Let it snow...

C&I Issue 24, 2011

Snow is not, as sometimes thought, simply frozen rain. Snowflakes form inside clouds when tiny ice crystals collide and stick together. For water vapour to turn into ice crystals, water molecules must stop moving enough to align in a latticed, hexagonal array. An ice ‘nucleator’, such as particles of ash or dust, can help kick-start freezing by attracting the water molecules and slowing them down. No two snowflakes – which are always hexagonal – have ever been found to be identical.

Conditions have to be just right for snow to form. The atmospheric temperature has to be at, or below, freezing, and there has to be a minimum amount of moisture in the air, according to the National Ice and Snow Data Center (NISDC) in Boulder, Colorado, US. If the ground temperature is also at or below freezing, the snow will reach the ground and settle. Snow can fall when the ground temperatures are above freezing as long as atmospheric temperatures are below freezing and the air contains a minimum moisture level.

As the Inuits, living in the Arctic region of Canada, know, there are lots of different types of snow. For example, when it is very cold ice crystals do not stick together easily and the snow is fine and powdery. At warmer temperatures near freezing point, however, large snowflakes form, especially if there is no wind, and this produces ‘wet’ snow.

Artificial snow

With mountaintop temperatures rising, snowfalls in ski resorts have become less heavy and occur later in the season. So resort managers, in Canada and the US, for example, are increasingly turning to artificial snow production.

Snow-making machines were invented in the 1950s and there are now two types. In one, air guns internally mix water and compressed air supplied by pipes. The compressed air atomises the water and then forces it out of nozzles where, if it’s cold enough, the water freezes and falls as snow. In the other design, a mist of water is sprayed into a fan, which then blows the water upwards into the air, where it turns into snow.

Early snow-making machines required near freezing temperatures to operate, but new technologies claim to be less dependent on the weather. However, there’s no escaping the fact that if ambient temperatures are too high, the snow will melt.

One way to optimise snow making is to add chemicals to the water that supplies the snow machines. These chemicals – nucleators – improve efficiency by providing a centre for snowflakes to form around and allowing the water to freeze at lower temperatures – just like dust or ash in natural snow.

One additive that acts as an ice nucleator is Snomax. Sold by Johnson Controls of Milwaukee, Wisconsin, US, Snomax comes as a freeze-dried protein powder. The protein is derived from Pseudomonas syringae, a common bacteria found on the surfaces of plants.

In the 1970s, plant pathologists studying the frost sensitivity of corn plants at the University of Wisconsin, Madison, US, discovered that P. syringae produce a surface protein that has ice-nucleating properties (Nature, 1976, 262, 282). The bacteria bind water in such an orderly way that water droplets freezing around a microbe almost mirror the natural lattice formation of ice.

According to its manufacturers, source water treated with Snomax contains between 1000 to over 100,000 more nucleation sites than untreated water. This means that every droplet of water has a site for ice crystals to form. This is important because the key to efficient snow making is to freeze as many droplets as possible before they hit the ground. Another feature of Snomax is that, according to the manufacturer, it initiates freezing at higher temperatures, at up to almost -3°C. Most natural water additives are not effective at temperatures above -6°C, they say.

In another attempt to make snow making more efficient, Aquatrols of Paulsboro, New Jersey, US, has patented Drift, a liquid additive formulated from organosilicone surfactants (polyether-modified siloxanes). The company claims Drift modifies the flow dynamics of snow-making water so it freezes faster and more thoroughly. This means snow makers can operate more efficiently at marginal temperatures, it says.

When Drift is dispersed in water at 3–5ppm, it lowers water’s natural cohesive tendencies, the company explains. As water tension is lowered, the droplets become flatter. The result is that the same volume of water forms a ‘thinner’ mass with a much greater surface area, and this freezes more rapidly and thoroughly.

Meanwhile, Israeli company, IDE Technologies has announced a new technology – the Vacuum Ice Maker – that produces snow regardless of air temperature.  IDE was set up originally to develop a freeze desalination process. Freezing salt water at a critical temperature can produce ice crystals of fresh water; it is then possible to separate the crystals from the solution using a mechanical method and remelt the crystals to obtain fresh water. Now IDE has found an application for its freezing know-how in snow making.

IDE’s all weather Snowmaker works by exposing water to a high vacuum in a freezer. The vacuum forces small part of the water to evaporate, while the remaining water freezes. The resulting mixture of water and snow is pumped out from the freezer to a machine that separates the water from the snow crystals and extracts high-grade snow. In order to maintain the deep vacuum in the freezer, the water vapour is compressed and fed into a condenser. Condensing of the vapour requires cooling water at about 5°C.

The Snowmaker can be used at all ambient temperatures and weather conditions, IDE claims, whereas existing snow-production solutions require sub-zero air temperatures. It also says the Snowmaker uses 80% less power than other weather-independent snow-making technologies, and does not require additives or chemicals.

Some concerns

But some scientists are concerned that artificial snow making threatens drinking water supplies and ecosystems in fragile areas, such as the high altitude basins in Europe’s Alps. They are concerned that, as temperatures rise at high altitudes, snow producers will fabricate more and more snow in a losing battle against global warming.

Artificial snow making consumes up to 4700m2 of water per hectare, according to Carmen de Jong, research director of the Mountain Institute of the University of Savoy, France. Water supplies can no longer keep pace with this demand, she claims, as water is extracted on an increasingly permanent basis from sources such as streams, reservoirs, springs and groundwater. The problem is that surface water, as well as groundwater,
is already scarce in high-altitude mountain basins.

The belief that all water extracted for snow production returns to the environment is wrong, argues de Jong. Her measurements in the Alps and Morocco’s High Atlas mountains indicate that about 30% of the water transformed into artificial snow is permanently lost to the area. The evaporated water, depending on atmospheric conditions, may be carried into the next basin or into the next country, she says.

Evaporation occurs when making artificial snow as water is expelled into the air; evaporative cooling allows the heavier ice crystals to condense and fall as snow, de Jong explains. Further water loss occurs because wind velocities at mountain altitudes are three times higher than at ground level. What’s more, since the temperature of artificial snow is closer to freezing than natural snow, generally at –4°C, it can frequently melt and refreeze during the day. The resulting molten water lying on impermeable snow cover forms another source of evaporation.

‘Although natural snow also evaporates and sublimates, artificial snow originates partially from groundwater and from water concentrated artificially at the surface so that these artificial snow-related losses are irreversible,’ she points out. ‘The process of making artificial snow is not comparable to natural snowfall; it is more like irrigation, yet nearly three times more water-intensive.’

Faking it – on set

Ski resorts may love machine snow, but it’s not often used for special effects on film sets because it melts and doesn’t look flaky when it’s falling. Alternatively, several products may be used in combination, or with machine-made snow, to create the desired effect. Fake snow can be made out of all kinds of materials, including paper, starch, cellulose, polystyrene, urea formaldehyde and marble dust.

One UK company has built a successful business around producing fake snow and ice. Snow Business, based in Gloucestershire, holds the world record for the largest area ever covered with artificial falling snow. Its product range includes paper, micro-fine white cellulose powder, and different forms of plastic.

‘It all depends on what you want the snow to do,’ explains managing director Darcey Crownshaw. ‘Paper, for example, is great if Tom Cruise has to lie on snow for six weeks of filming. We have developed a very special paper that is machine-torn into tiny particles. The edges of these particles are like Velcro and lock together so that the result is natural-looking snowdrifts. The camera loves it but, importantly, so do the actors because paper is a natural insulator. Laid on top of real snow, it keeps them warm.’

One problem with paper is that it has to be cleared away at the end of the shoot. ‘In contrast, natural cellulose derived from wood pulp and milled very fine gets washed away when it rains,’ Crownshaw explains. ‘It’s good at giving the effect of a light dusting of snow or frost.’

‘For shop window displays, a plastic product is better because it is durable, easy to keep clean, won’t soak up condensation or burn in a fire,’ Crownshaw continues. Snow Business’ newest product is Polymer 007, and like many of their products, its chemical make-up is confidential. ‘Other polymers look like wet ice or slush, but Polymer 007 is much more snow-like and can simulate free-flowing high-altitude powder snow,’ he explains. ‘It starts as a dry powder but expands up to 40 times its original size when mixed with water.’

Snow Business also makes real snow. ‘For something like an indoor ski show in a contained space,’ Crownshaw says, ‘we make real snow by adding liquid nitrogen to the snow-making water fired out of small snow cannons. We could make real snow anywhere – even in the Gobi Desert – it just wouldn’t last very long.’

Maria Burke is a freelance science writer based in St Albans, UK.

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