The element, thorium, was named after Thor, the Norse god of thunder. It stands to reason that Arc'teryx attached the same moniker to the Men's Thorium AR Hooded Down Jacket. This down jacket will protect man from the evils of winter and the perils of packing when space is at a premium. The combination of 750 fill European grey goose down and specifically situated synthetic insulation offers aggressive warmth when this jacket is worn over a thermal baselayer if you're working hard, and when it's worn under a hardshell if Thor is flexing his muscles and throwing storms in your face with gleeful abandon. Down is a fowl mix of feathers and clusters that work cooperatively to create air pockets in and amongst the fiber structure. Air pockets are little barricades, preventing warm air from escaping and cold air from entering. Synthetic insulation works the same way, except instead of using feathers, it uses man made materials to create the air pocket barricade. The big difference between natural and synthetic insulation becomes evident when moisture is an issue. Wet down is dead down, and you'll go down quickly from the cold. Synthetic insulation resists water penetration and dries quickly when water wins out, reducing your exposure to cold, wet conditions. The designers at Arc'teryx exploited the differences in insulation to your benefit areas that are the first line of exposure to the elements, like your shoulders, the hood, and the collar, are filled with Coreloft instead of down. The athletic fit and low profile cuffs cut a trim silhouette, and if it's 60F when you're putting together gear for a cold weather adventure, the Thorium squishes down into a cylinder not much bigger than a large coffee for easy packing. Throw it in your bag and still have room left over for essentials, like chocolate bars and beef jerky.