WHY IS IT SO

As natural snow gets scarcer, ski resorts increasingly rely on snow-making machines. JONICA NEWBY explains how they work.

— How to make snow

IF YOU’VE ever watched a skiing event in the Winter Olympics, or visited the slopes yourself, the odds are that much of the snow you’ve seen isn’t natural but made by machines. So how?

All snow is a type of ice crystal – but manufactured snow is formed quite differently from the natural stuff and so has a different structure. Natural snow starts life as molecules of water vapour, floating high in the atmosphere, in temperatures that hover around freezing point or below.

When the vapour encounters what’s called a “nucleator” – usually a speck of pollen or dust – it transforms from a gas to a solid. In other words, it freezes, forming a six-sided hexagon-shaped ice crystal: a baby snowflake.

Over time, this tiny crystal bumps into more water-vapour molecules, which attach and freeze. Gradually it grows into a beautiful ice crystal lattice. The fully formed snowflake is also generally sixsided, because the molecules bond naturally into the ice crystal’s hexagonal structure.

Not all snow looks like a classic snowflake, though. For example, very cold, dry air produces small powdery flakes that don’t stick together – great for “powder skiing”. When the temperature is warmer, flakes melt around the edges, forming “wet” sticky snow, which is better for making snowmen.

Natural snow is all our planet knew for billions of years, but as global warming begins to take its toll on seasonal snowfalls, the skiing industry is turning increasingly to technology to make its own.

The earliest snow machines were basically just giant hoses. Today’s machines boast sophisticated onboard weather stations, adjustable nozzles and complex software, able to maximise snow output with every change in temperature or humidity. A single state-of-the-art “snow gun” can make enough to fill 10 trucks in an hour.

Natural snow is made from water vapour. Human-made snow crystals are made from freezing liquid water. In most other ways, however, the process is similar.

There are two main kinds of snow-making machines: the lance snow gun and the fan snow gun. Pictured on the opposite page is a fan snow gun, which looks something like a giant hairdryer with a fan in the middle and an outlet ringed by metal teeth.

Most of the teeth are tiny nozzles that spray fine droplets of water. Nestled among them are nozzles that work as nucleators.

Just as with natural snow, the water droplets sprayed by the snow guns need some help from a particle to initiate the freezing process. But instead of spitting out dust or pollen, the mechanical nucleators make tiny ice particles to do the job. Compressed air is shot up through the nozzle, where it meets water and splits it into tiny droplets.

The sudden loss of pressure robs the system of heat, rapidly cooling the water to form a tiny ice pellet – a snow “seed”.

The giant fan now propels these seeds into the air alongside the fine mist of water droplets. As the droplets encounter the seed, they stick to it and begin to freeze.

It takes a while to freeze a snow crystal, which is why snow guns are designed to send their icy bullets so high into the cold air. Height also allows additional time for evaporation, which helps freeze our baby snow.

Because human-made snow is made from droplets rather than vapour, it has a different shape. It forms a ball that freezes from the outside in – a bit like freezing an egg. The result is a tiny rounded grain instead of a flake.

So can we still call these little ice balls snow? That’s a good question. It certainly looks a bit different when it first falls. But once on the ground, the structures of both natural and manufactured snow continue to change as they bond and mix with other snow crystals to become a continuous snow pack. They end up pretty similar.

In fact, the human-made stuff is arguably better for groomed ski runs – where snow is smoothed and compacted using a tractor. Human-made snow makes a more durable ski slope and is slower to melt, and hence is usually preferred for slalom courses. JONICA NEWBY is a science writer, broadcaster and former veterinarian.

IMAGES 01 Goshiva /Getty Images 02 Heath Korvola / Getty Images