Snow from the Sky

Six-sided snow crystal with lines indicating hexagonal symmetry — Although they vary in shape, all snow crystals have hexagonal (six-sided) symmetry. Image adapted from Alexey Klijatov on Wikimedia Commons (CC-BY-SA-4.0)

Snow consists of tiny ice crystals that form in clouds. The terms ice crystal and snow crystal mean the same thing, and you might see both terms used to describe snow particles. Snowflake, on the other hand, is a more general term for falling snow particles. A snowflake can be one individual crystal or many crystals clumped together. Snow crystals form when water vapor (the gas phase of H₂0) freezes directly into ice without melting into water first. In order for snow crystals to form, certain conditions must exist.

The birth and growth of a snowflake

It is hard to get a cloud to snow. Even at low (-20°C/-4°F) temperatures, water vapor doesn’t freeze easily, and when it does, the minute ice crystals are too light to fall out of the cloud due to updrafts. Fortunately, there are also particles of dust, salt, clay, and even microbes in the clouds, and these particles, which are called cloud condensation nuclei (CCN), help in the freezing process. Silver iodide also works as a CCN, which is why it is used in cloud seeding.

A few examples of Cloud Condensation Nuclei

Image showing flat flake-like structures clumped together

Scanning Electron Microscope (SEM) image of clay (kaolinite). U.S. Geological Survey Image (Public Domain)

Scanning Electron Microscope (SEM) image of volcanic ash. U.S. Geological Survey Image / Kristi Wallace (Public Domain)

Scanning Electron Microscope (SEM) image of volcanic ash. U.S. Geological Survey Image / Kristi Wallace (Public Domain)

spoon and test tube containing gray powdery substance. Test tube labeled "AgI"

Silver iodide (AgI) in its solid, powder form. Photo by Ondřej Mangl on Wikipedia.

Diagram showing hexagonal structure created by Silver iodide molecules in solid phase

The molecular lattice structure of Silver iodide (AgI) is similar to that of water in its solid phase (ice), which makes it good at facilitating snow crystal formation . Illustration by Benjah-bmm27 on Wikimedia Commons (Public Domain)

Several processes in the cloud help embryonic snowflakes grow large enough to fall to Earth. The Bergeron effect allows ice crystals grow by attracting nearby tiny water droplets. Riming and aggregation help too.

Riming

One way that snow crystals grow is through the process of riming, in which liquid water drops freeze to them directly.

Aggregation

Snow crystals can also grow by the process of aggregation, in which they crash into each other and stick together.

A rimed snow crystal has a bumpy appearance. Photo by Matthew Sturm

When multiple snow crystals bump into each other and stick together, we call the resulting cluster a snowflake. Image adapted from W.A. Bentley (Public Domain) by Matthew Sturm.

Photograph of a snowflake formed by aggregation of snow crystals. Photo by Matthew Sturm

The Bergeron Effect

The Bergeron Effect describes a process through which snow crystals grow as water vapor is deposited on them. The amount that a snow crystal grows through this process depends on the proportions of liquid water and ice in the cloud. Animation by Matthew Sturm

Snow Crystal Shapes

The shape of the classic “snowflake” is a stellar dendrite, a six-sided star, but scientists now recognize 121 different forms of snow crystals (Kikuchi et al., 2013). By simply varying the whether a snow crystal grows faster on its basal or prism faces, and varying back and forth many times, a vast array of stunning snow crystal forms, all on a hexagonal motif, can be observed in nature. The form of a snow crystal is related to the temperature and the humidity (supersaturation) of the air that it falls through while growing.

“Snow crystals are letters sent from heaven.”
– Ukichiro Nakaya, 1936

Snowflakes are clues to what is happening in the clouds. The snow crystal morphology diagram created by Dr. Kenneth Libbrecht based on the work Ukichiro Nakaya describes which types of snow crystals form under different combinations of humidity and temperature conditions in the clouds. The next time it snows, try catching some crystals and looking at them with a magnifying glass to see if you can figure out what is going on up there!

Some pioneering snow crystal scientists

The earliest known study of snowflakes date back 800 years, and scientists have continued to be fascinated with these beautiful forms since. Three scientists, in particular, have advanced our understanding of snowflake physics: Wilson Bentley, Ukichiro Nakaya and Ken Libbrecht, and their websites are a wonderful resource on the topic.