Big storm, tiny magic: The science of snowflakes

It snowed in Minnesota today. Beautiful "stellar dendrites" floated gently earthward overnight, piling up and glinting in the pre-dawn streetlight reflection.

It turns out temperatures in the atmosphere were just right last night to produce some high quality snow crystals. With temperatures around 5 degrees across Minnesota last night, conditions were ideal for stellar dendrites and hexagonal plates to crystallize out of thin air and drift gracefully earthward.

Tiny magic in a huge storm 

The big-picture weather story nationally this week? The East Coast braces for an onslaught of heavy wet snow, wind and storm surge. But if we look inside at the smallest elements of the storm, we find the individual snow crystals have a beauty all their own.

What is a snowflake?

What we think of as snowflakes are really tiny snow crystals. I've interviewed North Dakota native and snow crystal expert Dr. Kenneth Libbrecht of Cal Tech about the science of snowflakes. Here's a good description of what makes a snowflake from his amazing site snowcrystals.com.

When people say snowflake, they often mean snow crystal. The latter is a single crystal of ice, within which the water molecules are all lined up in a precise hexagonal array. Snow crystals display that characteristic six-fold symmetry we are all familiar with. The picture on the left shows a snow crystal.

A snowflake, on the other hand, is a more general term. It can mean an individual snow crystal, but it can also mean just about anything that falls from the winter clouds. Often hundreds or even thousands of snow crystals collide and stick together in mid-air as they fall, forming flimsy puff-balls we call snowflakes.

Calling a snow crystal a snowflake is fine, like calling a tulip a flower.

The morphology of snow crystals

I believe the snowflake is as close to god as we can come on earth. The process of growing snow crystals in the atmosphere is simply magical.

Snow crystals grow from water vapor directly to ice in the atmosphere, skipping the liquid water phase. That's pretty incredible if you think about it.

It turns out temperature and humidity in the zone where snowflakes form are critical to determining the type of snow crystal that develops. If the so called "dendritic growth zone" is below zero, plates and columns form. At 5 degrees above zero, large beautiful stellar dendrites and hexagonal plates.

At temperatures around 20 degrees, a magical variety of prisms, columns and needles flutter earthward. Dendrites and plates are again favored as temperatures approach the freezing mark.

The Snow Crystal Morphology Diagram

The way snow crystals grow depends strongly on the temperature and humidity in the clouds. This is summarized in the Snow Crystal Morphology Diagram shown on the right. This is also called the Nakaya Diagram, after Japanese physicist Ukichiro Nakaya, who discovered this behavior by growing snow crystals in his lab in the 1930s.

Go ahead, click on the diagram for a closer look. It shows that the largest, most photogenic stellar snow crystals only grow in a narrow temperature range around -15 C (5 F). Needles and columns are best found around -6 C (21 F). Capped columns appear when the temperature changes as the crystals grow. Remember these are temperatures in the clouds; it is often substantially warmer on the ground.

You can also see that more elaborate, branched crystals grow when the humidity is high. Simple prisms grow when the humidity is low (or when the crystals are tiny).

Exactly why snow crystals grow this way remains an unsolved scientific puzzle. The growth behavior of ice depends on the molecular structure and dynamics at the crystal surface, and this is all so complicated that no one really understands it.

Complexity and variety

It is said that no two snowflakes are exactly alike. That is probably true in nature, though certain types of crystal formation are identical in a laboratory setting. Here's more on the variety of complex and symmetrical shapes of snowflakes in nature from snowcrystals.com.

A stellar snow crystal begins with the formation of a small hexagonal plate, and branches sprout from the six corners when the crystal grows larger. As it tumbles through the clouds, the crystal experiences ever changing temperatures and humidities, and each change makes the arms grow a bit differently.

The exact shape of the final snow crystal is determined by the precise path it took through the clouds. But the six arms all took the same path, and so each experienced the same changes at the same times.

Thus the six arms grow in synchrony, yielding a complex, yet symmetrical shape. And since no two snow crystals follow the exact same path through the clouds as they fall, no two look exactly alike.

Large scale weather chaos is about to unfold with the east coast mega-storm this weekend. Remember to look deep inside the storm at the tiny magic of the individual snow crystal.