More Weird Ice Weather

It seems like we should understand water pretty well. All kinds of scientists have studied it. It covers most of the surface of the Earth.  All life depends on it. And yet there is much we don't understand about how water behaves. Water is weird.

We have continued to have freezing nights here in Seattle, and so have been able to observe some of water's weirdness in its solid state. Last week I wrote about the weirdness that is hair ice. Since then, we've had a chance to notice some other strange ice effects.

Hiking on Cougar Mountain the other day, we saw hair ice again, but not nearly as much as at Squak Mountain last week. We also noticed our footsteps crunching on the trail. Sometimes the ground would even sink a centimeter or two beneath our feet. Some of the rocks in the trail also appeared to have burrowed down into the ground. 

This effect was actually due to the fact that water in the soil had frozen and lifted up the level of the soil's surface, leaving the rocks behind and below. The ice formed in this process is called needle ice. It grows as slender, vertical pillars, which form as capillary action draws water up through the soil. Scientists think that as the ice freezes, nearby supercooled water is drawn toward it, creating more vertical needles, all tightly packed together. As the needles grow vertically, they push the surface of the soil up. 

This process of forming the separate needles is called ice segregation, and it is the same process that is thought to form the packed-together hairs in hair ice. The needle ice forms in layers that represent freeze-thaw cycles. In our case, each cycle is probably a day, since our afternoons have been above freezing. So the needles form overnight and stop growing in the warmer part of the day. Then a new layer of needles form underneath as the temperature drops again at night.

The picture above shows four freeze-thaw cycles. I love the crystal, layer cake look.

As I was reading about needle ice, I came across references to pebble ice. It's similar, but forms on top  of rocks. The rocks have to be porous enough that water can move through the rock itself by capillary action (which often means sedimentary rocks). The vertical needles form on top of the rocks as the water escapes from the rock and freezes. Pebble ice, I read, can also form on porous old bricks. Suddenly I knew why I had been seeing this in the back garden for years.

Some, but not all, of the bricks in the patio are porous enough to form pebble ice! The more you know, the more you notice! Just like with the needle ice under the soil, the pebble ice on the bricks builds up in layers over each freeze-thaw cycle.

In my research, I also read about frost flowers. And then the next day I found them! 

Once again the more you know, the more you notice. Frost flowers are similar to hair ice, but grow from non-woody stems that still have active roots. I had just cut back some of my perennial flowers, so the stems were still drawing moisture up from the roots. In plants that have enough vessels radiating out through the stems, long filaments of ice can be produced as the water leaves the plant. These particular stems are from chocolate Joe Pye weed. Not all plants produce frost flowers, maybe due to the arrangement and/or size of the vessels radiating out from the center of the stems (called xylem rays). 

Unlike hair ice, no fungus is known to be involved. However, like all the the weird ice formations mentioned so far, the process of ice segregation forms the long filaments. My frost flowers are pretty, but modest. Here is a gallery of more impressive frost flowers. I think one or two of these might actually be hair ice; they are very similar looking effects.

Our temperatures at night are supposed to get even lower in the coming days. What weird ice will we notice next?

A very helpful article: "Flowers and Ribbons of Ice," James Richard Carter, American Scientist