December 26 2013, shortly after noon.
Tag Archive: winter
Merry Christmas (or happy whatever solstice holiday you celebrate.)
In honor of the season I’m posting one of my Geophysical Christmas Carols, and to help the words go with the music, a recording of my singing it. I’m not much of a singer, but this might help you hear how it goes together. (I’m still trying to figure out how to do this sound thing, so you may have to go through several clicks.) Meanwhile, to see the words while listening to the song, open the blog in two windows, click the audio link in one and then switch back to the other. (And you’ll catch me making a couple of mistakes that way!)
In the air, vapor’s swirling,
On the pond, folks are curling,
The vapor makes drops, the drops freeze and pop,
And six-sided snowflakes fall down.
On the lake, skates are gliding,
Overhead, clouds are hiding,
Ice in the sky is growing, oh, my,
And six-sided snowflakes fall down
Snowflakes could be square or five pointed,
Or octagons, or spherical, you know,
But water with water is jointed
So that only six arms can grow.
On the slopes, skiers swish on,
Snowflakes hide stars to wish on,
They fall through the air, and catch in your hair,
The six-sided snowflakes fall down.
(The snowflakes were Photoshopped from those included on the CD-ROM with Bentley’s book.)
Here are the sixth through the tenth placings in the 2012 World Ice Art Championships, along with a few shots of the general layout of the competition site.
Sixth place went to “Geoflames,” sculpted by a USA team.
Seventh place was “Playin’ in the Garden,” again by a team from the USA. This one had a “viewing window” (actually three atop each other) to indicate the best place to see the sculpture. This photo was taken through that window, also carved from ice.
Eighth place went to “The Super Raven Guide.” The two carvers were from Russia, and it took me a while to see the raven’s head. I was trying to make it an aurora, and it may have been intended to evoke one. Most of the sculptures were done by teams of four. This and the fourth place “The Land Calls” a couple of days ago were carved by 2-person teams.
Ninth place and Artist Choice was “Spring.” I’ve used this one before, but it’s worth using again. The artists on this one were from China.
Tenth Place and Governor’s Award was “Olde #7 On The Bridge To Nowhere.” The artists were from the USA.
The ice as cut from the pond has a layer of cloudy ice near the surface, where snow and ice intermingle. Although most sculptors cut this off, it can be used with interesting effect, as on this pedestal.
Finally a wider view of part of the competition area. Snow is being blown off “Mother” in this picture. I’ll have at least one more blog on the kids’ area of the ice park by daylight, and I might get to see it at night, lit up, next week. If I do, I’ll get some pictures. The sculptures are incredible bathed in colored lights in the dark. If you’d like to see a particular piece under the colored lights, leave a comment. No promises, but I’ll try.
Why do plumes flatten?
Sometime the plumes from smokestacks just keep gong up until they disappear, but at other times they will stop and start spreading out once they have reached a specific height. Why?
One common explanation is that the plumes have reached a capping inversion and can’t go any higher. There is some truth in this, but it is a major oversimplification. Inversions aren’t something you reach, for one thing. Here in Fairbanks, and throughout the country at night, inversions often start at the ground. Further, they may extend for a considerable distance – up to a mile or more – vertically.
So just what is an inversion?
Under normal circumstances the air gets colder with height. One reason is that air cools as it rises. This is due to conservation of energy: the air is essentially swapping heat for potential energy. For dry air, the cooling rate is about 1°C per hundred meters, or 5.4°F per thousand feet. This is called the adiabatic lapse rate. When water is condensing, such as in a cloud, the air doesn’t cool quite as fast as it rises, because the condensing water adds heat to the air.
In an inversion layer, the air gets warmer with height. The rise can be only a degree or two occurring over a height of a few tens of feet, or it can be up to 20°C or more extending for a kilometer or two. But it is a layer of finite thickness, not just a flat plane at a specific height.
Here in Fairbanks, power plant plumes are generally visible because they contain water, which at low temperatures condenses. As a result it is very obvious when they reach a particular height and suddenly flatten out. But why?
They rise to start with because they are warmer than the surrounding air. They stop rising at the height where their temperature is the same as that of the air around them.
Part of this is because they cool as they rise. But if this were all that were involved, they would rise quite a long way. Suppose the plume temperature when it left the stack were 100°C – the boiling point of water. If it were simply cooling off at the adiabatic rate, it would take 10 kilometers just to reach the freezing point – and I guarantee that the air at 10 kilometers over Fairbanks in the winter never gets that warm.
So why do they flatten out and stop rising?
Because all the time they are rising, they are mixing with the surrounding air. This is actually visible in the photograph – the plume gets wider with height, due to the mixing in of cold environmental air. This works even for very small plumes. I have seen diesel trucks with stacks trailing a plume that flattens only a foot or two above the top of the stack, or stationary sources whose plumes flatten several hundred meters up. It’s a question of how fast the environmental air mixes with the plume as well as the initial plume temperature.
So if you see a plume flatten out, you can be pretty sure it is in an inversion, but not that there is a particular inversion at that particular height.
In the air, vapor’s swirling,
On the pond, folks are curling,
The vapor makes drops, the drops freeze and pop,
And six-sided snowflakes fall down.
On the lake, skates are gliding,
Overhead, clouds are hiding,
Ice in the sky is growing, oh, my,
And six-sided snowflakes fall down
Snowflakes could be square or five pointed,
Or octagons, or spherical, you know,
But water with water is jointed
So that only six arms can grow.
On the slopes, skiers swish on,
Snowflakes hide stars to wish on,
They fall through the air, and catch in your hair,
The six-sided snowflakes fall down.
The rhyme above can be sung, to the tune of “Winter Wonderland.” But it’s also a fairly good outline of why snowflakes look the way they do.
A water molecule is made up of one oxygen atom and two hydrogen atoms. The hydrogen atoms are not in a straight line with the oxygen atom, but are angled, like a bent line with the oxygen at the bend.
Ice, being crystallized water, is made up of water molecules in three-dimensional order. The water molecules in an ice crystal are held together by what are called hydrogen bonds — each hydrogen atom links not only with the oxygen in the water molecule, but with the hydrogen atom of a neighboring molecule. Given the shape of the water molecule, the easiest way the molecules can form an ordered structure is a hexagonal lattice. I’m not going to try to draw it, but there is a good drawing in this reference.
Most snowflakes actually start out as water droplets in clouds. A few droplets encounter ice nuclei as the temperature drops below freezing, and freeze into ice droplets. Sometimes the droplets explode to make many ice particles as they freeze, and each bit of ice can nucleate another droplet.
If ice and water are side by side at subfreezing temperatures, the ice will suck up water vapor from the water. The growth on the ice will be strongest at the sites where the crystal lattice juts out farthest, so the frozen droplet rapidly grows into something like a very short bit of a hexagonal pencil. The edges and corners of this hexagonal prism grow fastest, and sometimes even sprout arms.
Why are snowflakes often symmetrical, but different from each other? The type of growth is determined by the temperature and moisture of the air at the moment of growth. As each snowflake follows a slightly different path through the cloud, it will encounter a different sequence of growth than any other snowflake. At the same time, all of its six arms see the same sequence. The result is a snowflake that is fairly symmetrical but different from any other snowflake.
Very simple snowflakes – usually simple hexagonal plates or needles – may look very similar to each other. But the more complex dendritic snowflakes are generally one of a kind, because each has had a unique path through the cloud that spawned them.
We have snow of the ground now, here in Fairbanks, and many other areas farther south will soon. If you live in snow country, invest in a small hand lens and enjoy the myriad shapes of the snowflakes.
(Photos are from Bentley’s collection of snowflake images.)
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