EVERY EVENING, when skiers have left the slopes, the "piste-bashers" set off for the nightshift, illuminating the mountains with their headlights as they repair the damage done to the ski-area during the day. The most obvious result the fine "corduroy" grooves in the surface of the pistes the following morning; but the piste-bashers - tracked vehicles with a bulldozer blade at the front and sort of giant rake behind - actually carry out a number of different tasks.

First, the snow has to be spread out to cover and restore steep slopes: an average skier displaces 907kg of snow a day, which has to be pushed back up the mountain. To improve its durability, the snow is then compacted, ideally "to a hardness which resists penetration by a steel shovel" (according to the Canadian Handbook of Snow, published in 1981). That gives the surface greater "ski-ability", as does the next process, of chopping and smoothing the surface to remove bumps. The final stage of grooming the snow is that of cutting the corduroy grooves.

The invention of snow-grooming equipment is usually attributed to one of the creators of Winter Park in Colorado, Steve Bradley, who in 1952, developed a roller that could be drawn by a tractor across heavily used slopes to flatten "moguls" (bumps). The technology was developed to the point where, in 1993, the US Beartrac company (once owned by John De Lorean of Ulster car-making infamy) produced a range of seven different snow- groomers. Now, such machines are an essential tool for piste-maintenance in every resort - and, it must be said, a determining factor in making the natural environment of the mountains both safer and more sterile.