This Superlette Primaloft® insulation jacket that can be used all season Use is surprisingly hot. We use Primaloft® insulation in the Rainier Jacket for its warm, compression capacity and soft comfort. While the human body uses energy to keep warm, Primaloft® insulation uses a patented microfiber structure to help the body maintain heat and save energy. The ultrafine fibers of Primaloft® insulation form a tight collection of air pockets that use the heat from your body to keep the cold out and provide an excellent heat-to-weight relationship. When the insulation gets wet, these air pockets lose and thus its insulating properties. Primaloft® Ultrafine Fibers are designed to be waterproof so that your insulation gets drier and maintains its insulating properties even when wet. Wear the jacket like a lightweight outermost layer or combine with a rain jacket to get a functional 2-in-1 combination. This versatile jacket with lots of pockets and hidden hood can easily be compressed in a pocket.