Mysteries of the snowflake: The curious world of the ice-crystal experts
Everybody loves snow, right? But not many of us are obsessed, like the scientists who study these icy enigmas. Nicola Gill enters the curious world of 'dendrites' and 'plates'
Saturday 05 January 2013
Dr Chris Westbrook works in deepest Hampshire at the Chilbolton Observatory, home to the world’s largest steerable radar dish, at a whopping 25 metres across. Inside his laboratory, lights blink and instruments receive continuous feedback from the giant dish pointed skywards and looming ever-present outside the window.
But even on the hottest summer day, while the other denizens of Chilbolton parish are enjoying Pimm’s on their sun-loungers, Dr Westbrook is buried deep in snowflakes. “The radar dish sends out microwave pulses into ice clouds high up in the atmosphere where the temperature is always well below freezing – whatever it is down here,” he says. The ice crystals, nestling in the ice clouds as unborn snowflakes, bounce those microwaves back and the echoes which return are pored over and analysed by Dr Westbrook and his team.
“"We have the most sensitive equipment for studying ice clouds in the world," he says. Westbrook is one of just a tiny handful of snowflake researchers in the world, a group of obsessives who live and breathe snow – fixated on chasing the perfect flake and understanding exactly which weather conditions will produce the many different formations. “It may seem slightly odd that I’ve devoted myself to studying snowflakes when the UK isn’t renowned as an especially snowy place,” he continues, “but, in fact, the vast majority of precipitation in this country starts as snow, which melts high above us and then falls as rain, which we certainly do have a lot of. So if you want to predict precipitation you need to study snow and how it forms.”
So far, so dispassionate; ask Dr Westbrook if he likes making snowmen and he rather frostily replies that he’s as keen as the next man (“but I have a degree in physics and electrical engineering and where others see a winter wonderland I see physics in action”). But ask him about the way snowflakes are formed and fall to earth and the amazed child inside emerges as he describes the physics-meets-fairytale element of his work.
“The aerodynamics of snowflakes have an inherently mysterious quality we’ve yet to crack,” he enthuses. “We classify their falling style in four unique ways: the ‘tumble’ is a sort of head-over-heels action, the ‘spin’ is a vertical downwards motion with a built-in rotation, the ‘pitch and glide’ is best described as a zig-zag and the ‘twirl’ is how we describe a snowflake that’s descending while spinning and rotating at an angle. Which they do depends on how fast they fall and their size, but it’s a puzzle that’s not solved and we don’t know why they behave as they do all of the time. As for the intricate formations of individual flakes, I defy anyone not to be amazed.”
Of course, it’s those spectacular shapes – some like icy fireworks caught mid-explosion, others frozen, fantastical many-armed sea creatures – that fascinate the rest of us non-scientists. Nearly all snowflakes (or snow crystals as scientists insist on calling them, as a large flake can actually be made up of several crystals that clump together on their drift earthwards) have six-sided symmetry, though three- or 12-sided crystals also fall. You will never see a snow crystal with four, five or eight sides. It was ancient Chinese scholars who first noted their sixfold symmetry and they made beautiful complex categories and charts detailing their infinite variety and grouping them into types; as no two snowflakes can ever be identical.
Broadly speaking (there are several competing classification systems), the classic, celebrated Christmas-card snowflake is categorised as a dendrite (meaning tree-like, with branches and side-branches). These are the iconic superstars of the snowflake world, hogging all the glory and most of the photo-opportunities. They can be sub-categorised as stellar, radiating or fern-like. As if winning the beauty contest weren’t enough, dendrites’ supermodel qualities (they can be extremely thin and light) also mean they make the best powder snow for skiing.
Next in line, the supporting cast, are the plates (stellar, sectored or split) with 12-sided flakes bringing up the rear. The ugly sisters, which in reality make up the vast majority of snowflakes, are the rather dull, hollow and capped columns, needles, simple prisms, bullet rosettes and asymmetrical specks, doomed forever to be the boring, bitty, non-showbiz flakes we brush off our sleeves with nary an “ooh” or an “aah”.
The categorisation of snowflakes has a long history. In 1655, Robert Hooke published a f large volume called Micrographia, containing his sketches of snowflakes viewed for the first time under the new invention of the day, the microscope. American farmer, Wilson ‘Snowflake’ Bentley, devoted most of his life to capturing images of snow crystals and his famous book of that name is still in print to this day. Japanese physicist Ukichiro Nakaya created the first truly systemic classification scheme for snowflakes in 1934, in which he subdivided falling flakes into 41 individual types which meteorologists Magono and Lee almost doubled by producing a chart of 80 different types in 1966. Mathematician and philosopher René Descartes is one of many fine minds through the ages to be fascinated by snowflakes and to ponder how such perfection could be created.
While every flake really is a law unto itself, other supposed snow ‘facts’ are not quite so true. The oft-quoted idea that it’s ‘too cold to snow’ is nonsense (it snows at the South Pole where it’s rarely above -40C), and even the apparent truism that snow is white turns out to be slushy logic. Ice crystals are clear, like glass, but when they form a large pile, light is reflected off the surface, bounces around and eventually scatters back out. Since all colours are scattered roughly equally, snow only appears to be white.
These, and many other reasons, are why world-renowned snowflake obsessive, California-based Ken Libbrecht, has made it his life’s work to study, photograph and ‘grow’ snowflakes. The author of several beautiful books showcasing his favourite flakes out of the 7,000 he has photographed, he lives and breathes dendrites, rosettes and plates. “There is something magical about snowflakes,” he says from his laboratory in Pasadena. “You don’t often see such complex symmetry in nature and that makes them extraordinary. The whole intriguing structure of a snow crystal simply arises quite literally out of thin air, as it tumbles through the clouds. The way the crystal grows depends on the temperature it is shaped in – a simple enough idea to grasp – but the underlying physics is fiendishly complicated and has remained a puzzle. I spend a lot, and I mean a lot, of time thinking about this.”
As Libbrecht explains, the life of a snowflake is a hidden, epic, scientific journey in which it transforms through liquid, gas and solid states. “Snowflakes begin life as water vapour in the air – evaporated from oceans, plants, even your breath – and when air cools down at some point the water vapour will condense out. Near the ground it could, for example, be as dew, but higher up it condenses on to airborne dust particles into countless minute droplets. A cloud is just a huge collection of these water droplets suspended in the atmosphere.”
The next stage is where it gets exciting, say Libbrecht. Depending on conditions, these droplets could fall as dreary rain, sleet or hail, or descend as mist or fog. But when conditions are right, the alchemy occurs and these minute droplets metamorphise into something more impressive. “At around -10C, the droplets gradually freeze into minuscule particles of ice,” he says. “When humidity is high enough, water vapour condenses on to its surface, gradually building a snowflake. At first they are very small and mostly in the form of simple, hexagonal prisms – but as they grow, the branches sprout from the corners to make ever more complicated shapes.”
By growing crystals in his lab, Libbrecht has learnt how the multitudes of varying shapes depend almost entirely on the temperature and humidity. For example, thin plates and stars grow around -2C, while columns and slender needles appear near -5C. Plates and stars form around -15C and a combination of plates and columns are made at around -30C.
Libbrecht’s devotion to dendrites has led him halfway around the world and he thinks nothing of basing holidays with his wife and two children exclusively around snowflake sightseeing. On one trip, he took his young children to Japan, where snowflakes are virtually a national craze. “Snow-crystal tourist spots are popular with the Japanese and I flew my family over for a winter holiday to the northern island of Hokkaido, home to the Museum of Snow and Ice, where even the doorknobs are in the shape of snowflakes. Admittedly, it’s not your usual family getaway, but my children know all about capped columns and other snowflake forms. They’re both in college now, but my daughter definitely gets a kick out of telling friends her dad is a snowflake scientist.”
At dinner parties, when asked what he does, Libbrecht says, “I like to lead with the science,” but admits that people are really only interested in his photographs and the pretty patterns of individual flakes, and unlikely to want to hear about the convection chamber where he conjures snowflakes into existence. “Basically, it’s just a cold chamber about a metre tall, with two containers of heated water on the bottom. Convection mixes the water vapour into the cold air creating super-saturated conditions for growing snowflakes. We nucleate crystals by dropping a speck of dry ice in the chamber and the crystals float until they grow to about 10-100 microns in size, when they fall to the bottom of the chamber.”
Inevitably, though, the most common question is, how can Libbrecht be so sure no two snowflakes are ever identical? He likes to tell people that physics has a Zen-like answer, “which is that it depends largely on what you mean by the question. The short answer is that if you consider there’s over a trillion ways you could arrange 15 different books on your bookshelf, then the number of ways of making a complex snowflake is so staggeringly large that, over the history of our planet, I’m confident no two identical flakes have ever fallen. The long answer is more involved – depending on what you mean by ‘alike’ and ‘snowflake’. There could be some extremely small, simple-shaped crystals that looked so alike under a microscope as to be indistinguishable – and if you sifted through enough Arctic snow, where these simple crystals are common, you could probably find a few twins.”
If you thought snowflakes were the ultimate in nature’s micro-level majesty, ice crystals have one more trick up their sleeve, one that almost none of us will ever see, unless we find ourselves at the South Pole. Ice crystal halos are produced in the same way as rainbows, except that the sunlight (or moonlight) refracts from ice crystals instead of water. In other words, instead of being rainbows, they are ‘snowbows’, and, says Libbrecht, “simply exquisite”.
Does he ever wonder, staring for years on end at the so-far-impenetrable and wondrous beauty of his subjects, if only a higher hand could have made them? “No,” he says bluntly, the scientist firmly back at the helm. Of course there’s still one obvious question that always come up before pudding that he’s more than happy to elaborate on. Why does he do it?
“Humans usually make a thing by starting with a block of material and carving from it,” says Libbrecht. “Computers, for example, are made by patterning intricate circuits on silicon wafers, but in nature things simply assemble themselves. Cells grow and divide, forming complex organisms. Even extremely sophisticated computers like your brain arise from self-assembly. Your DNA does not contain nearly enough information to guide the placement of every cell in your body, most of that structure arises spontaneously as you grow.”
The snowflake is a very simple example of self-assembly. “There is no blueprint or genetic code that guides the growth of a snowflake, yet marvellously complex structures appear, quite literally out of thin air.” As the electronics industry pushes toward ever smaller devices, it is likely that self-assembly will play an increasingly important role in manufacturing, and Libbrecht’s work could contribute to that. But neither he nor Westbrook care much about that, they just revel in the joy of unravelling the tantalising mystery of snowflakes.
“Einstein didn’t worry about the practical applications of relativity, he just wanted to understand how nature worked. Snowflakes are remarkable structures that simply fall from the sky. With over six billion people on the planet, surely a few of us can be spared to ponder the subtle mysteries of snowflakes.”
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