Ice Question for the Scientifically Inclined

[SkunkedAgain]

Let me set the stage for this question, location Cutler Reservoir, Temperature range -3 F to 25 F  Ice sheet has been in tact since November... Went out Tuesday and noticed the ice is terribly deteriorated, like the surface has 2" of snow 1.5" of water, then a thin very grey ice layer, another water layer and then 2-3" of grey crappy ice, then open water below...  Temperatures have been below freezing for months, this should be good ice building temperatures, yet the ice seems to be ready to disappear for the year... What is going on?  Is this a result of solar radiation eating up the ice below, while the small snow layer is insulating the formation of new ice?  It isn't adding up to me, why with this much cold, I mean we have few hours where the temp might get above 32, and many hours closer to single digits... in the fall this formula means good ice building weather, now it seems to be melting away... Has the solar angle changed enough that the radiation is driving the ice condition?  I was just trying to figure this one out because it doesn't make sense to me from a heat freezing stand point... Later Jeff

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Google provided this, what do you think? J
Many elements drive ice melt on the lake, the most obvious of which is sunlight. Sunlight is the primary driver behind internal melting – melting that occurs within the ice sheet, at the triple junctions, grain boundaries, crystal structure imperfections, and within individual ice crystals. Internal melting can dramatically weaken the ice sheet without significantly thinning it. Tiny pores will open up in the ice as the meltwater drains through it, making it more fragile. Direct sunlight can also melt ice along the shorelines facing the sun, so it becomes difficult to return to land. In shallow areas the sun shining through the ice can actually heat the water, which then melts the ice from beneath.

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So me thinks the ice season is going to close before long, even though the temperatures would say otherwise.... What's your thoughts, anyone have enough experience to add more ice insight to this? Thanks Jeff

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Okay for those interested in ice thawing and the dangers ahead, this is a great read... My only question is what is the difference between small grain and large grain ice? S1, S2 ? http://lakeice.squarespace.com/thawed-ice/

[Piscophilic]

Hi Jeff,

The best description of S1 and S2 I found comes from the same site, but a different article: http://lakeice.squarespace.com/types-of-ice.

The S stands for secondary (ice formed under the initial or primary layer is secondary ice) and the 1 and 2 stand for small versus large grains in the ice. There didn't seem to be a lot of info on how small is small or how large is large.

From my experience with metal deposition in electronics (very pure materials) grains are small pieces of single crystal material in a larger area like a metal or ice layer. If the material cools very quickly formation of small grains occur. Slower cooling results in larger grains.

The triple junction idea comes from a location where 3 grains touch each other at the same point. The higher salt (or other impurity) concentration found at a triple junction comes from the fact that impurities tend to be forced out of a grain as it forms because they disrupt the perfect lattice structure that is the lowest energy state for the solid. At a triple junction the impurity is being pushed to a single location from three directions at once.

From an ice melting standpoint any imperfection in the crystal structure interacts differently with incoming light, usually absorbing it more and thus being heated by it. So a triple junction (or other imperfection) becomes a little hot spot leading to the ice around it melting.

This whole process occurs initially on a microscopic scale, less than millimeter sizes, but leads to the larger things we can easily see. However weakening of the ice can definitely be caused by the microscopic changes. Ice becoming cloudy results in part from the invisible imperfections getting larger and more numerous.

I didn't read far enough to get to something useful to predict ice safety, but knowing the weather history is clearly useful to try to stay safe.

One item I didn't see mentioned, that is likely more prevalent in shallow water bodies is the simple fact the the ground is constantly being heated by the higher temperatures of the earth deeper down. A cold event lowers the surface temperature, but if those low temperatures are not maintained the surface (the ground and the water) will gradually heat from underneath. Nothing stays constant.

I guess this discussion in general is not very clear. It is easier to just say clear ice is better than cloudy and new ice is stronger than old ice.

[SkunkedAgain]

Thanks Jim, appreciate the additional insight... very interesting how the impurities aid the melting by gathering the heat... Until I got a camera, I always assumed it was dark under the ice, especially with snow on it.. but my camera has shown me how much light actually gets through the ice, so it's a lot easier to see how the solar heating is melting the ice from within... I'm learning a lot that I hadn't really considered before, but it all makes sense when you stop and think about it... I think I have forgotten or didn't learn the first time in material science what makes up a grain? I know they like to go to the space station to produce really large grain sized materials and like you mentioned the rapid cooling (quench) makes the small grain size... I just don't remember what makes grain boundaries and why can some be large or small with the same material??? I think I'd learn a lot more out of that lecture today than when I was just trying to pass the class... Guess it's time for another google search... Later J

[RockyRaab2]

At the other end of the temp scale the fast/slow and small/large grain process also occurs in lava. If you've ever noticed the octagonal columns of basalt just south of Pocatello along I-15, that's the result of slow cooling.

[Piscophilic]

Thanks Jim, appreciate the additional insight... very interesting how the impurities aid the melting by gathering the heat... Until I got a camera, I always assumed it was dark under the ice, especially with snow on it.. but my camera has shown me how much light actually gets through the ice, so it's a lot easier to see how the solar heating is melting the ice from within... I'm learning a lot that I hadn't really considered before, but it all makes sense when you stop and think about it... I think I have forgotten or didn't learn the first time in material science what makes up a grain?  I know they like to go to the space station to produce really large grain sized materials and like you mentioned the rapid cooling (quench) makes the small grain size... I just don't remember what makes grain boundaries and why can some be large or small with the same material??? I think I'd learn a lot more out of that lecture today than when I was just trying to pass the class... Guess it's time for another google search... Later J

As far as how a grain forms, the easiest way for me to visualize it comes from thin films, again in electronics. You take a Si wafer and put it in a vacuum chamber and then heat an Al source hot enough for it to melt and then evaporate. Then you end up with a flux of evaporated Al atoms moving rapidly out from the source. They are individual atoms, but they are gaseous because of their temperature. Most of that energy is converted into kinetic energy which is why they move so fast.

When they hit the wafer surface, which usually is at room temp, their kinetic energy is absorbed by the large mass at room temp and the "stick". However the very surface of the wafer is getting hotter by by absorbing that kinetic energy. As more and more atoms hit the surface they either stick to another stuck Al atom or a bare spot on the surface. When several Al atoms stick to each other you get what's called a nucleation site or the beginning of a grain. As a grain grows larger it will spread across the surface and encounter stuck atoms which get absorbed into the grain. 

As the process continues the very surface continues to heat up which allows the grain to continue to grow and it will grow as a single crystal as long as enough energy (heat) is present. Gradually the whole surface becomes covered with growing grains and each new atom helps a grain to grow as it lands on top of it. 

When the surface begins to be covered with grains their edges begin to push against other grains and their size becomes limited. This pushing against each other is at the grain boundaries. If there is enough energy, some grains will be able to consume others and get larger. but most will not.

If however, we start the process by heating the wafer to say 350 degrees C, something different happens. When an atom hits the surface it doesn't instantly stick, but wanders around on the surface slowly losing kinetic energy. Because the average time for sticking is longer, fewer nucleation sites are formed and more material moves into the lattice of the few grains that are forming. This results in fewer but larger grains.

Of course with ice the formation of grains is 3 dimensional, but the basic ideas apply. I don't even try to think in 3 dimensions so I like the surface example. 

You don't necessarily add heat to ice to get larger grains, but the temperature remains nearer to the ideal temp longer and the grains get larger.

The one that is hard to predict is the thawing and re-freezing thing. In some conditions it causes grain growth and in others it causes breakdown of the existing structure.  The thing that is important for us is that the conditions that lead to weakening are far more common or likely that the ones that lead to strengthening!

BTW, I don't remember all that much from the classes either. There are a few things that I have been re-exposed to over the years that now make them easier to think about. A form a reviewing you might call it.

[SkunkedAgain]

Great explanation thank you... I like your last line, I think that need to know goes a long ways toward making things relevant to us so we pay attention to it... That's when we really learn something isn't it? Thanks Jeff

[MrShane]

Great explanation thank you... I like your last line, I think that need to know goes a long ways toward making things relevant to us so we pay attention to it... That's when we really learn something isn't it?  Thanks Jeff

I always believed in a shallow reservoir such as Cutler that lake ‘turnover’ happens a couple times a day, therefore keeping the cap from getting thicker, by weakening it from bottom up.
The earth at bottom of lakes heats up, accelerated by sun’s rays plus earth temp, then raises that warmer water up till it sits below ice cap.
Then sun goes down and top portion of column now cools quickly and sinks to earth and then repeats by heating and cooling.
Kind of a vertical current in a way.
A deep body of water would still have this action but not nearly in the same amount of cycles in a 24 hour period.
But…..I never spent a single day in school past high school so I could be 100% wrong about this.