Taken from website: The Rho Heavyweights – our warmest base layer tops – are made for cold days and mixed output. On the move, the Polartec® Power Stretch® Pro stretches for outstanding freedom of movement. During rest phases – or when it gets really cold – the fleece’s soft brushed interior traps heat to keep you warm. The fleece’s durable nylon outer has a smooth face for comfort under layers, the tall collar extends warmth, and the deep front zipper aids heat dump and makes on-and-off easier.