It’s a known fact that layers are important when it comes to weatherproof clothing, especially in the snow. Mid Layers are responsible for providing insulation and warmth and are often paired up with a base layer for wicking sweat and an outer layer for protection from the elements.
Most Mid-Layers are not designed to be worn in isolation up at the snow and hence are made of softer and more comfortable materials than Ski Jackets or Pants. There are a number of options when it comes to insulation materials. Down (or Feathers) has been used for an eternity and still offers the best warmth to weight ratio. The Patagonia Down Sweater is the most iconic technical down jacket around. 800 Fill Down is incredibly warm and the light face material is a beaut.
Most Down Jackets struggle in wet conditions and aren't as good as synthetic insulation options. The Patagonia Nano Air Jacket and North Face Thermoball Jacket have proven that synthetic insulators are worth looking at. Both Warm and Breathable no matter how wet they get.
Fleeces are a great option for adding a bit of warmth underneath a ski jacket. They also double as great apres jackets and their simple styling is known around ski towns. Fleeces have the advantage of being warm even when wet and dry quickly if they do get a bit of rain/snow. Check out the Classic North Face Glacier Fleece or the retro Patagonia Synchilla Women's Fleece