Alpine skiing (or downhill skiing) is a recreational activity and sport involving sliding down snow-covered hills with long skis attached to each foot. ---------------------------------- Alpine skiing evolved from cross-country skiing when ski lift infrastructure was developed at mountain resorts to tow skiers back to the top of slopes, thus making it possible to repeatedly enjoy skiing down steep, long slopes that would be otherwise tiring to climb up. The sport is popular wherever the combination of snow, mountain slopes, and a sufficient tourist infrastructure can be built up, including parts of Europe, North America, Australia and New Zealand, the South America Andes, and East Asia (mainly South Korea and Japan, although the popularity of skiing is increasing in China as well). -------------------------------------- The main technical challenges faced by skiers are simply how to control the direction and speed of their descent. Typically, novice skiers use a technique called the "snowplough/snowplow" to turn and stop by pointing one or both skis inward, but more advanced skiers use more difficult but more elegant and speedier methods. One popular method of turning is called parallel turn; it involves keeping both skis parallel to each other while altering the weight distribution between them in order to turn them in any particular direction. The angle of the ski in relation to the slope (called edge angle) is also important as it determines the resistance (friction) created by the edges of the skis. Modern advanced skiing technique is dominated by "carving." To carve, a skier rolls his or her knees from side to side while keeping the upper body and hips facing down the hill, so that only the knees and feet are turned. This method of turning allows modern skis to turn using the radial properties of the edges of the ski without skidding or slowing down, creating a smooth arc. To maintain the line of a parabolic ski, one must lean one's center of mass into the turn. ------------------------------------- As skiers gain confidence, they may tackle steeper, longer and more uneven slopes (including off-piste and ungroomed runs) at higher speeds. In North America the easiest slopes are marked by green circles, and are typically fairly flat and smooth. Sometimes known as bunny hills, they are usually groomed by specially equipped snowcats every night. A blue square marks slopes of medium difficulty; they are steeper than green circles and may be left in a natural state rather than machine-groomed. A black diamond slope is steeper than a blue square and often involves challenging terrain such as moguls, double fall lines, or gladed sections. A double black diamond is for experts only; these trails are steep, rarely groomed and often left in a completely natural state. There is no standard for these designations, however, and each ski resort determines them relative to their own terrain difficulty. So, for instance, a blue-square (mid-level) trail at one ski mountain may be markedly more difficult than a black-diamond (expert) trail at another mountain. In Europe the system is based on colour alone. North American green circles, blue squares, black diamonds, and double blacks correspond to European green, blue, red, and black trails, respectively.