Are snowflakes only white?

Yes

The scenery when it snows is stunning, but what scientists and arts and crafts artists marvel at are the tiny, delicate patterns of snowflakes. More than a hundred years ago, glaciologists began to describe the shape of snowflakes in detail.

Tyndall, the originator of Western glaciology, described the snowflakes he saw on Roza Peak in his classic glaciology work: "These snowflakes... are all composed of small ice flakes, each one Each small ice flower has six petals. Some petals release beautiful little tongues like shansu flowers. Some are round, some are arrow-shaped, or zigzag-shaped, some are complete, and some are in a grid. shape, but they do not exceed the scope of the six-petal type."

In our country, as early as the era of Emperor Wen of the Western Han Dynasty more than 100 years ago, there was a poet named Han Ying who wrote a book. "Han Shi Wai Zhuan" clearly points out in the book, "Every plant and tree has five flowers, but snowflakes only have six."

The basic shape of snowflakes is hexagonal, but they are almost impossible to find in nature. Two identical snowflakes are just like there are no two identical people on 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, 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 earlier. 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. As long as we place them on black woolen cloth or black velvet, we can initially identify their shapes with the naked eye.

How big are the snowflakes?

The poet Li Bai had a famous poem when describing the snowflakes in Yanshan Mountain: "The snowflakes in Yanshan Mountain are as big as a mat." Are snowflakes really that big? Actually, snowflakes are very small. Not to mention that there is no record of snowflakes as big as a mat in the history of science. Even snowflakes as big as goose feathers are not easy to encounter.

In fact, the diameter of individual snowflakes we can see is generally between 0.5 and 3.0 mm. Such tiny snowflakes can only be weighed on an extremely precise analytical balance. About 3,000 to 10,000 snowflakes add up to weigh one gram. A scientist made a rough calculation and found that there are about 6 to 8 billion snowflakes in one cubic meter of snow, which is more than the total population on the earth.

The size of snowflake crystals depends entirely on the temperature conditions when water vapor condenses and crystallizes. The snow crystals formed in very cold weather are so small that they are almost invisible. Only when they sparkle in the sun can people find they exist like diamond powder.

According to research, temperature has an impact on the size of snow crystals: when the temperature is -36°C, the average area of ??snow crystals is 0.017 square millimeters; when the temperature is -24°C, the average area is 0.034 square millimeters. ; When the temperature is -18°C, the average area is 0.084 square millimeters, when -6°C it is 0.256 square millimeters, and when the temperature is -3°C, the average area of ??snow crystals increases to 0.811 square millimeters.

People have a wrong feeling. This feeling often comes from some literary works that like to describe the severe cold weather as "goose-feather heavy snow". In fact, "heavy snow" is the product of temperatures approaching 0°C, and is not a symbol of severe cold weather. On the contrary, the larger the snowflakes, the higher the temperature at that time. Heavy snowfall rarely occurs during severe cold weather. It is possible to get heavy snowfall only in late autumn and early winter or late winter and early spring. The so-called goose feather snow is actually not a single snowflake, but formed by many snowflakes stuck together. The maximum diameter of a single snowflake crystal is no more than 10 millimeters, which is at most the size of our fingernails. It cannot be called a goose-feather snow.

When the temperature is relatively high, snowflake crystals can easily connect to each other. This phenomenon is called the merger of snowflakes. Especially when the temperature is close to 0°C and the air is relatively humid, the ability of snowflakes to merge is particularly strong, and hundreds or even thousands of snowflakes can often merge into one large piece of snow. Therefore, strictly speaking, goose feather snow cannot be called snowflakes, it is just an aggregate of many snowflakes.

Working Snow

Since ancient times, it has always snowed when God is happy, and not when God is unhappy. Is there a way to make God let it snow according to human needs?

There is a way, and this is artificial snowfall.

For water vapor in the sky to turn into rain and snow, it must meet two conditions. One is that it must have a certain degree of water vapor saturation (mainly related to temperature), and the other is that there must be condensation nuclei. Therefore, artificial snowfall must first have clouds in the sky. Without clouds, it is like a skillful woman cannot make a meal without rice, and it cannot snow. Clouds that can make snow fall are "cold clouds" that are below 0℃. In the cold clouds, there are both small water droplets where water vapor condenses, and small snow crystals where water vapor condenses. But they are very small and light. If there are no conditions for continued growth, they can only be suspended in the air like smoke and dust, and it is difficult to fall down. We often see large chunks of clouds in winter, but no snowflakes are floating down. This is because the snow crystals that make up these clouds are too small to overcome the buoyancy of the air, and their precipitation ability is very poor.

If some particulate matter is sprayed into the clouds to promote the snow crystals to grow quickly enough to overcome the buoyancy of the air and fall down, this is the contribution of artificial snowfall.

What substances can be sprayed to promote the rapid growth of snow crystals? In the early days, people used many interesting methods to show their magic. These methods mainly include: setting fires on the ground and releasing large amounts of smoke and dust into the sky; using cannons to attack clouds; using kites to fly high into the clouds, and then electrifying the kites to discharge sparks; flying into clouds by airplane to spray liquid water droplets and dust particles . However, the results of these methods are not ideal. It wasn't until 1946 that people discovered that putting tiny particles of dry ice into cold clouds could form millions of snow crystals. On November 3 of that year, someone on a plane sprinkled dry ice pellets onto the top of altocumulus clouds with a temperature of -20°C, and found that snow fell from the clouds.

The dry ice mentioned here is not ice frozen by water, but a solid state of carbon dioxide, much like the solid snow in winter. The temperature of dry ice is very low, below -78.5°C. Spray the dry ice crystals into the cold cloud like a goddess scattering flowers. Each carbon dioxide crystal becomes a sharply cold center, prompting the water vapor, small water droplets and small snow crystals in the cold cloud to quickly gather around it and condense. Huacheng's larger snowflakes fell.

Silver iodide is now commonly used to produce artificial snow. Silver iodide is a yellow chemical crystal that is usually used as a photosensitive agent in photographic materials. The hexagonal monomer sizes of silver iodide crystals and snow crystals are very similar, the atomic arrangements in their monomers are also very similar, and the lattice spacing of the two is also very close (silver iodide is 4.58 angstroms, snow crystals are 4.52 angstroms). Therefore, if silver iodide particles are sprinkled into clouds with poor precipitation capabilities, allowing them to "fake" snow crystals, the water vapor and small water droplets in the clouds can condense and crystallize on the "fake" crystals and turn into snowflakes.

How to spread these condensation nuclei into the clouds? Most modern people use cannons, put chemicals in cannonballs, and then use the cannons to launch them into the clouds. However, this method sprays unevenly, wastes a lot of medicine, and increases the cost of artificial snowfall. Others put them in clay rockets and let the rockets fly into the clouds to spray them.

Generally speaking, artificial snowfall has a greater success rate than artificial rainfall. Artificial rainfall can increase rainfall by about 20%, while in high mountain and cold areas, artificial snowfall can increase rainfall by 30 to 40%. This is because in alpine and cold areas, where temperatures are low, water vapor can easily reach a saturated state. At the same time, snow crystals are easier to form than raindrops. As long as some crystallization nuclei are artificially added to the atmosphere, it will be easier to promote snowfall.