Is every snowflake in the world the same shape?

The basic shape of snowflakes is hexagonal, but there are almost no two identical snowflakes in nature, just like there are no two identical people on the earth. Many scholars have observed thousands of snowflakes with microscopes. These studies have finally shown that snowflakes with exactly the same shape and size and complete symmetry of each part cannot be formed in nature.

Among the snowflakes that have been observed by people, no matter how regular and symmetrical the snowflakes are, there are also deformities. Why are snowflakes deformed? Because the water vapor content in the atmosphere around the snowflake cannot be the same in all directions. As long as there is a slight difference, the side with more water vapor content will always grow faster.

There are many collectors of snowflake patterns in the world. They collect all kinds of snowflake photos like stamp collectors. There was an American named Bentley who spent his life taking nearly 6,000 photos. Soviet photography enthusiast Sigshan was also a photographer of snowflake photos. His ecstatic works are often used by industrial artists as models for structural patterns. The Japanese Ukichiro Nakatani and his colleagues worked hard for twenty years in a cold room in the laboratory of Hokkaido University in Japan and in a tent on the snowfields of northern Japan, photographing and studying thousands of snowflakes.

However, although the shapes of snowflakes come in all shapes and sizes, they remain the same, which is why scientists can summarize them into the seven shapes mentioned above. Among these seven shapes, hexagonal snow flakes and hexagonal prismatic snow crystals are the most basic forms of snowflakes, and the other five are just the development, metamorphosis or combination of these two basic forms.

As early as the Western Han Dynasty BC, "Han Shi Wai Zhuan" pointed out: "Every plant and tree has five flowers, but snowflakes only have six." The basic shape of snow is hexagonal. But under different circumstances, it can show various forms.

There are many snowflake pattern collectors in the world, and they collect various snowflake patterns. Someone spent his whole life taking thousands of photos of snowflakes and found that there were nearly 6,000 different kinds of snowflakes. However, before he died, he thought that these were just a small number of snowflakes that nature had fallen into his hands. So much so that some say that no two snowflakes are exactly the same size and shape.

Why are the basic shapes of snowflakes hexagonal flakes and columns?

This is related to the crystal habit of water vapor sublimation and crystallization. Snowflakes formed by the condensation of water vapor and ice frozen by natural water both belong to the hexagonal crystal system. We are easily attracted by the pure and transparent crystal in the museum. Crystals and ice crystals are both hexagonal crystals, but crystals are crystals of silicon dioxide (SiO2), while ice crystals are crystals of water (H2O).

The hexagonal crystal system has four crystallographic axes, of which three auxiliary axes are on a base plane and intersect each other at an angle of 60o. The fourth axis (main crystal axis) and the three auxiliary axes form The base is vertical. The most typical representative of the hexagonal crystal system is like a regular hexagonal cylinder in geometry. When water vapor desublimates and crystallizes, if the main crystal axis develops slower than the other three auxiliary axes and is very short, then the crystal will form a flake; if the main crystal axis develops quickly and extends very long, then the crystal will form a columnar shape. . The reason why snowflakes are generally hexagonal is because the crystal growth rate along the main crystal axis is much slower than along the three auxiliary axes.

Various shapes of snowflakes

For a hexagonal snowflake, due to its different surface curvatures (convex, flat and concave), the saturated water vapor pressure on each surface is also different. Therefore, a mutual water vapor density gradient is generated, causing directional transfer of water vapor. The direction of water vapor transfer is convex surface → plane → concave surface, that is, it moves from the surface with large curvature to the surface with small curvature. The six corners of a hexagonal snowflake have the largest curvature, followed by the flat surfaces at the edges, and the smallest curvature in the center. In this way, the hexagonal snowflakes are always in the process of directional water vapor migration. Since water vapor on the edges and corners is transported to the edges and the center, the saturation degree of water vapor near the edges and corners decreases, resulting in sublimation. As the central part receives a steady stream of water vapor, the ice surface reaches saturation, resulting in sublimation. This process of sublimation and crystallization continues, and the hexagonal snow flakes gradually evolve into hexagonal prism-shaped snow crystals. (Schematic diagram of water vapor migration on snow flakes: fig42)

This is an ideal situation assuming that no water vapor is transported from the outside. In fact, things are closely connected with their surrounding environment, and there is always more or less water vapor in the air. If the surrounding air inputs less water vapor, which is not enough for the edges and corners of the snow flakes to transport water vapor to the center, then the development process of the snow flakes into columnar snow crystals will continue. In high latitudes and polar regions where the temperature is very low and there is little water vapor, columnar snow crystals often fall for this reason.

When the water vapor saturation level in the air is high, another situation occurs. At this time, the surrounding air continuously transports water vapor to the snow flakes, causing the snow flakes to rapidly sublimate. Sublimation reduces the density of water vapor in the air layer surrounding the snowflakes, which in turn promotes the transport of water vapor from the outer layer to the interior. In this way, the snowflakes grow quickly. When water vapor is rapidly transported to the snow flakes, the six vertex corners bear the brunt, and the water vapor density gradient is the largest. The water vapor that has no time to transport to the interior of the snowflake condenses and crystallizes on the top corners; at this time, some protrusions and branches will appear on the top corners. When these branches grow to a certain extent, they will branch again.

The secondary branches maintain an angle of 60° with the parent branch, thus forming a six-pointed star-shaped snowflake.

In clear weather in the mountains or in the polar regions, you can also see a kind of ice needles, shining like gems, people call them diamond dust. There are two situations for the growth of ice needles: one is the result of spontaneous crystallization of water vapor in severe cold conditions (below -30°C) when the humidity is very low, and the other is the result of spontaneous crystallization of water vapor when the temperature is high (around -5°C) and the humidity is high. The product of particularly rapid growth at the top angle where one of the auxiliary axes of the snowflake is located is the abnormal development of the snowflake.

After various snowflake crystals are formed in the sky, when their diameter reaches 50 microns, they can overcome the buoyancy of the air and begin to make an obvious downward movement, continuing to grow and change while falling elegantly. As a result, snowflakes come in a wide variety of forms.

We just need to