The iceman cometh
As we approach Christmas, we’ll be taking a look at the world of artificial snow. You might be spraying a bit of white foam in your windows to fake the stuff, or you might be lucky enough to be off on a Christmas break at a winter resort to have a bit of a slide around on the ‘real’ thing. We’ll take a look at how they fake snow in TV and film, then we will look at how some surprisingly interesting science goes into producing artificial snow to furnish the winter sports resorts that are struggling to maintain their pistes as climate change creeps ever upwards. They are past cloudbusting these days (still love you Kate Bush though, I trust you are reading this). I have heard of mothers who freeze snowballs collected in winter so the kids can have a throw-about in the warmer months, but this is not really an option for the resorts. We will learn a little about the role of chemistry, physics and even biology in creating slippery matter to keep the enthusiasts sliding.
There are plenty of films and TV series that need a bit of snow. It’s not my world, but I sense that hanging around for the weather you need to get a crispy white backdrop probably isn’t an option for a production team. Down here on the balmy south coast of Sussex, England, snow is by no means a guaranteed winter event, especially as the world warms. Early on, Corn Flakes painted white were
used. I do not envy the – presumably – junior set staffer who was given the job of painting the stuff, but the advent of audio in movies turned out to be a cereal killer for this particularly crunchy stand‑in. Over the years, snow has been represented by flour, firefighting foam, instant potato flakes and gypsum salts. Marble dust was used as snow for the 1965 film Doctor Zhivago. I am not sure how healthy for the lungs a set strewn with mineral dust would be, but we will see in a moment how things have moved on.
These days, paper flakes are used as snow. It comes in a range of grades of weight and texture and can be baled for easy handling. Fired through a hose it can cover 37 m2 of ground per minute. A fancier sounding alternative is a shredded polymer such as sodium polyacrylate. Many of you (including me, and my evil editor) will have encountered materials like this many times without realizing it. It is a super-absorber and can take on 800x its own weight in water! So… if you’ve changed a nappy, chances are it contained a super-absorber. For the film director, there is the benefit that this material (the polymer, not nappies) can be made wet, frozen and heavy as required to help realism at the expense of the comfort of the actors.
The special effects people first only wanted the appearance of snow, then sound started to matter, then you could actually make the stuff cold, heavy and wet for realism. Let’s turn to the winter sports where appearance is not enough and physical properties really matter. Early efforts included just pulverizing solid ice, but the result was heavy and quick to melt. On the slopes now you will find snow guns that atomize water into droplets around 25 µm across (1/40 of a mm, 1/1000 of an inch). The smaller they are the higher the chances that they will just freeze naturally. This is not strictly snow, but it gives you tiny ice particles which do the job.
To maintain the cold, the water droplets are driven out by compressed air. On leaving the snow gun, the air expands and cools, helping to promote freezing. When water freezes, heat is released. This makes sense, to melt ice you give it heat; to freeze ice, heat must be thrown back out. The expanding, cooling air helps to soak up this heat, promoting freezing. Snow guns are placed away from the ground so that the latent heat released by freezing water does not melt the snow nearby. It helps if the droplets stay small so they lose heat efficiently, so a surfactant can be added to stabilize the small drops. That’s the physics and chemistry almost done…
A word about water – it does not necessarily freeze just because it is below 0°C. If there is no scaffold for ice to form on (as in very pure water), water can remain liquid as low as -40°C. We need cold to freeze water, but at higher temperatures we also need a scaffold – a sort of molecular jigsaw puzzle mat: something for the water molecules to align on while they link hands to form ice.
OK, so your snow gun is firing out tiny water droplets and you hope they will freeze. At -8°C, this might work fine. A few degrees higher means that you might need to freeze smarter. Now we bring in the biology. Meet Pseudomonas syringae, a bacterium with an impressive superpower. P. syringae has proteins in its cell wall having sites where water molecules are happy to sit along with sites where water molecules are reluctant to sit. The result is that water molecules are helped to align to form ice. Remnants of these bacteria are mixed into the water that is fired from the snow guns and promotes freezing at higher temperature than would otherwise be seen.
It seems this microbe uses its ability to promote ice formation to breach the cells of the plants that it wants to infect. One account described an ‘ice spear’, but I failed to find an image to confirm this method of attack (if you have one, send it in). After all, the name syringae suggests something pointy. Other accounts just describe the ice disrupting the membrane of the cell to be conquered. P. syringae is found in natural snow all over the world and is routinely detected in the atmosphere. It is suggested that microbes like this might have a crucial role in climate and weather.
There are environmental concerns, as seems almost inevitable with any technology, with artificial snow on the pistes. Even natural snow causes the growing season to be held back by several weeks when pistes are kept in place for winter sports. Nitrate salts are used to enhance the snow surface for races, but nitrates are powerful fertilizers that encourage the growth of aggressively growing plants at the possible expense of more reticent alpine plants.
We have seen how special effects from breakfast cereal to nappies have been used to suggest snow on the small and large screens. Winter sport resorts routinely rely on artificial production of a fine ice snow substitute, but they use a little help from physics, chemistry and even biology.