The chemical art behind snowflakes
This image illustrates how water molecules interact with each other to form the hexagonal, 6-sided shape present in snowflakes.
As mentioned before, chemistry is in everything and snowflakes are no exception! Crystallography allows us to study the arrangement of atoms in a snowflake crystal. Although they all start the same, once this crystallization process begins, it is true that no two snowflakes are alike. A snowflake originates as a grain of dust floating in a cloud. Water vapor in the air sticks to the dust grain and the resulting droplet is converted directly into ice. Crystal faces form on this ice droplet that then transforms into a hexagonal prism with six faces, a top, and a bottom. A hexagon forms because water molecules chemically bond into a hexagonal network (see image on the right).
A cavity then forms in each face of the prism because ice grows fastest near the edges. This accelerated growth on the corners causes six branches to sprout. When the air temperature reaches 9 degrees Fahrenheit (-12.8 degrees Celsius), new growth at the branch tips narrows. At 6 degrees Fahrenheit (-14.4 degrees Celsius), side branches start to sprout on each branch. If the crystal encounters in its descent to the ground a quick blast of warmer air followed by cooler air, even more side branches sprout. As this ice crystal gradually warms to 7 degrees Fahrenheit (-13.9 degrees Celsius), the tips of the snowflake grow long and narrow. At 8 degrees Fahrenheit (-13.3 degrees Celsius), the growth of the crystals slows that results in widening of the tips. Ultimately, this unique and fragile structure falls to the ground along many other unique snowflakes. Once snowflakes land and accumulate, the changes in temperature on the surface causes them to merge into layers of snow.
The evolution of a single snowflake is highly sensitive to the temperature and humidity of the immediate environment. It takes about half an hour for a snowflake to reach its final size, and during this time frame it can travel a mile or more experiencing a wide array of conditions. The shape of the snowflake's arm parallels the exact history of its travels. Since all six arms of a hexagonal snow crystal travel together, they all experience the same history and appear to grow in a synchronized manner. However, no two snowflakes follow exactly the same path in the turbulent atmosphere, and this is one reason why no two snowflakes are exactly identical. Another reason why it is highly unlikely that two snowflakes are alike is due to the estimated ten quintillion (1019) water molecules that make up a typical snowflake. Different environmental conditions can cause some snowflakes to melt, whereas others will continue to grow and change structure. One common misconception is that all snowflakes are perfectly symmetrical, but the reality is that symmetrical crystals are actually pretty rare (a frequency less than 0.1%). We typically see symmetrical snowflakes in the media because those are the ones that get the most attention, but it is not representative of the reality. Uneven temperatures and the presence of dirt in the atmosphere are some reasons why not all sides of a snowflake are the same.
Over the years, crystallographers have been classifying snow crystals into various categories based on their arrangement of atoms, more specifically the positioning of water molecules. By the 1930s, there were 21 classifications that a snowflake could be in. However, by 2013 this number rose to 121 different categories. Snowflakes are studied at the molecular level by sending X-rays through them. These rays then bounce off all the atoms in a snowflake and head in different directions. By seeing where these beams end up, the arrangement of atoms in a snowflake can be determined along with how it looks like at the atomic level. It was this same technique that allowed Rosalind Franklin to visualize the double helix arrangement of DNA.
Colder temperatures generally produce snowflakes that have sharper tips on the sides of the crystals and greater branching of the snowflake arms. Under warmer conditions, snowflake growth is markedly slower and result in smoother, less sophisticated shapes. The largest, most visually appealing snowflakes only form when the cloud temperature is around 6 degrees Fahrenheit (-14.4 degrees Celsius) in the clouds. Additionally, highly branched crystals are found when the humidity is high.
