4. Insulated Layer: Down feathers or synthetic ‘loft’ material stuffed inside nylon in the form of a jacket and pants, or perhaps a one piece suit (for Vinson or Himalayas). These layers are usually worn at breaks while climbing on a cold day, while standing on a cold and windy summit, inside the tent as a pillow or extra layer over sleeping bag to add warmth. These layers generally resist moisture, but are intended for conditions where any precipitation comes in the form of dry snow.