Though Arc'teryx named it after a radioactive element with a half-life of 10. 45 million years, the Women's Thorium AR Hooded Down Jacket isn't, thank goodness, radioactive; however, it does slow your decay in cold weather to give you an outdoor half-life longer than 10. 45 seconds. Crafted with a combination of 750-fill European goose down and strategically placed synthetic insulation, this jacket keeps you warm whether you wear it on its own during your chilly backpacking trips or under a hardshell whilst perusing some secret powder stashes. Down is a mix of feathers and clusters that work cooperatively to create insulating air pockets, trapping warmth and preventing cold air from passing. The Thorium's Coreloft synthetic insulation works the same way, creating an air-pocket barricade, but with one substantial difference: wet down doesn't retain heat. To ensure warmth when wearing the jacket, Arc'teryx strategically placed Coreloft synthetic insulation in high-moisture areas such as the hood, cuffs, and under the arms so you can stay warmer longer. The 750 down fill body of the jacket is protected from exterior moistures like rain from the DWR treatment that beads and repels light moisture. Technical bells and whistles are fun, but when it's all said and done, you want a down jacket that will handle the rigors of daily abuse, precisely why Arc'teryx opts for ripstop Colibri as the face fabric. So whether you hike year-round, ski in and out of bounds, or schlep from the dorm to class on inclement days when your fellow classmates bury themselves in bed, the Thorium AR hoodie is there keeping you warm and protected from the elements.