Recipe for a pretty snowflake

17th February, 2018

Mid-Hudson Valley, New York

Snow is promised this evening, and we are watching for those first chubby flakes to break the dark. When they come, they’ll drift in soft clusters, they’ll stick to our woolly gloves, and the kids will look skywards to catch them on their tongues. So we imagine.

Stellar plate, by Alexey Kljatov / CC BY-NC 2.0

The reality may be different: wet globules sticking to our eyelashes, pellets hurling themselves into our down-turned faces, or something so fine that we won’t at first be certain whether it’s rain or snow.

What’s happening up in those clouds right now? What type of snow are they making?

All snowflakes, the beautiful and the plain cold ugly, start out when water vapor crystallizes around a speck of dust or pollen in a cloud. The water molecules arrange themselves into repeating hexagons, one water molecule at every corner of every hexagon. Different crystals take different structures, but snow crystals always form from hexagons because of the forces within, and between, the water molecules.

Stellar dendrite, by Alexey Kljatov/CC BY-NC 2.0

From this basic pattern, snow crystals can grow into a variety of shapes: columns, needles, flat plates, and of course the 6-branched stars known as stellar dendrites that we most readily identify as snowflakes. Shape is largely determined by the temperature and humidity. For stellar dendrites, the cloud temperature will be around -15℃ (5℉) and the humidity will be high. As the growing six-sided crystals move through the cloud, water vapor catches on the corners, and branches sprout rapidly. More water vapor catches readily on the branches, which grow longer.

Capped column snowflakes/
by Alexey Kljatov/CC BY-NC 2.0 

In lower humidity at the same temperature however, the snow crystals collide with water vapour less frequently, so the crystal grows more slowly and the structure retains a neat plate-like appearance. Meanwhile columns and needles - the white splinters you sometimes see on your arm - tend to form in moderate humidity and slightly warmer conditions, around -6℃ (21℉). If the temperature drops partway through the column’s growth, plates can grow on either end to form a capped column.

Fernlike stellar dendrite, by Alexey Kljatov/CC BY-NC 2.0

The largest snow crystals, up to 5mm wide, are fernlike stellar dendrites. Because each branch experiences the same conditions, the branches are similar, but not, in fact, symmetrical. Take a look at the image for a game of spot the difference.

A snow crystal’s story doesn’t end in the cloud. Collisions on the journey to the ground result in the clumps of crystals that we call snowflakes, for example. Some crystals will be transformed entirely. Even the most stunning will reach land as sleet if they pass through a layer of warm air that partially melts them, and then a layer of cold air that refreezes them. These frozen rain drops - sleet - are not snow crystals at all.

Nobody watches for sleet. Let it snow.

Explore further:

The Science of Snowflakes, by Maruša Bradač, Ted-Ed video

Snowflake, from Slava Ivanov on vimeo, microscopic timelapse by Vyacheslav Ivanov

SnowCrystals.com, by Kenneth G. Libbrecht, Caltech professor