WHY IS IT SO

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

Cosmos - - Contents -

— 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

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