Boots

Nothing takes the fun out of winter trekking faster than when that bone-numbing chill seeps into your boots. If you're not familiar with winter-apparel construction, some of the fabrics, liners, and insulation mentioned in this guide may be as alien to you as "ramp travel index" is to the average snowboarder. Here's a thumbnail description of the most popular features, courtesy of The North Face, Columbia Sportswear, and other apparel makers.

Down: A somewhat misleading term, because "down" filler in a jacket or sleeping bag usually refers to a mix of actual down (the light, fluffy filaments, without a quill, that form the undercoating of waterfowl) and feathers (which include both down and the quill). The mix is typically represented in a ratio of down to feathers, like 80/20 or 60/40. The insulation quality of some bags and garments is indicated by a "fill power" number. That number corresponds to how many cubic inches of space an ounce of down would fill after it has been compressed and released. The higher the fill power number, the higher the quality of down. A fill power number in the high 500s is good; 650 is better.

Fleece: The fleece mentioned in this article isn't wool from sheep, but a synthetic material (like polyester) that provides greater warmth with less weight than cotton or wool, dries quicker, and is very breathable. Fleece generally comes in varying weights, so you can choose the level of insulation you need to suit conditions.

Gore-Tex: The big daddy of outdoor fabrics, Gore-Tex is so popular because of its combination of waterproof, yet breathable protection. (If a fabric doesn't breathe, you'll sweat yourself silly--and cold--in a matter of minutes.) Gore-Tex is actually a membrane, permanently bonded to outerwear shell fabrics, that contains billions of microscopic pores. Each pore is 20,000 times smaller than a raindrop, but 700 times larger than a molecule of water vapor, so perspiration can pass through the membrane whereas water can't.

Gore-Tex for Boots: The concept of the waterproof/ breathable membrane is the same, but in order to pass the company's waterproof standards for footwear, a Gore-Tex boot must pass rigorous tests. Testing includes submerging 75 percent of the boot in water and repeatedly flexing it, measuring how many flexes the boot can withstand before it starts to leak. A minimum of 300,000 flexes (equal to five days of nonstop flexing) is required for a boot to pass Gore's muster.

Mini-Faille FD: Columbia's matte-surface nylon with four fabric plies for durability and abrasion-resistance.

Thermolite Micro: Thermolite is an insulation developed by DuPont, and the Micro version is a blend of microfibers that traps body heat to block radiant heat loss. It provides more warmth-per-inch than down, so it insulates without bulk.

Thinsulate: Developed by 3M, Thinsulate is an insulation material that traps air--and therefore warmth--in its microfibers. When equal thicknesses of insulation are compared, Thinsulate provides about 11/2 times the warmth of down and double the warmth of other high-loft insulating materials, according to 3M.