Special snowflake question about actual special snowflakes

January 26, 2015 3:34 PM

Look at these weird snowflakes! How do they work? Photo one and two.

Around 2pm at 28 degrees F I was walking around NYC during the first hours of the snowpocalypse. The snow was crunchy and dry and actually looked like fake confetti snowflakes being tossed from the clouds. I could not believe it actually looked like snowflake-shaped snowflakes to the naked eye! I'm not that familiar with snow as I've only lived up north four years now. Is this normal? What could cause the crystals to keep their form so well? They took several minutes to melt and the ground was covered with them. From the cursory research I've done the prevailing opinion is that you can only see the crystalline form at this size under a microscope. Can someone who understands the science of snow explain this to me? Is it actually common?

posted by Juicy Avenger to Science & Nature (7 answers total)

Those flakes are known as rimed crystals. They are just normal snowflakes onto which the tiny water droplets that make up clouds have frozen. Rime ice being the type of ice that forms when water drops freeze onto cold surfaces (look for photos of Mt. Washington, or the spray off Niagara Falls for larger-scale examples).
posted by plastic_animals at 3:40 PM on January 26, 2015 [4 favorites]

For what it's worth, I not-uncommonly saw snowflakes like that when I lived in western Mass.
posted by MadamM at 3:58 PM on January 26, 2015

I live upstate where it snows every day for weeks at a time, and I see flakes like that sometimes but not often. I think of it as a beginning-of-the-storm thing, maybe because the whirling flakes have has less of a chance to knock the corners off of each other or agglomerate into giant flakes.
posted by tchemgrrl at 4:26 PM on January 26, 2015

Now that I'm off my phone, this link shows the various types of snow crystal forms. At the same time you saw these flakes you may have also seen their close cousin - graupel. Graupel forms in the same way, supercooled water droplets touching a snow crystal, but so much riming takes place that the crystal turns into a soft little pellet of snow.
posted by plastic_animals at 4:29 PM on January 26, 2015 [2 favorites]

I did considerable snowflake observing during the years I lived in Colorado Springs, Denver, and Boulder, and never saw classic six-pointed stars even approaching that size -- how exciting!

My guess is those grew from water molecules which were deposited essentially individually and directly from water vapor in the air rather than the super-cooled suspension of droplets of liquid water which produce rime ice, because as soon as the snowflake grew well beyond the size of the average droplet of fog, I don't see how the water in a randomly encountered droplet could have coated all parts of the crystal evenly before it froze, and I think that process would have resulted in the amorphous graupel plastic_animals mentions instead of such beautiful symmetries.
posted by jamjam at 6:55 PM on January 26, 2015

I am an ice scientist! I know way too much about the science of snow. To answer what I think is your main question, those are particularly large and well-formed specimens, but it is not at all unusual to see snowflake-shaped snowflakes that are easily visible to the naked eye.

plastic_animals is correct that those are rimed snowflakes. Here's a blog post I wrote about them. (as you can see, those snowflakes are also pretty big.) The six-sided crystalline form comes from water vapour--since those are so large, I think they would have formed in an environment that was highly supersaturated with moisture. They look like branched "dendritic" crystals. The specifics of a snowflake's shape come from the changing microenvironments it encounters as it moves through a cloud, which is why all six points of a snowflake grow more or less the same (they form in the same environment, being attached to each other) but every individual snowflake is slightly different (since they all follow different paths.) These snowflakes formed in those nice classical snowflake shapes, then fell through a region of super-cooled water droplets that froze instantly as they contacted the flake and resulted in the "fuzzy" appearance of these flakes. (Did you know pure water droplets that don't have anything to freeze onto can get as cold as -40 F before freezing?)

Thanks for the snowflake photos, I hope you find more! Check out the link that plastic_animals posted and visit some of the other things on the sidebar there, it's a great snow science page that also has lots of beautiful photos (although for some reason it's not working for me at the moment.)
posted by fermion at 8:32 PM on January 26, 2015 [16 favorites]

Response by poster: Amazing! Of course askmefi provides an actual ice scientist!! Thank you so much fermion for sharing your blog post and everyone for weighing in. I loved the Cal Tech page and am now digging into this American Science article. The physics of snowflakes are fascinating. I wish I had had my macro lense and DSLR on me today but alas I was commuting. I look forward to more snowflake hunting now that I know these beauties exist.
posted by Juicy Avenger at 10:16 PM on January 26, 2015 [2 favorites]