Light, semi-puffy, and extra warm, the Xenon is a packable mid-layer piece absolutely critical for cool-weather racing. Its Primaloft One insulation is toasty on the core, and it stays warm (unlike down) even when wet. The jacket's hood ensconces like a heated hat. The Xenon's outer face, a thin Pertex fabric, feels fragile to the touch but has proven to be tougher than imaginable in the mountains and thick brush. Best part: The Xenon weighs just 11 ounces and stuffs into its own chest pocket to the size of a grapefruit to save space inside your pack.