Outdoor Research Women's Helium II Jacket;

Built for flash-storm protection, the 100% waterproof, super-breathable Helium II Jacket weighs an incredibly light 5.5 ounces. An ultra-compressible piece you'll never leave behind, it's so light and packs down so small you'll forget it's clipped to your harness until you throw it on to ward off a sudden mid-route downpour. Compressible to the size of an apple, the Helium II keeps you completely protected from unfriendly weather on the trail but doesn't waste space or add more than a feather's-worth of weight to your pack.