Take about a cup of water, place in a glass flask, then plunge in liquid nitrogen. Watch.
What happens in that video is that the water starts to freeze from the bottom and sides of the flask, and the ice also spreads across the top surface. When the water turns to ice, it expands by about 10% and so the remaining water must be pushed aside. If a hole remains in the top ice layer, the water gets pushed out the top.
What happens next depends a lot on various, poorly studied features of the situation. But sometimes, the out-flowing water forms an ice spike like that shown above. Sometimes it doesn't. In a previous post (02/06/2011), I describe a vase-like shape that formed in an outdoor bathtub:
The trick of putting the flask in liquid nitrogen (about -200 C) came from Larry Wilen back in about 1989 when we were at the University of Washington. I later did the trick a few times at the University of Arizona, but I thought I'd try to perform it during a talk I gave in January, 2013. So, I went to see the chemistry-department demonstration guy, Eric Camp, at the UW, and we tried to replicate the demo. Now, in the past, I could get the spike pretty much every freeze, but Eric and I had the hardest time with it. So, in the end, I got the video, but didn't attempt the stunt at my talk.
This recent front-page article caught my eye:
The writer is an avid skier-snowboarder, and thus concerned about the future of his sport. The facts he relates paints a grim picture:
- In the past 47 years, a million square miles of spring snowcover has disappeared from the Northern Hemisphere.
- Since 1970, the winter warming-rate has been triple the rate of the previous 75 years.
- The Alps are warming 2-3 times faster than the world-wide average.
- Potential Winter Olympic venues shrinks from 19 (now) to 6 by 2100.
and many other facts. Avid skiers like spring skiing. For March, the data (from Rutgers University Global Snow Lab) show a decline in area covered by snow.
In early January, while visiting a cold, dry region, I saw this frost on a wooden fencepost.
The pattern resembled a cluster of butterflies. In the shade, these "butterflies" were blue, reflecting the blue sky. In the sun, they were bright white:
These are a type of hoar-frost, and though hoar-frost grows by the same processes as snow crystals, they can take on an even greater variety of forms. That they have more forms is a consequence of the fact that they can experience much greater levels of humidity, that is humidity relative to their temperature. This greater degree of humidity produces faster growth. Frost forms can also be more unusual because the proximity of the crystals alter the vapor gradients.
When water droplets land on ice, what happens?
If the ice is at least several degrees below zero (celcius), the drops freeze quickly, and build up a whitish, bumpy surface.
When the ice surface is heated to melting, the droplets vanish into the melt.
When the ice is instead very close to zero, the droplet spreads out, but not completely. You can still see the boundary of the droplet.
In the early 90s, there was some scientific debate about the ice surface near zero. Some said it had a thin, liquid-water surface, a layer that allowed us to ice-skate and make snowballs. Others said that if that were the case, then a droplet placed on the ice would spread out and vanish. Experiments showed the droplets didn't vanish. The jury is still out on the nature of the ice surface.
Bending bending bending
The fir branches are bending
They are waiting for more snow
The famous Japanese poet Basho wrote a poem like the above, except it was about bamboo bending under the snow load.
I was reminded of Basho's poem yesterday, as we just had our first snowfall of the season. Here in Redmond, about 2 inches, just enough to bend some of the fir branches.
And not only the branches were bending.
I took the opportunity to check out the pond. It had an ice covering before the snow, but with the snow and warming trend, the ice had thinned, and gotten pressed down under the snow load. The glaze over the pond was bending. Where a hole appeared in the ice, water got pushed up and out, flowing in dark channels over the icy glaze, forming a spider-like or branch-like pattern in the now slushy snow. I climbed up a tree on the shore and took a few shots.
It had been a continued spell of cold days and nights, the ground snow-free, the air clear and dry. No film-frost on the cars in the morning, and no spikes of hoar frost sticking up on grass or post. The only signs of ice had been the frozen pond and the needle ice, making the ground crunchy underfoot.
And yet in the woods and lawn, hoar frost still lurked.
When I last wrote about the classification of falling snow and ice (02/14/2010), I discussed the 1966 Magono-Lee system. At the time, this system was the most recent one, and as such, the one generally used in meteorology. And to those who wondered how many types of icy precipitation exist, Magono and Lee would tell you 80 types.
Although 80 sounds like a lot, Magono and Lee did not include at least one very common type as well as a few other interesting types. This major omission was snow-crystal aggregates, generally known as “snowflakes” by meteorologists. These are individual snow crystals that have stuck together during their fall and arrive as large open clusters of crystals, often with tens–to–hundreds of crystals in a big, round blob. (To meteorologists, a snow crystal is not a snowflake; rather, a snowflake consists of many snow crystals. It is like the difference between a trundled rock and a landslide.) In fact, the snowflake is probably the most common type of snow precipitation in most areas that receive snow. Several other omissions of the Magono-Lee that I mentioned in my 2010 posting included spearheads, seagulls, and the both the 18- and 24-branched forms.
If you were waiting for someone to fix the classification, well wait no more: we now have a new classification scheme that includes all of these and more. The new system has not one, but three types of snow-crystal aggregate (snowflakes), which are given the symbol “A” (for aggregate). There is also four types of spearhead crystal (symbol “CP8”), five types of seagull crystal (symbol “CP9”), and both the 18- and 24-branched crystals (P5e and P5f). In all, the new system has added 41 new types. In one big graphic, here is the new system:
The architects of this new classification scheme published the above table in summer 2013*. There were four authors (K. Kikuchi, T. Kameda, K. Higuchi, and A. Yamashita), making it harder to name the system after the originators, but hopefully their names will become known, just as Magono and Lee’s have.
I will discuss various aspects of this new system in subsequent postings. But for now, I point out that the number of basic types has increased over the years, from 10, to 40, to 80, and now 121. When will it ever end?
Of course, it may just keep increasing. In fact, if we plot the numbers vs the years, and look at the pattern, we might conclude “never”.
My sister recently sent this photo of a frozen puddle, a little over a foot across. Something broke out a piece of ice in the upper right, but it’s mainly a complete glaze over the top. (The white dots are rimed snow crystals. Click to zoom in and see.)
Notice how it has drained – the light color is due to the air underneath. The lines are roughly concentric with the shape of the puddle, though there can be considerable variation. On larger puddles or ponds, you might just see these thick curvy lines just near the shoreline.
While riding my bike home the other day, I saw what appeared to be a patch of light snow.
It was the only such patch around. Looking closer, I could see that it consisted not of snow, but of chunks of partly melted rime deposits. (Note how the pieces are long and narrow, like little icicles.)
Just above were the branches of a huge Douglas Fir tree. Perhaps there had been an over-active squirrel or two up there. Other trees still had their rime. Hard to see why only this tree would have its rime fallen off.
The Pacific Northwest has been foggy a lot lately, but the fog droplets have been subzero, or supercooled. When such fog droplets hit an object, they almost always freeze. The resulting frozen aggregate is called rime. Freezing fogs make rime.
The resulting rime formations may look like hoar frost, or even snow, from a distance. But close up the special features of rime become apparent.