Cerium is the most abundant rare earth metal, decomposes slowly in cold water and rapidly in hot water. The Arc'teryx Mens' Cerium LT Hooded Down Jacket is not quite as prevalent as its namesake, but does slow your decomposition in cold weather. The combination of 850 fill European goose down and strategically placed Coreloft insulation keeps you warm whether you wear the Cerium alone or under a hardshell when you're confronted with a winter tempest most storm chasers avoid. Bigger numbers are fun when it comes to betting odds and down insulation. A higher fill power down, like the 850 fill used in the Cerium, refers to how much space a single ounce of down occupies. The more space taken up by the down means more air is trapped between the fibers. Trapped air creates a barrier between warm air from your body and cold air in the environment, so you stay warm when it's cold outside. Taking this logic one step further, if one ounce of a certain down takes up more space than a different type of down, it stands to reason you'll need less of the 'bigger' down to keep you warm, thus reducing the weight of your jacket. In essence, this jacket's high down fill makes it both light and warm. Since down doesn't maintain its warmth when it gets wet, the folks at Arc'teryx placed synthetic insulation in the spots that are most likely to get a bit damp. Still, while Coreloft insulation does resist moisture better than down, and an ominous drizzle won't leave you a cold, soaking mess, it's best to pair this with a waterproof shell if you know the weather is coming. This superlight hooded jacket comes with its own stuff sack so you can easily bring it along and so your pack stays organized. The same 850 fill down fills the low profile StormHood so your melon stays warm. Mom was right if you keep your head covered, you'll stay warmer. Two zippered hand pockets hold your keys and phone, trail map and candy, or your partner's ID and lip balm.