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Mountain Equipment says:
A very warm yet incredibly packable hi-loft fleece for cold weather mountaineering, ski-touring, and polar expeditions.
Proven on some of the greatest expeditions of the modern age and made from POLARTEC® Thermal Pro® Hi Loft fleece, the Hispar is about as warm for its weight and bulk as it is possible for fleece to be. On top of that its ultra-breathable and fast drying nature make this a perfect cold weather midlayer.