Unsurprising fact of the day: mountain storms come out of nowhere and move really quickly. What was a mellow bluebird day can suddenly turn into a serious alpine experience, and you won't be able to stay dry unless you can move as fast as the storm. Weighing in at just 10. 8oz, the Arc'teryx Alpha SL Men's Jacket is a tough and compact rain shell that'll keep you one step ahead of the storm. As part of Arc'teryx's Alpha collection the SL stands for superlight, this shell is designed to be clipped onto your harness or backpack, so you're always prepared on long climbs and alpine expeditions. Made of lightweight nylon and waterproofed with a Gore Tex PacLite membrane, the SL sports a trim fit with e3D ergonomic and articulated construction to keep you sending during the storm, and provides clean sightlines to your harness so you can grab the right protection without hassle. To keep carrying simple, the SL packs into its own pocket and clips to your harness via a webbing loop, keeping it out of the way but close at hand should the weather turn. If it gets really nasty, the StormHood covers your helmet and adjusts with a one handed cinch, and a WaterTight zipper stops sneaky moisture from finding its way into your jacket. Maybe best of all, the removable HemLock foam insert sits around the hem of the SL, keeping it from slipping up underneath your harness and leaving exposed skin. If that's not thoughtful design, nothing is.