The RoS in Castelli's Alpha RoS Light Jacket stands for Rain or Shine, as in the jacket is wind- and waterproof but also breathes well enough to manage moisture when the clouds part and the sun shines through on a late-November day. Depending on conditions, the jacket is actually well suited to Castellie's suggested temperature range of 46 - 59 degrees Fahrenheit. We don't always trust vendor recs, and we know that seems like an extraordinary claim, but, well, it's an extraordinary jacket. If it's raining and windy, 50 is actually comfortable; if it's a bright winter afternoon, then temperatures in the mid-40s range are fine—though we should disclaim that some of us need a thermal baselayer to push down into the low-40s.
Gore's Windstopper 150 membrane is the key to the jacket's versatility, but Castelli's cycling clothing know-how is why it's ideally suited for serious cold-weather training and racing. Part of that is the inclusion of a rear panel of Castelli's Nano Flex Xtra Dry, a thermal material with a DWR treatment that dumps excess heat, improves breathabillity, and maintains a water-resistant surface across the entire jacket.
The Windstopper layer is separated from a thermal layer across the front panel, so you can unzip the protective shell but leave the thermal layer intact, letting you moderate exposure on long climbs without exposing your bare chest to a frigid, wintery blast. Skiers and alpinists use Windstopper indiscriminately: their goal is to stay warm and dry at all costs. Cyclists' needs, on the other hand, change over the course of a ride depending on intensity, climbing, descending, etc., so we need the protection to change with us in real time. The Alpha solves that problem.