Grid fleece is a very advanced fabric that works like a thermo-regulator for your body. The special grid construction consists of high-loft thick fleece (for warmth) that is surrounded by special channels (for maximum breathability and wicking). So not only does it keep you as warm as possible, it is extremely quick-drying and breathable. All of this works to keep you as comfortable as possible during both the cold idle times and high-output active times.