body-Sensors MicroSilkweight EC2 bottoms ($28; Kit) look like standard-issue polyester, but the fabric carries a mild negative charge that pulls positively charged water molecules off your skin. Shocking, but effective: The Body-Sensors remained dry even when our tester zipped up to force overheating.
Keying on the idea that if you maintain a cool body temperature, you'll ascend faster, the creators of Craft's Micro-climate underwear have incorporated corduroy-style air channels to radiate body heat. Since less material touches your skin, heat readily escapes. Choose the windproof-front WindBlock Crew Neck ($60; Chuck) with the tapered back for winter road rides.
Unlike polyester, which absorbs and then disperses water, polypropylene is hydrophobic, meaning it won't load up like a wet mop. Shake it out, tuck it in your sleeping bag, and in five minutes it feels like it just came out of the dryer. Unfortunately, polypro is known for absorbing body odor, but Helly Hansen's new Lifa Sport Lightweight bottoms ($29; Chuck) don't stink, because they're made from a breathable polyfilament.

MIDWEIGHT
Skiing, Snowboarding, Backcountry Touring, Snowshoeing
Thin polyester wicks but doesn't insulate; fleece insulates but doesn't move moisture well. So, working with Polartec, Patagonia designed Regulator—;highly compressible fleece that grabs moisture like a sponge and then moves it to the outer fabric, where it spreads out and dries. The result? Patagonia's lightweight Regulator R.5 top ($59; Chuck) excels under high exertion in cold temperatures.
Although the box says Expedition Weight, we found Duofold's two-layer, four-way-stretch Varitherm bottoms ($50; Chuck) to be more appropriate for the (normally) less extreme temperatures associated with lift-served snowboarding. Constructed from hollow-core DuPont Thermolite, which traps body heat within each fiber, Varitherms are warm without being bulky (ideal for action sports).
If you sweat excessively, wool gets clammy and heavy. But SmartWool's Next-to-Skin-Wear ($60; Kit) is warm wet or dry (perfect when there's no way to avoid getting soaked) and doesn't stink after four days of hard use. The garment is 100 percent itch-free merino wool; the fibers absorb moisture, pulling it away from your skin, while its natural loft insulates. Want more color and style options? Ibex and Woolrich also make excellent merino base layers.
More like a tight than a traditional long-underwear bottom, Hot Chillys' four-way-stretch Lycra and polyester Micro-Elite Base Layer ($45; Kit) can be worn alone when snowshoeing in moderate temperatures, or under a shell when weather moves in. The density of the weave keeps warm air in and cold wind out.

HEAVYWEIGHT
Winter Camping, Ice Fishing, Playing Pond Hockey, etc.
Bouldering in April? Sleeping in a snow cave in January? The North Face Cirque Top ($60; Chuck) is a cold-weather workhorse. It has ample wicking power for light aerobic exercise and the insulation of a sweater. Buy it snug so you can wear it as a base layer; a ten-inch chest zipper lets you vent.
L.L. Bean's Power Dry Performance Long Underwear bottoms ($40; Chuck) are fleece pants with an excellent warmth-to-weight ratio. Although they're thin enough to wear under a shell, we found the two-layer Polartec fabric to be too warm for all but the coldest ski days. But they're ideal when you're standing around camp, or ice-skating by moonlight.
Mountain Hardwear's Transition Zip T ($135; Kit) doesn't look like underwear, and as you can tell from the price tag, it costs more too. What gives? Part shell, part thermal, the Transition T is two
layers in one. Mountain Hardwear has sandwiched a windproof laminate between two moisture-moving polyester knits, creating a temperature and humidity gradient that forces moisture through the fabric.
Marmot's Polartec Power Stretch Tights ($80; Kit) have a velour inner surface, spandex in the weave for a close fit, and a smooth nylon outer surface that moves freely with a shell. Overheating? Lose the shell. Unlike velvety fleece, the nylon finish doesn't hold snow. We were equally impressed with similar PowerStretch bottoms from Cloudveil and Lowe Alpine. —Marc Peruzzi

Where to Find It: Nike (800-292-6453, www.nike.com); Terramar (800-468-7455, www.terramarsports.com); Craft (781-631-9544, www.craft-usa.com); Helly Hansen (800-435-5901, www.hellyhansen.com); Patagonia (800-638-6464, www.patagonia.com); Duofold (800-448-8240, www.duofold.com); Smartwool (800-550-9665, www.smartwool.com); Hot Chillys (800-468-2445; www.hotchillys.com) Ibex (800-773-9647, www.ibexwear.com); Woolrich (800-995-1299, www.woolrich.com); The North Face (800-362-4963, www.thenorthface.com); L.L. Bean (800-441-5713, www.llbean.com); Mountain Hardwear (800-953-8375, www.mountainhardwear.com); Marmot (707-544-4590, www.marmot.com); Cloudveil (888-763-5969, www.cloudveil.com); Lowe Alpine (800-366-0223, www.lowealpine.com)