Ever since the Gabba swept through the peloton, Castelli has been the go-to for weather-resistant cycling kit. In order to one-up itself, the brand went were no cyclist has gone before: the sea. The crafty Italians recently got their hands on some of the threads used in the sails on world-class racing yachts, material that's strong enough to stop wind, block water, and propel a boat while doing it. The result is the Men's Vela Vest, which is named for the Latin word for a ship's sail. The Vela situates that maritime wonder material on the front panel, between you and the elements, for a completely weatherproof barrier that still packs down to less than a half pocket when the sun comes back out from behind the clouds.
The material at the Vela's heart is called Dyneema, and in addition to sails and this vest, it also feature in climbing ropes and bulletproof vests. Given those applications in particular, you might assume that it's what people turn to when failure really isn't an option. You'd be right. And that's part of the reason why it makes sense here. We should disclaim that, when we imply that it's the nuclear option in anti-weather fabric, we mean it—and that does mean it won't breath as well as your tissue-thin summer jersey.
To offset the Dyneema-equipped protection, Castelli builds in panels of stretchy, breathable material on the sides and rear in order to help keep the internal climate from turning into a clammy, boggy swamp during hard efforts or when the storm relents. This is particularly useful when the bottom half of a descent is warm enough that you don't need the protection but damp and winding enough that you can't spare a hand to fiddle with the zipper.
Finally, and though we mention it above, the Vela's pack-ability is worth another call-out. It compacts down to about half a man's palm, so when we say it fits in half a jersey pocket, we're erring on the side of understatement. More a like a third of a pocket, meaning you have more room for gloves, warmers, headbands, and any other accessories you find yourself wishing you'd left in the kit drawer.