Graupeling with Seattle Snow, or, Rime and Reason

Seattle is enjoying one of its all-too-rare episodes of snow—well, I’m enjoying it, at least. Upon leaving the house this morning to walk to work, I found the sidewalks lightly dusted with an interesting type of snow we in the biz call graupel.

Most snowflakes, the six-sided ones you probably think of when you think of “snowflake”, grow by the condensation of water vapor. The individual water molecules attach to the snowflake in an orderly fashion, like building a structure out of Legos, and you end up with a regular crystalline shape.

Sometimes, though, the snowflake will pass through a cloud of water droplets as it falls down to the ground. As it hits the snowflake, the whole water droplet will freeze almost instantly, retaining its rounded shape. The snowflake bounces around in the cloud of water droplets, accumulating more and more, and your orderly Lego structure starts to look like someone has been pelting it with spitballs. These frozen water droplets are called rime. When so much rime has accumulated that the underlying shape is no longer visible, the snowflake has become a pellet of graupel.

In the picture below (sorry for the questionable quality, it’s a cell phone camera) you can see quite a few snowflakes; I’ve circled one on the left that retains the crystalline snowflake shape, and one on the right that’s still clearly six-sided, but so covered in rime it looks like it’s wearing a fur coat. (My advisor called this a “textbook” rimed snowflake.) Elsewhere in the picture you can see a few pellets of shapeless graupel.

Rimed and unrimed snowflakes

Circled on the left: a classic snowflake. Circled on the right: a snowflake covered with rime (frozen water droplets.)

Incidentally, this also goes to show that you don’t need a microscope to appreciate snowflakes; your eyes will do just fine. I’ve found that a good method is to go out when the snow is falling and catch snowflakes on a fuzzy hat, either faux fur or knitted with especially fuzzy yarn; the snowflakes will be caught on the fibers, where they can be more easily examined without melting.

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4 Responses to “Graupeling with Seattle Snow, or, Rime and Reason”

  1. Aspen Says:

    I noticed that yesterday morning, too. I’m not in the biz, but my mother taught me to call it graupel, too- or corn snow. I’m trying to think if there was any difference between the two and not coming up with any.

  2. psychroteuthis Says:

    I think corn snow is also sometimes used to mean snow that’s melted and refrozen somewhat, making the individual snow grains bigger. One of the reasons I find snow so fascinating is that each single grain has such a varied history–formation, riming, melting, refreezing, growing by vapor transfer, and so on and so on.

  3. Chicken Lady at Locust Lane Says:

    So how many words are there for snow?

  4. psychroteuthis Says:

    Depends on if you mean nouns or adjectives, I guess. There are at least a dozen different kinds of snowflakes–classic six-armed ones, prism-shaped ones, capped columns, structures made of several flakes stuck together, you name it. And then once it’s on the ground you have yet another vocabulary for it. There’s a lot, is what I’m saying. I should try and list them all sometime.

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