The baselayer sits directly against your skin and determines how you feel on the ascent. It immediately absorbs sweat and wicks it away to the outside, so you're not standing in wet clothes when you change at the summit. It's the foundation of every ski tour: no other layer works without it.

The mid-layer keeps you warm on the ascent without overheating. It wicks moisture away from the base layer and gives your shoulders and arms the freedom of movement you need when using poles and making kick turns.

The insulation is your reserve for everything that isn't part of the ascent: changing clothes at the summit, removing skins, eating a snack in the wind. It stores heat when your body is no longer producing it, and then disappears back into your backpack, packed down small.

The top layer is your outer shell against wind, snow, and moisture. On a ski tour, you need it where it gets uncomfortable: on exposed ridges, in snowfall, and on the descent when speed turns into cold.

For your legs, you don't work with multiple layers, but with a single pair of pants that has to do both. On the ascent, it wicks away moisture and can be regulated via ventilation; on the descent, it keeps out wind and snow. It also needs to be robust in areas where edges and crampons rub against it.