Why Are Snow Crystals Hexagonal? Nakaya's "Letter from the Sky" and How to Photograph Snow with a Phone

What a snow crystal actually is — the letter that arrives from the sky

A snow crystal is a single ice crystal that forms when water vapour turns to ice inside a cloud and then keeps growing. It is not a frozen droplet of water: it is built up molecule layer by molecule layer through deposition, in which gas (water vapour) changes directly into solid (ice). That is precisely why such regular shapes emerge.

Crystals are born in clouds and grow as they fall

Inside a winter cloud, droplets often remain liquid even below 0°C — a "supercooled" state. When a tiny airborne particle (a mineral grain, a biological particle and so on) acts as an ice nucleus, water vapour freezes onto it and a small ice particle is born.

What happens next is the interesting part. At the same temperature, the saturation vapour pressure over ice is lower than over supercooled water, so the surrounding droplets evaporate and their vapour collects onto the ice crystal. The ice grows one-sidedly at the droplets' expense. Named after its discoverers, this is the Bergeron process (the Wegener–Bergeron–Findeisen process), and it is the origin of most snow and rain in the mid-latitudes, Japan included.

A grown crystal falls slowly, at speeds of a few tens of centimetres up to about one metre per second. It can take tens of minutes to reach the ground from a cloud several thousand metres up, and throughout that journey it keeps recording the temperature and humidity of every layer it passes through. If conditions change on the way down, the shape changes too. That is why "two-storey" crystals — a column at the centre with plates on the outside — are sometimes found.

Why hexagons? The molecular structure of ice

The reason lies in the shape of the water molecule itself and in how molecules bond. A water molecule is bent, with two hydrogen atoms attached to one oxygen atom at an angle of roughly 105 degrees. When these molecules link up through hydrogen bonds, hexagonal rings with oxygen atoms at their vertices line up in planes, and those planes stack. This is the structure of everyday ice, and because it belongs to the hexagonal crystal system it is called "ice Ih."

As a crystal grows, water vapour can only attach in directions dictated by that atomic arrangement. Growth along the plane of the hexagon (the a-axis) yields plates and dendrites; growth perpendicular to it (the c-axis) yields columns and needles. What decides which direction grows faster is temperature and the amount of water vapour — and that correspondence is exactly the Nakaya diagram described below.

"Snow" and "snow crystal" are not the same thing

Before you start observing, there is a distinction worth making. Most of what we call "snow" is not a crystal that fell on its own, but a snowflake (a cluster of many crystals stuck together) or graupel — a crystal made white and opaque by frozen droplets. Whether you can photograph one clean crystal is decided almost entirely by that day's temperature and the quality of the snow.

Around 0°C, crystals stick to each other and form flakes, and they start melting the instant they hit the ground. A day when the temperature is −2°C or below and dry, powdery snow is falling is the very best for photography.

Ukichiro Nakaya and the "letter sent from the sky"

It was the physicist Ukichiro Nakaya who turned snow crystals from "something beautiful" into "something that can be read." That shift was achieved through patient observation in Japan's snow country and through the world's first artificial-snow experiment.

Born in Kaga, he studied snow at a northern university

Ukichiro Nakaya was born on July 4, 1900, in Katayamazu, Enuma District, Ishikawa Prefecture (present-day Kaga City). He studied under Torahiko Terada in the Department of Physics at Tokyo Imperial University, graduating in 1925. After a period at RIKEN he studied abroad at King's College London, becoming an assistant professor at Hokkaido Imperial University's Faculty of Science in 1930 and a full professor in 1932.

Nakaya himself wrote, in effect, that he took up snow research because there was simply so much snow after he came to Hokkaido. Choosing snow as a research subject was close to an accident of circumstance, but from there he accumulated and classified more than 3,000 microscope photographs of natural snow, searching out the regularities common to snow in Japan.

March 12, 1936: making snow for the first time

No amount of observing natural snow can prove that "this shape forms under these conditions," because the conditions in the sky cannot be measured. So Nakaya chose a different route: build a cloud inside the laboratory, set the conditions yourself, and grow the crystals.

Inside an apparatus placed in the low-temperature laboratory, water vapour was supplied and a fine fibre was suspended to give the crystal a foothold. The nucleus used was rabbit hair: the microscopic projections on its surface act as ice nuclei, and crystals grow there. After much trial and error, on March 12, 1936, an artificial snow crystal appeared inside the apparatus — the first time in history that snow had been made by human hands.

The significance of artificial snow is not that snow could be "made." It is that temperature and vapour supply could now be varied one at a time, so that the shape produced under every condition could be mapped systematically. The accumulation of these experiments produced the Nakaya diagram discussed in the next section. In 1941, on the basis of this artificial-snow research, it was decided to establish the Institute of Low Temperature Science, which opened in 1943.

The sentence written in <em>Snow</em>

In Snow, published by Iwanami Shoten in 1938, Nakaya expressed what his research meant in the following words.

Snow can be said to be a letter sent from the sky

― Ukichiro Nakaya, <em>Snow</em>, Iwanami Shoten, 1938

The sentence is often quoted as poetry, but Nakaya's intention was far more concrete. The shape of a crystal is "writing" put down by the temperature and humidity aloft, and reading that writing reveals a state of the sky that cannot be measured directly — the metaphor of a letter was a practical declaration to that effect. In an era when neither radiosondes nor weather radar were widely available, a new means of knowing the sky had been presented.

What matters is that anyone who knows this "way of reading" can receive the sky's news. No special equipment is needed. A crystal that lands on your palm will tell you about the air far above, at this very moment.

The Nakaya diagram — reading the upper air from a crystal's shape

What Nakaya assembled from his artificial-snow experiments was a chart with temperature on the horizontal axis and vapour supersaturation (how much excess water vapour the air holds) on the vertical axis, showing which crystal shapes form where. This became known as the Nakaya diagram, and it is the starting point of snow crystal science.

Two axes decide the shape

The principle the diagram reveals is remarkably simple. Temperature decides whether the crystal becomes plate-like or column-like, and the amount of water vapour decides how elaborately it grows. At the same temperature, scarce vapour yields a crisp-edged hexagonal plate, while abundant vapour yields a dendrite branching again and again.

For example, the beautiful six-petalled dendrite familiar from textbooks forms at roughly −15°C when there is ample water vapour. Around the same −15°C but with little vapour, you get a plate with no branches. At around −6°C with plenty of vapour, slender needle crystals appear.

What "reading the letter" means in practice

Suppose you find a great many dendrites at ground level. Tracing the diagram backwards, you can infer that there was a moist layer aloft at around −15°C. If needle crystals are mixed in, there was probably a humid layer near −6°C as well. If graupel dominates, there was a thick cloud containing abundant supercooled droplets.

This is not merely an intellectual game. In actual meteorological research, the temperature and humidity distributions computed by numerical weather prediction models are checked against the crystal shapes observed at the ground. Crystals are a physical record you can use to mark the model's answers. This matters especially for snow brought to the Kanto region by south-coast cyclones, where forecasting rain versus snow, and accumulation versus none, is notoriously hard.

How later research widened the map

The Nakaya diagram is not a finished, final form; it has been tested and extended ever since. In Tenki, the journal of the Meteorological Society of Japan, discussions have appeared examining the relationship between the diffusion-type method of making artificial snow and the diagram, exploring how the resulting chart changes with experimental technique. That a chart drawn more than 80 years ago is still the basis of discussion is itself a measure of the work's reach.

Classifying snow crystals — from Bentley to the Global Classification

People often say no two crystals are alike. Strictly speaking, crystals matching down to the last detail of every branch essentially do not exist — yet the broad shapes fall into clear types. Efforts to organise those types began alongside the development of photography.

Wilson Bentley's 5,381 photographs

The first person to capture snow crystals on film was Wilson A. Bentley (1865–1931), a farmer from Jericho, Vermont, USA. Enchanted at fifteen by snow crystals seen through his mother's microscope, he built his own rig by coupling a bellows camera to a microscope and, on January 15, 1885, took the world's first photomicrograph of a single snow crystal.

He went on photographing throughout his life alongside his farm work, reaching a total of 5,381 images. In the autumn of 1931 a photographic collection titled Snow Crystals, containing 2,453 of them, was published; shortly afterwards Bentley died of pneumonia. His work made the diversity of snow crystals visible to everyone for the first time. When Nakaya began his research, Bentley's collection was also the largest photographic resource available to him.

From 7 types to 121

Based on his observations of natural snow, Nakaya systematically classified the crystals seen in Japan. Then in 1966, Choji Magono and C. W. Lee published a finer classification of 80 types (the Magono–Lee classification), which long served as the international standard.

In 2013, a research group including Katsuhiro Kikuchi, Takao Kameda, Keiji Higuchi and Akira Yamashita published a new classification system in an international journal, based on observations from the mid-latitudes to the polar regions. This is the Global Classification, comprising 8 major classes, 39 intermediate classes and 121 sub-classes. Of those 121, ten are ice crystals 0.2 mm or less in diameter and six are solid precipitation such as ice pellets, sleet and hail; restricted to snow crystals alone, the count is 105.

The basic shapes to learn first

There is no need at all to memorise 121 types. To begin with, being able to tell these six apart is plenty, and most of the photos you take will fall into one of them.

• Stellar dendrite: six branches dividing again and again — the most popular shape. Forms near −15°C with abundant vapour
• Hexagonal plate: a flat hexagon with no branches. Forms where vapour is scarce
• Sector plate: fan-shaped plates spreading in six directions; intermediate between dendrite and plate
• Column and bullet: an elongated column like a pencil or a bullet. Grows in cold layers
• Needle: a cluster of fine needles. Forms near −6°C with plenty of vapour
• Rimed crystal: densely covered in tiny white droplets — evidence that it passed through a cloud rich in supercooled water

Photographing snow crystals with a smartphone — what to prepare

Now for the practical part. Photographing snow crystals sounds as if it requires a microscope, but today's smartphone cameras can capture crystal detail perfectly well once you add a macro lens costing a few hundred yen. Kentaro Araki, a cloud researcher at the Meteorological Research Institute, has repeatedly pointed out that beautiful crystal photos can be taken with a 100-yen-shop macro lens for smartphones.

You only need three things

The single biggest tip is to put the cloth and the phone outside in advance so they chill. If you catch crystals on cloth brought straight from a warm room, they melt into round droplets the instant they land. Put them outside 15–30 minutes before shooting. Chilling the phone too much drains the battery fast, so a realistic balance is to keep it lightly warm in a pocket and take it out just before you shoot.

Good days and bad days for shooting

However well you prepare, you cannot get clean crystals if the snow is wrong. Here are the conditions to aim for.

• Temperature at or below −2°C: near 0°C crystals melt on landing and stick to each other
• Dry, powdery snow: snow that does not turn watery on your palm
• Light wind: strong wind rolls crystals across the cloth and breaks them
• From the start of the snowfall and while it is falling: lying snow has already changed and lost its original shape
• Just before the snow gets heavy: the period when small crystals fall individually is prime time

Conversely, warm days with sticky snow, windy days, and trying to scoop up lying snow to photograph are the classic failure patterns. Snow on the ground gradually rounds off and bonds together (metamorphoses), so the rule is to catch it fresh as it falls.

Shooting technique — focus, burst mode and scale

Once you are ready, all that remains is practice. A crystal holds its shape only briefly; depending on conditions, its corners begin rounding within tens of seconds. Shoot decisively, all at once — that is the key.

The seven-step shooting procedure

1. Spread the pre-chilled dark cloth on a flat, sheltered surface (a bench, a car bonnet, an umbrella)
2. Catch falling snow for a few tens of seconds; sparsely scattered grains are ideal
3. With your eyes or on the screen, pick one well-formed crystal that landed on its own
4. Attach the macro lens and set the camera to maximum zoom
5. Rest your wrist on the cloth or ground to steady it and move within a few centimetres of the crystal (about 10 cm without a lens)
6. Focus by raising and lowering your body slowly; changing distance is more reliable than chasing focus with your finger
7. The moment focus locks, shoot a burst. Don't aim for one shot — take dozens and choose later

Steps 5 and 6 matter most. Held in mid-air, the shot will always blur. Plant your wrist or elbow as a makeshift tripod, and focus by moving the phone back and forth — observing just these two points raises your success rate dramatically.

Three common pitfalls

One more surprisingly common failure is breaking the crystal by touching it. To reposition one, use the tip of a fine brush, never a finger — body heat melts it instantly.

Turning a photo into a record

A well-shot photo is a pleasure in itself. But adding a little information turns it into scientifically valuable data. Four things are needed.

Note that location data may be embedded in photos automatically. When posting to social media, check your settings so your home address cannot be pinpointed. Information down to the municipality is generally sufficient for research.

When your photo becomes research — the #KantoSnowCrystal Project

A photograph you took yourself becoming data for meteorological research — such a scheme genuinely exists in Japan. It is the "#KantoSnowCrystal" Project, launched in fiscal 2016 by Kentaro Araki of the Meteorological Research Institute, Japan Meteorological Agency.

Ultra-dense observation born of smartphones and social media

Snow in the Kanto Plain is often brought by south-coast cyclones and is notoriously hard to forecast. The rain–snow boundary is delicate, and a temperature difference just a few hundred metres up decides whether snow accumulates. Yet the means of measuring the upper air in fine detail are limited: there are only a dozen or so radiosonde sites in all of Japan.

The idea that emerged — inherited directly from Nakaya — was that collecting the shapes of crystals that reach the ground, in large numbers, allows the state of the upper air to be estimated across an area. Now that high-performance smartphone cameras and social media are ubiquitous, many people can observe simultaneously, each in their own location, with no special equipment. Participants simply photograph a crystal and post it with the hashtag "#関東雪結晶".

November 24, 2016: the day 5,100 photos arrived

The project's first major success came with the Kanto snowfall of November 24, 2016. On that day more than 5,100 snow crystal images were submitted by citizens, achieving the world's first ultra-dense, wide-area snow crystal observation through public participation. Snow accumulating in central Tokyo in November is a rarity in the observational record, and that rare event was documented simultaneously by a great many eyes.

Data has continued to accumulate with every snowfall, reaching a cumulative scale of over 100,000 images. The submissions are classified by crystal type and checked against the temperature and humidity distributions reproduced by numerical prediction models. Each individual's single photo becomes material for improving Japan's snowfall forecasts. The work also earned the Japanese Society of Snow and Ice Encouragement Award for fiscal 2016.

What to keep in mind when taking part

The barrier to entry is low, but whether the data proves useful depends on the care taken in recording it. Add the date, location, size and temperature described in the previous section — and don't discard the photos that didn't come out well. Even the information that there was nothing but graupel and flakes has value.

At the same time, safety and privacy need attention. Photographing on roads or roadways in snow carries real risk. Choose a safe spot where you can check your footing and surroundings, and review your location-sharing settings before posting.

This kind of effort — in which non-specialists take part in observation and classification and contribute to science — is called citizen science, and it now spans fields from wildlife surveys to stargazing. Umi LAB has gathered Japanese examples in What is citizen science? Wildlife surveys anyone can join, and how the data gets used, which gives the broader picture.

Snow and the sea are connected — the water cycle beyond the crystal

The tiny crystal on your palm is one scene in a grand journey of water. That water came from the sea, and in time it will return there. Take snow crystals as your entry point and the whole water system of Japan comes into view.

How the Sea of Japan becomes a snow factory

Heavy snow on the Sea of Japan side is produced by cold Siberian air and the relatively warm sea surface. When cold air blows across a warm sea, huge amounts of heat and water vapour are supplied from the surface to the atmosphere. This process, called air-mass transformation, converts dry cold air into moist snow clouds. In other words, the water in the snow falling on Niigata or Kanazawa was seawater in the Sea of Japan moments earlier.

What causes particularly heavy snow is the linear snowband stretching across the Sea of Japan known as the JPCZ (Japan Sea Polar Air Mass Convergence Zone). In February 2022, a research team from Niigata University, Mie University, the National Fisheries University and others captured the JPCZ's internal structure for the first time through shipboard observation. At the time of observation the sea surface temperature was 14°C and the air temperature 3°C — a difference of 11°C — with winds of 17 metres per second. It was shown that vapour evaporated from the broad sea surface concentrated into the JPCZ through airflow convergence, transporting an amount equivalent to two metres of snowfall per day.

The warmer the sea surface, the more vapour it supplies. In recent years the Sea of Japan's surface temperature has been rising over the long term, which is the background to the seemingly paradoxical possibility that even as temperatures rise, individual snowfalls may grow heavier. For persistent high sea temperatures, see What is a marine heatwave?

Snow is stored water — and it returns to the sea

The decisive difference between rain and snow is that snow stays where it lands and runs off later. Snow that piles up in winter is stored in the mountains and melts slowly from spring into early summer, feeding the rivers. That is largely why stable water is available for puddling rice paddies, and why rivers do not run dry in summer.

Meltwater then flows down the rivers, dissolving nutrients such as nitrogen, phosphorus and iron out of mountain soils, and enters the coast through river mouths. Those nutrients nourish phytoplankton and support rich fishing grounds. A single grain of snow that fell on a mountain ultimately feeds life in the sea — this chain is exactly what the phrase "the forest is the sea's lover" points to.

How warming will change snow

As temperatures rise, the first thing to change is the form of precipitation. If the same amount of water increasingly falls as rain rather than snow, less is stored in the mountains, and the spring melt peak arrives earlier and smaller. Summer drought risk and impacts on agricultural and hydroelectric water are the concern.

At the same time, because warmer sea temperatures increase the atmosphere's moisture content, it has been suggested that short-duration snowfall may become more intense when cold air arrives. The shift is "fewer snowy days, but heavier dumps when it does snow." Year-to-year variation in snowfall is also strongly tied to El Niño and La Niña, explained in How El Niño and La Niña change fisheries and weather.

And what is needed to verify such changes is precisely a record of what kind of snow actually fell. The crystal photographs citizens accumulate become valuable observational material for checking models. A single photo taken as a hobby joins the foundation of climate research.

To enjoy it further — places to visit and next steps

You don't have to wait for a snowy day to touch the world of snow and ice. Finally, here are places and methods for taking your observations a step deeper.

The Ukichiro Nakaya Museum of Snow and Ice (Kaga City, Ishikawa)

Katayamazu, Nakaya's birthplace in present-day Kaga City, is home to the Ukichiro Nakaya Museum of Snow and Ice, which honours his achievements. It opened in November 1994, with architecture by Arata Isozaki. Facing Lake Shibayama, the building itself is built on a hexagonal motif — an homage to the snow crystal.

Alongside Nakaya's research materials and microscope photographs of natural snow, the museum offers hands-on exhibits such as diamond-dust demonstrations and ice pendant making. Another attraction is the chance to encounter the many sides of Nakaya, known not only as a physicist but as an essayist, documentary filmmaker and painter. Even in a snowless season, you can experience the science of snow.

What to do on days when it isn't snowing

• Photograph frost crystals: on a clear winter morning, the frost on dead grass or a car window is also hexagonal ice. It lasts longer than snow, making it ideal practice
• Look for window frost: in cold regions, feather-like ice patterns form on window glass — good macro subjects
• Observe frost in your freezer: the frost inside is ice formed by deposition too. The same physics is happening in your home
• Sort through your past photos: rearranging your collection by crystal type reveals correspondences with the weather
• Read Snow: Nakaya's book is available on Aozora Bunko. The struggles and joys of the experiments are written in plain language

Frost photography is especially recommended. Unlike snow, there is time before it melts, so you can practise macro focusing calmly. Get used to it here and you won't panic on the day it snows.

Small habits for keeping records going

The trick to observing for the long haul is to reduce the burden of record-keeping. Make an album each time you shoot and jot down the date, place and temperature — that's enough. After a few seasons you will begin to see your own local patterns, such as "sector plates dominate here during south-coast cyclones".

Once you reach that point, you are already a reader of letters from the sky. Applying the correspondences Nakaya confirmed in his laboratory to the sky above your own patch of ground — that, differing only in scale, is a continuation of the work he began more than 80 years ago.