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Post by mrsonde on Jun 4, 2013 7:21:40 GMT 1
Mr S: Supercooled water is not ice. Ice is not supercooled water. We scientists use words very precisely, and don't like to have them misused by amateurs. Not understanding the significance of supercooling can kill you in real life, or get you laughed at in a science forum. I know what supercooling is, thankyou - that's why I distinguished them.
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Post by alancalverd on Jun 4, 2013 7:28:41 GMT 1
Then why did you imply that frozen and supercooled were the same? Or does "indeed" mean something else on your planet?
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Post by mrsonde on Jun 4, 2013 7:30:58 GMT 1
I think we can agree that NOAA is authoritative: And what is it about the physics of sea water that so radically changes its behaviour from that when it's fresh? You still haven't explained why. You're asserting that the volume of the ice under the water is less than the volume taken up when all the floating ice is melted - including all the ice above the water. On what grounds?
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Post by mrsonde on Jun 4, 2013 7:34:57 GMT 1
Then why did you imply that frozen and supercooled were the same? I implied no such thing - on the contrary, the implication is that they are not the same, else I wouldn't have had any basis for drawing the clarification. My statement was that the table was recording temp and density in an ascending fashion. (Just because water has been supercooled does not mean it can't be ice, incidentally!)
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Post by fascinating on Jun 4, 2013 7:41:43 GMT 1
I found that NOAA quote that mrsonde provided. It is at oceanservice.noaa.gov/facts/oceanfreeze.html. As he says, I think we can take that as authoritative. Therefore I think mrsonde is correct to state that the melting of sea ice will not cause a drop in sea level.
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Post by mrsonde on Jun 4, 2013 7:45:30 GMT 1
So melting floating ice will lower the temperature of the oceans and the density, making them shrink. Pretty neat. But not true. If as you say fresh water is densest at 4 degrees, then the liquid sea at, say 2 degrees (for most of the time the Arctic ocean is at least this cold), will become denser when ice floating on it melts (which it does at four degrees.) And the temperature will rise at the same time. Though you've still to show why this would make the sea levels fall. For that you have to show that this density fall is more than the volume of water added by the melting floating ice.
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Post by mrsonde on Jun 4, 2013 7:57:18 GMT 1
I think alan provided it - but your link doesn't seem to work. I fail to see why it would, and no one has yet explained why it should. I have an inkling that the disagreement centres around this idea of density. At first I thought it was a complete red herring, I must admit, but in that it seems I was probably wrong. However, I'm still pretty sure that the relative densities of the waters is not the pertinent issue, as far as I can see. It might be if we were talking about liquid waters in both cases - but we're not, we're talking about turning ice into water. And ice doesn't float merely because it is less dense - it also floats because it is buoyant: in sea water at least. (In the ordinary two phases that we encounter ice in the Arctic, anyway.) Otherwise the Arctic pack would be a completely featureless flat plain, and polar bears would spend all their lives merely skating about on a frozen pond.
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Post by alancalverd on Jun 4, 2013 13:41:23 GMT 1
Chemistry. It doesn't even taste the same as fresh water, so why should it behave the same?
It could be, but it isn't. Supercooled water is a liquid, ice is a solid. Big difference when you are flying thorough it.
We clearly do inhabit different planets! At sea level standard atmospheric pressure, ice melts at 0 deg C: it is half of the definition of Earthling Celsius's scale of temperature.
We all say strange things in our dotage, Mr S, but if you don't retract that statement, I will have to classify you as an incorrigible fool - or a direct descendant of Aristotle, perhaps? Thanks for the laugh, anyway!
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Post by marchesarosa on Jun 4, 2013 14:52:05 GMT 1
Years ago I think I read somewhere, can't remember where, that melting sea ice would cause a local sea level drop in the vicinity of the Arctic for reasons related to what you have been discussing, I suppose. But I don't understand why cos I don't understand the physics.
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Post by mrsonde on Jun 4, 2013 17:40:59 GMT 1
Chemistry. It doesn't even taste the same as fresh water, so why should it behave the same? It's its ionic content, if that's what you're hinting at: they become strongly hydrated, thus increasing the density of the water (and the temperature of its maximum density, and the point of its phase transition.) Having re-read all the classic papers on the subject this afternoon- Debye, Bernal & Fowler, Fuchs, Lennard-Jones, Pople - and done some preliminary checking of recent research, it would appear that this anomalous property of water - its expansion below four degrees - has still not been adequately explained. The closest theoretical account seems to depend on the same cause - it's the varying amounts of hydrogen ions in water that produces this extraordinary behaviour. As thermal agitation decreases the standard structure of water becomes more and more closely packed, due to the molecular electrostatic polar interactions. The H3 and to a lesser extent the H1 ions then more readily jump from molecule to molecule (the extra or missing proton, that is), bringing the tetrahedral H2O structures still closer, until, at around four degrees, or just under zero in sea water, ice structures begin to form - still the standard tetrahedral structures, but now arranged in hexagonal sheets rather than the liquid pentamers. The anomalous increase in volume is a result of an enormous fall in its dielectric properties, very probably resulting from the consequent lack of rotational freedom of the ice molecules - in particular of the lone pairs. So, its electrostatic repulsion that accounts for this curious slow decrease in density before a full phase transition. Incidentally - it's never struck me before, but this is a remarkable breach of the standard nonsensical way of interpreting the second law of thermodynamics. All life does the same of course, but water is not normally considered "alive". But here we have an everyday example of "the universe" progressing, without any additional imput of energy (on the contrary), from a lower to a higher state of order. Or from a higher entropy in the case of liquid water to a lower entropy in the form of ice, if you want to make the mistake of conflating the energetic meaning of entropy with that of "order". And moving from there to Ice II and to Ice III only increases the degree of "order". Well, not really. Ice can easily be crystallised from supercooled water. ;D A joke, I presume? Well, that would depend on how strictly you want to define "it" and "melt". Ice at near absolute zero still contains a lot of liquid water, which continually melts the ice at its interface, and then freezes again. But in terms of the bulk formations - it always has a proportion of uncoordinated structural crystals, just like liquid water - the transition is mostly between 2C and just under 4C. That's the energy required to break the lowest-energy structure of ice. You don't realise that ice contains a lot of space? Full of gases, of course - including all those loose protons (hydrogen) that have been passing down the chain of molecules. It's standard form is like a multiply sheet. What do you think the fundamental difference between Ice II and Ice III is? (I was at college once with an anthropologist who told me that the Inuit have separate words for them. I don't know if that's true, but it sounds likely.)
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Post by buckleymanor1 on Jun 4, 2013 21:16:52 GMT 1
the previouse floating ice and sea water. So melting floating ice will lower the temperature of the oceans and the density, making them shrink. Pretty neat. But not true. If as you say fresh water is densest at 4 degrees, then the liquid sea at, say 2 degrees (for most of the time the Arctic ocean is at least this cold), will become denser when ice floating on it melts (which it does at four degrees.) And the temperature will rise at the same time. Though you've still to show why this would make the sea levels fall. For that you have to show that this density fall is more than the volume of water added by the melting floating ice. I'ts been explained. Ice freezes and melts at around 0 degrees if it's fresh water. Sligtly less if it's sea water. Melting ice caps at 0 degrees, which are made of fresh water lower the temperature of the surrounding sea water. So if the surrounding sea water starts with a temperature of about 3.5 degrees as described above and the melting fresh ice water at 0 degrees is added to it then some of the heat from the sea water is added to the colder ice water. The result being an overall drop in temperature of the now new sea water mix. Because sea water becomes more and more dense as it gets colder this new sea water mix takes up less room in the oceans than the previouse ice and seawater.
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Post by buckleymanor1 on Jun 4, 2013 21:26:07 GMT 1
Of course he realises that ice contains a lot of space. So do steel ships, boats, balloons and sponges.But the overall density of any floating object is less than the substance it's floating in, that is why it floats it's less dense or buoyant.
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Post by alancalverd on Jun 5, 2013 0:40:19 GMT 1
No.
Supercooled water may be invisible, a faint mist, or a rain shower. Looks harmless but may solidify instantly on a nucleating surface like a wing and rapidly cover the entire aircraft in hard, dense ice glaze that thickens without limit. The effect ranges from shit-scary to fatal.
Ice crystals look like a twinkling fog and with any luck, just bounce off the plane without settling. Larger accretions (snow) tend to clump and fall off in noisy chunks, provided they don't block the air intakes.
Hail, encountered at flying speeds, is unforgettable but generally survivable, though large stones can puncture the skin of the aircraft.
Bollocks. Look at a phase diagram.
Since neither ice II or ice III exists below 200 MPa (2000 atmospheres) it's unlikely that many Inuit have seen either. There are however many Nordic and Inuit words to describe different forms of snow, ice fog, and other ambient phenomena.
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Post by marchesarosa on Jul 16, 2013 9:33:47 GMT 1
JURY STILL OUT ON CAUSE OF MELTING ICE SHEETS Date: 15/07/13 Graham Lloyd, The Australian Another decade of satellite observations was needed to determine whether a recent acceleration in ice melt from the Greenland and Antarctic ice sheets was the result of long-term trends or short-term natural variability. A new paper, published today in Nature Geoscience, said melting ice was the major factor to determine future sea level rises. Some researchers claim the melt has been underestimated by the UN’s Intergovernmental Panel on Climate Change. And the new paper said a lack of consensus among scientists and a high level of uncertainty remained. “Although the recent observations show an increasingly negative mass balance for both ice sheets, the scientific community has not reached a consensus on whether this is owing to variability (“noise”) in the ice-sheet climate system or reflects a secular signal,” the paper said. Author Bert Wouters, from the University of Colorado, Boulder, said the findings “underscore the need for continuous satellite monitoring of the ice sheets to better identify and predict melting and the corresponding sea-level rise”. The research said although there was almost enough satellite data to detect an acceleration in mass loss of the Antarctic ice sheet, another 10 years of satellite observations may be needed to do so for Greenland. “This finding challenges the notion that recent accelerations in melt will be sustained into the future, and urges caution in extrapolating current measurements to predict future sea level rise” a statement accompanying the release of the paper said. Researchers said the uncertainty was due to the inability of current ice sheet models to incorporate all processes governing ice loss, in particular “complex dynamical changes of the marginal glaciers and the forcing at marine margins”. Full story www.theaustralian.com.au/news/health-science/jury-still-out-on-cause-of-melting-ice-sheets/story-e6frg8y6-1226679300880
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Post by marchesarosa on Jul 16, 2013 9:34:31 GMT 1
RATE OF FUTURE ICE LOSS REMAINS UNCERTAIN Date: 16/07/13 Darren Osborne, ABC Science At least another decade of satellite observations are needed before scientists can determine the rate at which ice sheets in Greenland and Antarctica are melting, according to a new report. Until then, it isn’t yet possible to determine whether the melt is due to long-term trends such as global warming or short-term natural fluctuations, and whether the current rate of melting will change in the future. The study, which appears today in the journal Nature Geoscience examined data collected by the Gravity Recovery and Climate Experiment (GRACE) satellite from 2003 to 2012. GRACE measures changes in the gravity field above the ice to determine the ice sheet’s mass distribution, as well as movement into the ocean. Study lead author, Dr Bert Wouter of the University of Colorado, says the satellite observed “the ice sheets are losing substantial amounts of ice — about 300 billion tonnes each year — and the rate at which these losses occurs is increasing.” “Compared to the first few years of the GRACE mission, the ice sheets’ contribution to sea-level rise has almost doubled in recent years.” The researchers compared the satellite data with reconstructions — based on ground measurements — of about 50 years of mass changes to the ice sheets. “We find that the record length of space-borne gravity observations is too short at present to meaningfully separate long-term accelerations from short-term ice sheet variability,” the researchers write. The uncertainity surrounding the rate of ice-loss means current models of sea-level rise due to melting ice sheets by 2100, could be out by as much as 35 centimetres, say the reserchers. But it may not be too long before enough data is available to better predict the rate of ice loss. The researchers suggest that there may be enough data to detect a speed-up in mass loss of the Antarctic ice sheet, and that another 10 years of satellite observation is needed for Greenland. Predicting the future While there is strong evidence that sections of the Greenland and Antarctic ice sheets are melting, climate scientists are reluctant to predict what they will look like by the end of the century. Full story www.abc.net.au/science/articles/2013/07/15/3802146.htm
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