A snowfall is on the way: scientists explain what a snowflake is and the factors that determine its unique pattern.

The Birth of a Snowflake

Snow is frozen water vapor that falls to Earth in the form of atmospheric precipitation – snowflakes. In most cases, they form in high-layer clouds at mid-level altitudes ranging from 3 to 6 kilometers above the Earth's surface. The temperature in these tropospheric layers is optimal for snow crystal growth.

When water molecules settle on a pollen grain, dust particle, or any other speck in the sky and freeze instantly, an ice crystal is formed. As it falls to the ground, water vapor deposits and crystallizes on the nascent structure.

“The complex shape of a snowflake is determined by the atmospheric conditions the ice crystal experiences during its descent. Its arms may begin to grow in one direction, but then, after a certain period, slight changes in temperature or humidity cause the crystal to grow in a different direction. The stronger the upward air currents and the more moisture in the cloud, the larger the snowflakes can become,” explains Yegor Razumovsky, a graduate student at the Department of Mechanics of Composite Materials and Structures at PNIPU.

Why Does a Snowflake Have Six Arms?

At low temperatures, water molecules group into sets of six, forming a perfect hexagon known as a "dendrite," with internal angles of 120 degrees. This structure is characteristic not only of snow but also of melted water and ice.

“To be more specific, one water molecule consists of one oxygen atom and two hydrogen atoms. Due to hydrogen bonds, they arrange themselves at the vertices of a perfect hexagon, creating the main crystal. From this central structure, side branches grow. The temperature and humidity of the air surrounding the snowflake are uniform, making it initially perfect and symmetrical in its shape,” Yegor Razumovsky clarifies, a graduate student at the Department of Mechanics of Composite Materials and Structures at PNIPU.

What Shapes Does Snow Take?

According to the expert, in 1951, the International Association of Hydrological Sciences proposed a classification distinguishing seven main crystal forms:

Plates – flat and thin hexagonal snowflakes without arms. Star-shaped dendrites – the favorites among snowflakes, where six arms radiate from a central ice crystal. Due to their many branches, they trap air in the snow cover and create the fluffiest snow.

Spatial dendrites – these are voluminous snowflakes resembling flakes. They form when several crystals merge.

Columns – the most common form. This type tends to form in severe frosts, hence they do not stick together. They can be seen in loose snow. Crowned columns – these are columns with plates growing at their ends. They form when crystals enter a zone with a different temperature.

Needles – resemble thin threads. This structure allows flakes to cluster very tightly together, making such snow ideal for skiing or snowball fights.

Irregularly shaped crystals. A snowflake can encounter numerous adventures, potentially losing some of its branches or breaking entirely. Typically, many of these broken snowflakes are found in wet snow and during strong winds.

The Pattern Depends on the Weather

The expert states that the shape of a snowflake is influenced by atmospheric pressure, the intensity of electric and magnetic fields, wind, humidity, and temperature. These factors determine whether the upper or lower prism develops and alters or if only the lateral faces do.

“It is known that when the temperature is between minus 5 to 10 degrees Celsius, snowflakes form in the shape of needles or columns. At temperatures of 10 to 12 degrees, snowflakes take the form of plates or flakes. However, when the temperature remains between minus 12 and 18 degrees, the most beautiful branched hexagonal snowflakes – star-shaped dendrites – form, typically measuring five millimeters or more,” notes Yegor Razumovsky.

Snow Only Appears White

Each snowflake is not white itself but rather colorless and transparent like ice; however, our eyes perceive many such snowflakes as white. The essence lies in the fact that each light wave has its own length and frequency of oscillation. However, they do not possess color on their own. Color emerges when we perceive the reflection of waves with our eyes. The color of the surrounding objects is formed by the absorption and reflection of light waves. Some materials can absorb waves of certain lengths, while others reflect them. For instance, materials that absorb all waves appear black, while those that reflect all falling waves appear white.

“Snow lies in a chaotic arrangement and forms a loose mass. When sunlight hits the snowflakes, it refracts and scatters multiple times through the ice crystals, changing its direction. This leads to the even reflection of the entire visible spectrum of light, creating the perception of white,” reports the PNIPU expert.

Interestingly, snow can also reflect ultraviolet light, making it easy to tan against its backdrop and possible to get sunburned in the cold.

The Fall of Colored Precipitation

The composition of clouds and their geographical location influence the hue of the crystals formed. This is due to the fact that different types of dust particles and colors can serve as crystallization centers (nuclei). Generally, we still see such snow as white.

“When it comes to brown, gray, or black falling snow, its color is caused by cyclones that bring dust. For instance, particles of sand, rocks, chemicals from industries, and slate coal are swept up by strong air currents and lifted into the lower layers of the atmosphere, where constant air mixing occurs, moving in various directions. Some particles get trapped in these layers and then fall out with the precipitation,” explains Yegor Razumovsky, a graduate student at the Department of Mechanics of Composite Materials and Structures at PNIPU.

The Speed of a Snowflake's Descent

“Snowflakes fall to the ground at a speed that depends on several parameters: shape, size, and density. It is also important to consider turbulent air flows. On average, snow crystals fall at a constant speed of about 0.3 meters per second,” noted the expert from Perm Polytechnic.

So Similar, Yet So Different

Under a microscope, large and complex snow crystals can indeed show similarities. However, if you delve down to the molecular level, you'll find that they differ in the number of atoms and the ratio of isotopes – atoms with the same number of protons in the nucleus but different numbers of neutrons.

“A bit of snow theory of probability. One snowflake contains approximately 100 quintillion water molecules, and the number of combinations that can be formed from such a quantity approaches infinity, making the chance of two completely identical snowflake structures practically zero,” notes Yegor Razumovsky, a graduate student at the Department of Mechanics of Composite Materials and Structures at PNIPU.

The Largest Snowflake

The Guinness World Records documented the discovery of a massive snowflake in Fort Keogh, Montana, on January 28, 1887. Its width reached 38 centimeters, while its thickness was 20 centimeters. It is important to note that this was a spatial dendrite, meaning multiple crystals fused together during its descent.

The largest individual ice crystal was described by researcher Kenneth Libbrecht, who dedicated his life to studying and photographing snowflakes. He discovered the largest specimen on December 30, 2003, in the town of Cochrane, located in Ontario, Canada. The length of this star-shaped dendrite from one end to the other was 10 millimeters.

Why Does Snow Crunch Underfoot?

“The crunch of snow occurs when thousands of microcrystals break simultaneously. Below minus 18 degrees Celsius, snowflakes become harder and more brittle. They break under slight impact and friction. A gentle movement is enough for the crystal structure to start breaking down, making the crunch more pronounced and clear. At near-zero temperatures, snow contains a certain amount of water in liquid form. The energy transferred from the boot causes the crystals to melt, resulting in near-silent footsteps,” explains Yegor Razumovsky, a graduate student at the Department of Mechanics of Composite Materials and Structures at PNIPU.