The men's [ak] Helium Hooded Stretch Insulated Jacket is the midweight layer that should always be on the packing list. You can wear it hiking when it's cool and windy or under your shell to stay warm on a ridge top. It's stretchy enough for an unrestricted feel and breathable enough to wear when you're headed uphill. When you don't need it, it stuffs into its own pocket and easily stows in a small pack.