Light, warm, highly versatile insulation performs as a standalone and midlayer. Atom Series: Synthetic insulated midlayers. | LT: Lightweight. The Atom LT Vest is a proven performer across a range of seasons, landscapes and activities. Elevated with advanced insulation and material technologies, this versatile vest delivers its best, most versatile performance ever. Coreloft™ Compact synthetic insulation retains warmth if wet and withstands years of packing and unpacking, and the breathable, durable Tyono™ 20 shell resists wind and light weather. Stretch fleece side panels aid ventilation and freedom, and the entire vest packs into its own pocket.