Ultralight base layers (under 130 g/m²) are all about moisture management, not warmth. They're the thinnest option available—almost sheer—and their main job is to wick sweat away from your skin as fast as possible without adding bulk.
This weight class shines in specific situations: spring skiing when temperatures are mild, high-output backcountry touring where you're generating tons of body heat, or as a standalone layer during warm-weather training. If you're someone who runs hot even on cold days, an ultralight base layer paired with a solid mid-layer can prevent you from soaking through your kit.
The catch is that ultralight provides almost zero insulation on its own. If you're sitting on a cold lift in January, you'll feel every bit of that chill. Skip ultralight if you're a resort rider who runs cold or tends to take it easy on the slopes—it's really designed for active skiers in mild conditions who prioritize staying dry over staying warm.