Air is much less dense than a piece of fabric, so less heat is conducted through a layer of air. It's not the clothing that keeps you warm, it's the dead air trapped by the clothing. Polartec is a synthetic material made of polyester fibres that have been brushed on both sides. The brushing produces tiny pockets that trap air which is then warmed by your body to produce a layer of insulation. Along with providing insulation, the polyester fibres also help to pull moisture to the surface for quick evaporation.
Polartec fleece is lightweight, quick drying and compactable.
These features are important because Byron will be packing and wearing his gear over a three-month period. Since there are no washing machines or dryers on the mountain he'll want clothing that's quick drying and won't hold perspiration. At higher elevations, from Camp III and up, where the temperatures are extreme, Byron will abandon his fleece layer for a down layer. He will be wearing a one piece down suit. Down is nature s warmest insulator.