Posts Tagged ‘seattle’

Ice Needles (not to be confused with Space Needles)

December 2, 2014

Yesterday on my way to┬áthe office I noticed that the ground looked weird. Inspecting it more closely I discovered that the apparent “surface” was actually composed of lots of dirt clods and small rocks on top of two-inch-tall pillars of ice.

All of these rocks and bits of dirt are on 2-inch-tall ice columns

All of these rocks and bits of dirt are on 2-inch-tall ice columns

I had never seen anything quite like it before, at least not in person. The physics involved is actually quite complicated and I don’t fully understand it myself, but it boils down to something like this: if ice is freezing at some particular spot, it “pulls” water toward itself. You mostly see this effect in porous materials that water can flow through by capillary action, like soil.

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So the water gets pulled toward the ice and freezes on to it, pulling yet more water along behind it, and gradually more and more ice forms. In this case you end up with columns that lift pebbles off the ground, a.k.a. needle ice. In other cases, where the freezing is happening underground, you can get frost heaves or weird permafrost landforms (this is called ice segregation.

You can see the ground through the hole.

You can see the ground through the hole.

An ice column I plucked out to examine.

An ice column I plucked out to examine.

Under the bushes the columns got surreally twisted.

Under the bushes the columns got surreally twisted.

Side view of the ice columns lifting some pebbles and a small ice sheet

Side view of the ice columns lifting some pebbles and a small ice sheet

You can see there was a change in the freezing process; the short, whiter columns seem to have been lifted by the taller, darker columns underneath.

You can see there was a change in the freezing process; the short, whiter columns seem to have been lifted by the taller, darker columns underneath. The bottoms of the small columns have frost on them.

Side view.

Side view.

More images here.

Graupeling with Seattle Snow, or, Rime and Reason

January 17, 2012

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.