A base layer can make or break a winter trip. Get it wrong and no amount of fancy outerwear helps; get it right and you'll stay warm through a chairlift ride, dry on a skin track, and presentable at the lodge bar afterwards. Outside Online's test team clearly took this seriously, putting nearly 70 women's base layer tops and bottoms through a full winter of field testing — hut trips in British Columbia, resort skiing in the Swiss Alps and Italy, and backyard sessions in the Tetons, Sierra Nevada and Cascades — judging each on warmth, breathability, fit and style.

The top all-round pick was the Smartwool Thermal Merino Mock Neck ($140). Testers wore it almost daily, from −10°F mornings at Jackson Hole to milder afternoons, and liked that it doubles as an everyday top with jeans. The only gripe: the mock neck gets bulky under stacked hoods and gaiters.

For genuinely frigid days, the Norwegian-styled Kari Traa Saga Half Zip and Pants ($150/$140) came out warmest in the test — one tester wore the set on a 10°F morning at Jackson Hole while everyone else shivered. It's a knit wool kit that also works as après wear under a vest, though it runs hot for touring and stretches out with wear.

Budget shoppers should note Quince's merino top and bottoms at $46 and $56 — a price testers found hard to believe. They're not the warmest option (an extra midlayer was needed on an 18°F day), but the tights held up well from 15 to 40°F. Meanwhile, Helly Hansen's Lifa Merino set ($115 per piece) solves a real problem: a synthetic inner layer paired with merino outside means people who find wool itchy can still get its warmth and odour resistance, with excellent moisture-wicking thrown in.

Two more picks fill specific niches. Ortovox's Rock'N'Wool three-quarter bottoms ($110) avoid sock bunching and suit high-output days around freezing and above. And Le Bent's Featherweight Hooded LS Crew ($170, bamboo-merino-nylon blend) was the go-to for temperature swings on backcountry trips — one tester logged weeklong lodge tours in the Selkirks with it as her primary layer. A caveat: it stops at size L. Honourable mentions included a Stio midweight hoodie ($179) for cold-weather resort skiing, Wild Rye's Bassett half-zip and leggings (in an inclusive 0–18 size run), and PAKA's alpaca-blend thermal crew.

Why does this matter to a traveller rather than a gear obsessive? Because the wrong first layer is the most common reason people come home from ski trips, winter hikes or Northern Lights chasing feeling cold and miserable. A good merino set also packs small, resists smells for days, and means you can pack fewer clothes overall. The test's practical verdict: pick a versatile midweight merino for general travel, go heavier (Kari Traa) for standing around in deep cold, and go light (Le Bent, Ortovox) if you'll be huffing uphill. Budget around $50–$170 per piece depending on how specialised you need it.