Ukichiro Nakaya created the first artificial snowflake nearly ninety years ago, after conducting one of the most complete studies of ice crystals. He also photographed a large number of snowflakes, cataloging their different types. His observations are summarized today in a morphological diagram of crystals that predicts their shapes based on variables such as temperature and humidity. Like all of us have done at some point, he asked: Why do snowflakes almost always have a hexagonal symmetrical shape?
If there is something that characterizes the ice crystals that make up snowflakes, it is the variety of their shapes. Although generally all have hexagonal symmetry, it is practically impossible to find two identical. Why does this happen if they form within the same cloud, subject to the same atmospheric conditions? The secret lies, as we know today, in the temperature and supersaturation of water vapor at the moment the crystals develop.
The crystal begins to grow around a small chemical impurity—usually dust floating in the atmosphere—in the air. The molecular structure of the ice crystal is extremely sensitive to the aforementioned environmental factors, and small variations in some of those parameters result in completely different crystals. Today we believe we understand the physical principles that govern their development, although we remain incapable of predicting with any degree of certainty what the exact shape of a snow crystal will be.
A History of Fascination
People—and especially scientists—have been attracted to the shape of the ice crystals that make up snowflakes since practically the first time a human saw it snow. It is very likely that the first scientist to address this problem methodically was Johannes Kepler, who wrote the first "serious" treatise dedicated to the morphology of snow crystals. René Descartes also concerned himself with this question and, after a long investigation, made public in his treatise on meteorological phenomena: "Les Météores." But these studies were limited to describing the variety of existing crystals, without explaining their origin. We had to wait decades and decades of years until the end of the 19th century, when the invention of photography allowed a deeper analysis of ice crystals. That is how Wilson Bentley was able to create a catalog of more than 5,000 shapes and published most of them in a book edited in 1931. It is possible that the beauty of Bentley's images is responsible for snow crystals becoming a true winter icon. Finally, after trying for centuries, humanity was about to understand how these crystals formed.
It was the Japanese Ukichiro Nakaya who carried out the first laboratory studies. Intrigued like his predecessors by the way crystals took shape during their growth, he managed, in 1933, to obtain the first synthetic snow crystals. Nakaya conducted experiments in which he slightly varied the temperature and the level of water supersaturation, and observed the influence this had on the shape of the generated crystals.
The Hexagonal Secret
The general profile of all snowflakes is always hexagonal. Despite the almost infinite variety of crystals that can be generated, it is the shape of the water molecule that dictates its symmetry. Since H2O is a perfect equilateral triangle, each growth node of the crystal has no choice but to "attach" at an angle of exactly 60 degrees with respect to the triangle's vertices. This fact is responsible for the fact that, in each "layer" of growth, six of those "molecular triangles" form the base of the next growth, so the shape will always be hexagonal. However, knowing this is not enough to determine the shape of the resulting crystal.
We know that the static part of the process generates objects with hexagonal symmetry, but we can say little about their dynamic aspect: Will it form facets? Will it be unique or divide into smaller communities? All this depends on the spatial location of each of the atoms that are added to the structure, a process complicated enough to become unpredictable. This is why, when examining two snowfalls separated by just a few hours, one can find crystals with very different needles.
How can such different structures be produced from exactly the same material? Doctor Kenneth Libbrecht—professor of physics at CalTech—is one of the people who has best studied this process, and believes he has an answer: "Every column, needle, and star-shaped plate that falls from the sky begins as a simple hexagonal prism, which is the basic shape of a snow crystal. This basic shape has two facets we call basal and six prismatic facets. However, the final structure depends on the relative growth rates on the facet surfaces: vapor condenses more rapidly on the prismatic faces. The mere existence of columnar and plate-like crystals requires that the growth rate vary by a factor of 1000 under different conditions." The solution, it seems, lies in understanding the process of their individual growth rather than the final result, because that is where the differences originate.
This same reasoning led Ukichiro Nakaya, nearly a century ago, to cultivate and grow his own snowflakes. Inside his laboratory he could study the formation process under controlled conditions. His observations are summarized in a morphological diagram of crystals that shows the way temperature and humidity broadly determine the shape of crystals.
Reading the Crystal Blueprint
As you see in the figure above, temperature is represented on the horizontal axis and humidity on the vertical axis. Nakaya shows what happens when the scale exceeds 100% humidity, a situation known as "water supersaturation". The blue line shows the differences that appear in the design of crystals that form above and below it. It is evident that relatively simple crystals begin to become more and more complex as humidity increases. Some shapes, such as star-shaped or multi-branched ones, only form with very high humidity levels.
It is also seen how the shape varies dramatically as a function of temperature. The fact that crystals return to simple shapes when the temperature drops too much is particularly difficult to explain, and 85 years after Nakaya's analysis we still do not understand why those large morphological changes are caused by just a few degrees of difference. Today we know that atmospheric pressure also influences the final shape, so not only two variables intervene, but at least three. And perhaps more. Libbrecht explains that "in fact, the morphological diagram of snow crystals is a simple two-dimensional 'slice' of a much more complex and multidimensional reality. If we add a third axis—time—we will see that crystals become much larger and more complex as time passes."
It is possible that we will never be able to predict exactly what shape a given snowflake will have, but that obviously only increases its beauty. Each of the crystals that Nakaya surely dreamed of is unique, almost a work of art of nature, and that is what matters.
Learn more about Ukichiro Nakaya on Wikipedia.
