skiers use a grip wax (also called "kick wax") for classic-style waxable skis to provide traction with static friction on the snow that allows them to propel themselves forward on flats and up hills. They are applied in an area beneath the skier's foot and extending, somewhat forward, that is formed by the camber of the classic ski, called the "grip zone" (or "kick zone"). The presence of camber allows the skis to grip the snow, when the weight is on one ski and the ski is fully flexed, but minimize drag when the skis are weighted equally and are thus less than fully flexed. Grip waxes are designed for specific temperature ranges and types of snow; a correctly selected grip wax does not appreciably decrease the glide of skis that have proper camber for the skier's weight and for the snow conditions. There are two substances used for grip wax: hard wax and klister. Hard wax: a traditionally paraffin wax-based substance with admixtures—for snow comprising crystals that are relatively intact and not substantially changed by packing or freeze-thaw. The admixtures, which include a dye, rubber, rosin, resin and colophony, adjust the hardness of the wax to tailor the effectiveness of its grip for specific, discrete temperature ranges (from approximately -25 °F to +35 °F); waxes are graded and color-coded according to these temperature ranges. Harder grip waxes are designed for colder snow temperatures, but grip poorly in warm temperatures. Conversely, softer waxes in cold temperatures create enough friction and melting that the melt layer may accumulate and promote frozen accretion of snow. Klister: a sticky ointment, which may contain a combination of rosins, waxes, solvents and fats—with the formulation tailored for snow that comprises coarse crystals, having been transformed through freeze-thaw or being wind-blown, and adjusted for specific temperature ranges. Spray-on klister is more convenient than klister applied from a tube. An incorrect match of klister to snow conditions can also cause icing. Some skis are "waxless", having a fish-scale or other texture to prevent the ski from sliding backwards. Ski mountaineers use temporarily adhered climbing skins to provide uphill grip, but typically remove them for descent. Wax solvents Wax can be dissolved by non-polar solvents like mineral spirits. However, some commercial wax solvents are made from citrus oil, which is less toxic, harder to ignite, and gentler on the ski base. Health and environmental effects Health Ski wax