Once upon a time, making snow was a straightforward craft. One could simply grind up large blocks of ice and spread the pulverized material where desired or use a basic stand-in material such as cellulose powder or bits of paper. Nowadays, with the advent of better materials and machinery--and because the fluffy white stuff fascinates people no end--there are myriad ways to pull off a big snow job for indoor or outdoor use using machine-made snow or artificial snow.

Machine-made snow has been substantially refined by the ski industry over the years. Snowmaking serves to extend the ski season or can rescue a dry winter, but it also has become important for controlling snow conditions as the number of skiers has increased and the mode of enjoying the slopes has evolved to include tubing, sledding, and snowboarding. Machine snow is also used in labs to learn how to forecast avalanches.

To make snow, water cooled to just above its freezing point is pumped under high pressure through the nozzles of a "snow gun." Compressed air or electric fans are usually used to help atomize the water into fine droplets and to disperse them over a wide area where they hopefully will freeze before they hit the ground. If not, the snow will be too wet. Other ways to make snow include using a combination of water and compressed air that is frozen by liquid nitrogen, a method used primarily for indoor sports centers. Snow also can be made from CO₂.

Critical to snowmaking for skiing is getting the right combination of temperature and humidity--the lower the humidity, the higher the outdoor temperature can be to form snow. With untreated water, an air temperature of about - 8 �C (15-20 �F) is needed. Another important factor is the need to generate sufficient nucleation sites for ice crystals to form. Nucleation sites can be a few water molecules that coalesce alone; calcium, magnesium, or other ions; or an impurity such as a clay particle or organic matter.

When the temperature isn't quite cold enough--above about 5 �C (23-30 �F)--snowmakers need little helpers in the form of seed materials added to the water to generate nucleation sites. Silver iodide, kaolin, soaps and detergents, and fungi or lichens are among the materials that have been used.

Currently, the most popular additive is Snomax, a freeze-dried protein powder sold by York Snow, Victor, N.Y. Snomax is derived from Pseudomonas syringae, a common bacterium found on grasses, trees, and vegetable crops. In the 1970s, plant pathologists studying the frost sensitivity of corn plants at the University of Wisconsin, Madison, discovered that the bacteria were responsible for initializing ice crystallization [Nature, 262, 282 (1976)]. Various bacterial products are now used to help form ice layers on citrus and other crops to prevent freeze injury.

A newer seeding product taking the market by storm is called Drift, a liquid polyether-substituted trisiloxane produced by Aquatrols in Cherry Hill, N.J. Drift works as a surfactant to decrease the level of hydrogen bonding in water so the water can freeze more quickly, according to the company.

When it comes to artificial snow, ice, or frost, there are more than 100 different materials that can be used, according to Snow Business, a U.K.-based company that supplies ersatz snow for movie sets. Different classes of materials include paper, plastic, starch and cellulose, or foam.

On movie sets, several products generally will be used in combination or with machine-made snow to create the desired effect. Machine snow is usually avoided because it melts and doesn't look flaky when it's falling. Paper, starch, and cellulose are good materials for falling snow. They can be sprinkled down onto a scene and kept aloft by fans blowing air from the edges of the set. A problem with fans, however, is that the noise may interfere with dialogue. During snow scenes there often will be no dialogue, only music, or the dialogue will be dubbed over.

Paper is one of the most versatile materials because it's weatherproof. Starch and cellulose can give the effect of a light dusting of snow or frost on plants and the ground, but they can be slippery to walk on and can generate a sticky mess. Shredded plastic snow is good for small-scale uses in a studio, although it's more expensive. Firefighting foam works well for deep snow and is fast and inexpensive to use, but it can't be walked on.

A favored material is instant mashed potato flakes. From a distance, the flake snow looks pretty real. The drawback: If it starts raining or the ground somehow gets wet, there's mashed potato slush to slog through. Also, in a close-up shot, potato flakes look like potato flakes, and on moist lips they could present a problem--pass the gravy!

One final type of artificial snow is called dryslope. This is a group of wood, metal, or plastic materials, usually laid down as latticework with void spaces, that is used to ski on out of season or in regions where it doesn't snow. One downside is the hard materials can lead to a greater risk of injury.

A newer type of dryslope that aims to curb injuries is a multilayer polymer composite matting that resembles carpeting. Two products are Snowflex, made by Briton Engineering Developments, Yorkshire, England, and Powderpak, made by an Atlanta-based company with the same name.

Snowflex, for example, has a slippery polybutylene terephthalate fiber surface layer that sits atop a shock-absorbing pad that has a woven backing. Water piped through the layers exits recessed nozzles and mists the surface, which helps reduce friction even further. This new type of dryslope can be laid out like carpet and cut to fit features such as moguls. It has been used indoors and outdoors to make half pipes and short slopes for freestyle (acrobatic) or downhill skiing and snowboarding.