For cold, dry days in the mountains or the city, the Arc'teryx Thorium AR Jacket is a super warm 750 fill goose down and CoreloftTM mid layer. The collar is the perfect height for protecting the neck while still allowing for hassle free layering. We all know the one downfall of down insulation is that it often fails to keep you cozy when wet. Synthetic insulation, however, is much more resilient in moisture. For this reason, Arc'teryx Down Composite MappingTM Technology puts synthetic CoreloftTM insulation in areas that are prone to getting wet the shoulders, underarms and wrists.