Strange as it may seem, very little snow falls in the cold Arctic regions of the north. The precipitation in those parts is in the form of individual ice crystals, and at a rate equivalent to fewer than two inches per year - only slightly more than falls on the Sahara Desert. The vast ice cap continues to grow, of course, but only very, very slowly over millions and millions of years.
But another strange thing discovered by explorers in the Arctic is that skiing on Arctic snow is often difficult and sometimes even impossible. The reason is related to the fact that the process of skiing is facilitated by the friction of the ski sliding across the surface: the friction melts the ice crystals in immediate contact with the ski almost instantaneously, and the water thus produced acts as a lubricant to facilitate further sliding.
In extremely cold conditions, however, at a temperature of, say, 40 degrees below zero, it is too cold for instantaneous melting to take place and the skis, rather than melting the points of the crystals to make them slippery, simply roll them over and over.
The result is a harsh, dry surface that is more like sand than snow.
Cross-country skiing of any kind is one of those things which should not work but does, like, as somebody once said, the internal combustion engine, a piano, democracy, and the British constitution.
In the case of a downhill run, all that is needed is a decent slope. But to move forward on a level surface, it is necessary to achieve a sort of ratchet effect: the ski should glide forward easily when required to do so, yet be reluctant to slide backwards when holding the skier's weight, to allow him to advance the second ski. To some extent this tends to happen anyway. We have seen that when a ski slides forward, friction melts the ice crystals in contact with it and the lubricating water then facilitates further sliding motion. When the ski stops, however, the water freezes again, and a good grip is restored.
But this action can be facilitated by applying special kinds of paraffin or silicon wax to the under-surface of the ski. The grade of `klisterwax' - `klister' being the Swedish word for `glue' - is chosen very carefully. If its consistency suits the type of snow, it lowers the friction when moving forward, while decreasing the tendency for backward slippage when the ski is stationary.
Generally speaking, the smaller the snow grains and the lower the temperature, the harder the wax must be.