The Streamliner 3ST is Vanson’s fourth-generation thermal liner, and it’s specifically designed to serve as a warm long-sleeve base layer, a thermal vest, a safety garment, or as a standalone outer layer. It’s lighter than it looks, and it’s not much bulkier than a normal quilted liner while offering much better insulation. Some of that warming quality is due to the long elastic cuffs and tall collar that do a great job of keeping cold air from getting to your torso. What do I know about cold air living in Southern California? I packed the Streamliner on an early spring trip to Big Bear, when my 6am departure saw the mercury hovering just above freezing. After experiencing the insulative capabilities of the Streamliner, I’m confident that it’ll be coming with me on future trips to elevation.