A half-face balaclava covers your head, neck, and lower face below the nose, leaving your nose and mouth more exposed. It's a great middle-ground option when you want solid warmth but don't need full wind protection.
This style shines in moderate cold conditions or when you're skiing hard enough that breathability matters. Because your nose and mouth aren't covered, you'll breathe easier, talk to your crew without muffling, and—most importantly—your goggles are far less likely to fog up from trapped breath.
The tradeoff is that your nose and mouth are more exposed to wind and cold, so a half-face design isn't the best pick for bitter temps or high wind chill days. But for most resort skiing in typical winter conditions, it hits a sweet spot between protection and comfort.