The Arc'teryx Women's Cerium LT Hooded Down Jacket is the lightest piece in the Cerium series, and possibly the warmest, too. The one two punch of high tenacity Airetica face fabric and high warmth, low weight 850 fill European goose down insulation is a knock out when it comes to staying warm in cold and dry conditions. Backcountry touring along the Wasatch Front. Getting a bouldering fix on a cloudless January day. Strolling through dormant cordons in Burgundy. Whatever your pleasure, the Cerium LT jacket will keep you warm without being a burden, either on your back or in your pack. Bigger numbers are fun when it comes to the lottery and down insulation. A higher fill power down, like the 850 fill used in the Cerium LT, refers to how much space a single ounce of down occupies. The more space taken means more air is trapped between the fibers of the down. Trapped air creates a barrier between hot and cold, so warm air stays near you, and cold air stays away. Taking this logic one step further, if one ounce of a particular down takes up more space than other type of down, it stands to reason that you'll need less of the 'bigger' down to keep you warm, thus reducing the weight of your jacket. Since untreated down doesn't work when wet, the folks at Arc'teryx put synthetic insulation in places most likely to get wet. The Cerium LT is not a raincoat, and if you see people building boats, you will get wet and not stay warm. Yet, synthetic insulation on your shoulders, collar, hem, and armpits can withstand a light drizzle until you reach shelter, without sacrificing too much warmth. This superlight jacket comes with a stuff sack so your bag stays organized and the jacket is protected from pokes and prods that happen to luggage. The same down hugging your torso swaddles your melon, courtesy of the insulated low profile StormHood. Two zippered hand pockets hold your keys and phone, trail map and candy, or your son's toys and a baggie of animal crackers.