Ski conditions are defined as the amount and the state of the snow for the purpose of skiing. For this reason, the expressions ski conditions and snow conditions are often used interchangeably. An assessment of the applicable ski conditions is an important, sometimes primary consideration in both Alpine skiing and Nordic (cross-country) skiing as to how the athlete will approach the tactical decisions required for a particular course. In the third of the skiing disciplines, freestyle skiing, ski conditions are less important to the performance of the athlete because the freestyle skier competes on a manicured snow surface on a generally much shorter race course.
The condition of the snow will dictate how the skis will perform on the snow surface. The altitude at which the skier is performing, the temperature of the snow, and the effect of sunlight and wind will all play a role in how the snow surface will react to the forces directed against it. Snow conditions often vary dramatically from the top of the mountain to the bottom in Alpine skiing; on a 12-mi (20 km) Nordic course, temperatures may change by as much as 20°F (8°C) from the beginning of the event to the end, causing the snow to physically change as the race progresses.
How snow falls will determine how it initially forms on the ground. Snow is classified into various types. Powder refers to the softest form of snow, and usually accumulates in a dense, packed formation, and when untracked, powder is a surface on which Alpine skiers find that they can execute turns, control their downhill speed, and generally hold their edges with greater ease. When powder has been skied on, it is described as "tracked out," where the surface has various lumps and ruts that reduce some of the cushioning.
When the snow has been subjected to a freeze and thaw cycle over a period of time, the snow surface may often have a crust of ice form on top, which also becomes more granular in its texture. The same freeze and thaw temperature cycle in reverse can produce an opposite result, slush, where the snow becomes wetter and heavier and makes skiing movement more difficult. The most challenging snow condition faced by both Alpine and Nordic skiers is ice, created where a soft or wet surface freezes; icy ski conditions are not 100% solid water, but are sufficiently hard to create an extremely slick ski terrain.
Ski conditions will dictate the type of ski wax applied to the equipment of both Nordic and Alpine skiers, although the purpose of the wax is specific to each ski discipline. Ski waxes tend to be one of two general types: glide wax and kick wax. Wax for skis has a dual purpose, the primary of which is to create an optimum sliding surface between the snow and the ski. The secondary purpose behind the application of wax is to protect the surface of the ski from damage caused by the contact between the ski and the icy ruts and imperfections of the course.
Glide waxes, in an almost infinite variety of formulations, are used by both Alpine and Nordic skiers. Most skis are subjected to some form of factory pre-waxing; the skis will be subjected to very high-end waxing treatments in preparation for all training and competition runs. A number of glide waxes are applied through a heat process to create a better adhesion between the ski and the wax. The glide wax is generally composed of a mixture of either hydrocarbon (oil) byproducts, or a fluorocarbon base. Glide waxes function by creating a microscopically thin cushion of water between the ski and the surface as the skier races over the snow, which reduces the amount of friction between the ski and the surface.
Kick wax is a substance used exclusively by Nordic skiers to create a more effective skiing motion. The longer and thinner cross-country ski has two distinct areas that are often treated with different kinds of wax; how extensively the two types of wax are applied depends on whether the skier will be skiing in the classic style, where the skier moves the skis forward in alternating strides, or the modern skating style. Kick wax is applied to the ski surface under the boot bindings to make the ski move easily over the snow. On cold or very firm surfaces, the wax will be a hard wax, which permits a degree of grip onto the icy surface necessary to permit the skier to get the traction necessary to kick forward with every stride. When the snow is soft or wet, the wax substance will be a softer "klister" wax, which permits the skier to glide more efficiently in the heavier snow. In elite-level Nordic racing, the ski technician is a very valuable member of a ski team. The technician will often be required to change the waxes a number of times during an event to counter the changes in weather or snow conditions.