Is it true that no two snowflakes are exactly alike? Why?

It’s the season when snow is expected to fall all over the country. Has it snowed where you are?

The falling snow fluttered down, reminding me of the soul-searching question of Xie An, a famous official of the Eastern Jin Dynasty, recorded in Shishuo Xinyu: "What does the falling white snow look like?"

Since ancient times, people have liked to use beautiful words to describe snow. Snowflakes are like salt, sugar, goose feathers...

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If you pay close attention, you'll notice that most snowflakes have six petals.

Do all snowflakes have six petals? Why does snow form this shape when it falls? Are there two identical snowflakes? This starts with the process of snowflake formation.

01

Snowflake formation:

The "race" between the main and auxiliary axes of ice crystals

As early as the Western Han Dynasty, someone summarized it as follows: "Most flowers of plants have five petals, but snowflakes have six."

Now, with the help of advanced photography technology, we can observe that snowflakes come in a variety of shapes, but most have six petals.

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The essence of snowflakes is ice crystals, which are solid hydrates formed by the condensation and growth of water vapor on ice nuclei. They are the necessary medium for the formation of snowflakes and can be regarded as young snowflakes.

As the ice crystals grow, a variety of snowflakes are formed.

There is a saying in the proverb "It's not cold when it snows, but it is cold when the snow melts", which means that the moisture in the air releases heat when it gets cold and condenses into ice crystals, while the snow absorbs heat and turns into water when it melts, so the temperature when it snows is higher than when the snow melts.

Snowflakes do not appear out of thin air by themselves. They must rely on tiny dust particles in the air below the stratosphere that are invisible to the naked eye as their "crystal nuclei".

The crystal nucleus is a material group formed at a lower temperature with some dust as the center together with gaseous water molecules. Like a magnet, it can continuously absorb the surrounding gaseous water molecules.

Let the water molecules condense around it layer by layer, continue to grow and become solid, completing the phase change.

There are two trends in the shape of the crystal nucleus growth: one is a long and thin hexagonal columnar crystal, sometimes it has two pointed ends, like a needle, which we call "needle crystal";

The other type is very thin, hexagonal platelets that look like pencil shavings cut with a knife.

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Why do the crystal nuclei grow in two different directions? This can be traced back to the structure of ice crystals.

Like all other crystals, ice crystals have the most basic property of having regular geometric shapes.

In order to facilitate the determination of the positions of the crystal edges on the crystal, we artificially select a coordinate axis according to certain rules and call it the crystal axis.

There is a type of crystal called the hexagonal system, which has four crystal axes, namely one vertical axis and three horizontal axes.

Similar to the crystals we see in museums, ice crystals also belong to the hexagonal crystal system and also have four crystal axes, divided into three auxiliary crystal axes and one main crystal axis.

The three auxiliary crystal axes intersect at an angle of 60° to form a base plane, and the main axis is perpendicular to this base plane. The following picture can help you understand the structure of ice crystals:

Crystal axis of ice crystal, image source: Wikipedia

When water vapor condenses into ice crystals, and when the main axis condenses faster than the auxiliary axis and develops very long, the ice crystals form columns;

On the contrary, if the major axis freezes slower than the minor axis, the ice crystals will appear flaky.

Most common snowflakes are hexagonal, which is caused by the fact that the growth rate of ice crystals along the main axis is much slower than the growth rate of the other three auxiliary axes.

02

Condensation of ice crystal main axis

Affected by these factors

After understanding the growth mechanism of ice crystals, let's look at the factors that affect the growth rate of the snowflake axis. The growth of the snowflake axis is mainly affected by temperature and humidity.

First, temperature affects the speed at which ice crystals condense.

When the temperature is extremely low, ice crystals have no chance to grow and very small snow crystals are formed. Individual ice crystals are even difficult to observe directly with the naked eye.

At minus 30 degrees Celsius, ice crystals will condense into needles, while at close to 0 degrees Celsius, the base of snowflakes will mostly grow into hexagons.

Secondly, humidity will also affect the condensation rate.

Humidity is mainly determined by the water vapor content in the clouds. If the air humidity is low, ice crystals will grow very slowly, and will mostly form three basic shapes: columnar crystals, needle crystals, and plate crystals, such as flake and powdery snowflakes;

When the air humidity is high, the ice crystals will change shape as they grow, forming the familiar star-shaped snowflakes.

This way we can sort out the process of snowflake formation.

As mentioned earlier, ice crystals are formed by the condensation of water vapor in the air. When ice crystals grow, they consume the surrounding water vapor, causing the water vapor concentration around the ice crystals to decrease.

Water vapor diffuses toward the ice crystals. The new water vapor first encounters the protrusions and corners of the ice crystals and condenses there, causing the ice crystals to grow and the protruding parts to gradually grow into branches.

Afterwards, new branches and edges grew on the twigs and edges for the same reason, gradually forming the familiar star-shaped snowflakes.

Based on the above theory, the relative parts of star-shaped snowflakes should be symmetrical, and the shapes and sizes should be the same.

But in the atmosphere, snowflakes will not grow in an orderly manner as described above, but will be affected by airflow, air composition, etc., and their shapes will not be so regular.

In addition, ice crystals are in constant motion during the formation process, and the temperature and humidity they are in are constantly changing, changing from an environment suitable for forming one shape to an environment suitable for forming another shape.

For example, different parts of a snowflake come into contact with different amounts of water vapor. The places that are exposed to more water vapor grow faster, and the places that are exposed to less water vapor grow slower, thus forming various different snowflake shapes.

In addition, scientists have shown through experiments that ice crystals formed in a vacuum space with only water vapor are almost all single triangular prisms, which indicates that other gases in the air can also affect the formation of snowflakes.

Image source: http://www.snowcrystals.com

03

"Heavy snow"

This is how it was formed

The shapes of individual snowflakes are already varied, and as they fall, they may merge into larger snowflakes.

This may be because the collision and friction generate heat that sticks them together, or there may be water film on the snowflakes that sticks them together due to surface tension, or it may be that the snowflakes themselves have complex shapes and branches, and they are formed "hand in hand" with each other.

The journey from heaven to earth is very long. When conditions are right, snowflakes can become very large after multiple connections and mergers. What we call "heavy snow" and "willow catkins in the wind" are formed after multiple mergers. The largest snowflake ever observed by humans is 38 cm in diameter.

Of course, the adhesion process may cause some snowflakes to break, forming some deformed snowflakes.

However, one thing needs to be noted: snowflakes are very light. Five thousand to ten thousand snowflakes weigh one gram, which is much lighter than goose feathers. The so-called "heavy snowfall" is a slightly exaggerated statement.

Snowflakes are not "beautiful waste", they are very useful!

Our ancestors gave snow many beautiful names, such as "Jade Dragon", "Jade Dust" and "Silver Chestnut", expressing their love for the beauty of snow. There is also an agricultural proverb "Cover the wheat with three layers of quilts in winter, and sleep on steamed buns next year", which describes the contribution of snow to agricultural development.

Snowflakes can not only keep warm, but also provide moisture for the growth of crops in the coming year. No wonder they have the reputation of "auspicious snow heralds a good harvest."

At the same time, snowflakes have also given scientists a lot of inspiration.

On the one hand, snowflakes are called "messengers from the sky." Japanese physicist Dr. Ukichiro Nakatani once found out that the different crystal forms of snow depend on the high air temperature and the amount of water vapor, which can be used to infer the condition of the atmosphere.

On the other hand, snowflakes are essentially ice crystals, and the fragility and transience of ice crystals make them a challenging subject for scientific study.

As early as the 17th century, German scientist and polymath Johannes Kepler began to think about the structure of snowflakes, and later generations have continued to explore the factors that influence the shape of ice crystals, promoting the continuous development of atomic physics.

To this day, there is still a dedicated group of researchers dedicated to the nature of ice crystal growth.

Although we do not fully understand the factors that interfere with ice crystal growth, related explorations have made certain breakthroughs in condensed matter physics;

It has also played a positive role in the development of drug molecules, semiconductor chips, solar cells and countless other applications involving the growth of high-quality crystals.

Snow crystal growth model, image source: Wikipedia

The falling snowflakes are like a group of elves, adding a unique charm to the world and containing the mysteries of nature, waiting for us to explore.

Produced by | Science Popularization China

Author: Li Zhen, Changchun Institute of Optics, Fine Mechanics and Physics, University of Chinese Academy of Sciences

Producer｜Computer Network Information Center, Chinese Academy of Sciences

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