Any experienced expedition guide for high mountain climbs will value the integrity of the rope as the most important safety gear. Scaling jagged rocky crags and traversing field of ominous ice crevasses always has the immediate danger of slipping and falling, so climbers must always be attached by a rope. Climbing ropes must have great tensile strength for pulling and holding a load, and hikers seek out lines made of light materials for easier packing. Natural fibers for constructing ropes, such as hemp and hair, are very strong, but modern material such as polyproylene and kevlar are lightweight and resilient. Rope strength is rated in units of kiloNewtons (1 Newton = 1 foot times 1 pound per second of force). One kiloNewton equates to the force of 100 kg suspended against the earth's gravity at rest.
With the great strength of synthetic materials ropes can be formed into lengths of over 150 feet and woven to be thin and light. (Climing up or down a steep drop be sure to know how long the rope is before you start). These ropes are designed to stretch to absorb some of the impact on the rope when a climber falls. These ropes are constructed by kernmantel design, with a core shielded by an outer layer. The "kern" or the core of the rope holds 75% of the strength, and the "mantel" or the sheath protects the core and holds 25% of the strength. Rope diameters are categorized into single (9-11 mm single rope), half (8-9 mm used in pairs), and twin (8 mm and thinner used in pairs). Another important factor in the strength and weight of the ropes is keeping them dry. Rope are impregnated with water-resistant solutions to force out the moisture. Having wet ropes makes them heavier, reduces their tensile strength, and in the cold weather ice in the ropes can add danger.