The day after Christmas, the path into the woods was nearly bare. With morning temperatures at 28 degrees, it wasn’t unusual to see patches of white stuff on the ground. And, being Rhode Island, it wasn’t unusual for that to be the only snow on the ground.
But there was something different about this. The snow wasn’t a delicate powder, nor was it composed of ice crystals like hail. Instead, it was as if someone tore open a bag of tiny white beads and tossed them to the ground. Each bead was perhaps half the diameter of a BB, and the air temps insured each bead would remain pristine for the time being. This type of frozen precipitation is called graupel (the “au” in the word sounds like “ow”). But where did this strange phenomenon come from?
Normally, snow forms when moisture in the atmosphere encounters sub-freezing temperatures. When that happens, crystals begin to form, and when they grow to a certain size, their weight carries them to the earth as snow. But sometimes the process isn’t that simple. The earth’s atmosphere is composed of layers, each with differing temperatures and moisture contents. When several layers appear over a given area, the precipitation may change to different forms. So it is in the case of the mysterious frozen beads. The moisture begins to freeze, and snow begins to form. However, as the snow begins to fall, it passes through a layer of supercool air. This layer contains droplets of water as cold as -40˚.
But wait: if water freezes at 32˚, how can it exist at -40? According to the physics website Phys Org, Daniel Gabriel Fahrenheit, the Dutch-German-Polish physicist for whom our measurement of temperatures is named, had discovered the existence of supercool air and droplets that resisted freezing way back in 1724. However, the debate over why it was possible for those droplets to remain liquid at extreme temperatures continued to rage right up until 2010 when a French science agency came up with a possible answer.
While doing research in nanotechnology (manipulating molecules for a variety of uses), scientists discovered that the molecular structure of certain liquids were five sided. Because of their shape, these molecules can’t be put together to make a new shape without leaving empty space. For example, if you were to lay out a group of pentagons in a given area on a table, there would be no way to put them together in one new shape without leaving spaces between them. Only when the structures are disturbed through the introduction of square or triangular pieces could a new shape be formed. Therefore, these droplets remain liquid in this extreme cold because they are essentially missing the piece of the puzzle that will allow them to freeze together. (There is a fascinating video of this transformation here: http://phys.org/news191068504.html )
That snowflake is essentially the missing puzzle piece. As snowflakes form in the upper atmosphere, they sometimes drift through this layer of droplets. The flake fills in the empty spaces between the molecular pentagons, and they begin to attach. When just a few take hold, the flake retains its general shape, and it is classified as “rimed”. However, when enough of the liquid adheres to the flake, it takes on this beadlike shape and becomes graupel, a fluffy lump of snow.
By afternoon, the same path into the woods showed no trace of this novel phenomenon. But the winter is young, and surely graupel will make another appearance. And this time, it will arrive with a story of its seemingly impossible journey through atmospheric layers to the humble path of a backyard.
Hugh Markey is a freelance writer, naturalist, and teacher who lives in Richmond.