Powder skis, also called big-mountain or backcountry skis, have a wide waist width, making them ideal for places with heavy powder. The extra surface area helps skiers to float above premium powder. However, they can be difficult to use on slopes with less snow or groomed trails, especially for beginning to moderate skiers. Experienced skiers sometimes buy powder skis as an alternate pair, to be used when conditions warrant it. True backcountry skis have a waist width of 90 to 110 millimeters, while powder skis are the widest type, measuring from 110 to 140 millimeters.