The ice abides: Be careful out there

Late in the movie Titanic, a couple of the lead characters slogg their way through waist-deep water in the ship’s passageways as they try to escape. The flaw in these scenes is that the people show no signs of physical comfort – only emotional panic – when the reality is they would have been in utter agony.

I grew up on Lake Michigan and remember what it was like at the beach on days when the water temperature was in the mid- to high 50s. Step in and within seconds your feet would ache; you just had to get out and take a break. You could get accustomed to the water after a while and the pain would subside; you might even decide take a dip. But that first encounter with the water? Brrrrr! 

Now imagine how water at around 32 degrees would feel. And in the ice-strewn North Atlantic where the Titanic went down, the water might have been even colder than that. Dissolved salt lowers water’s freezing point; seawater freezes at 28 to 29 degrees F.

But I digress. The point I am getting to is that you want to be careful on your lake now that, most likely, ice has taken hold, as on Birch Lake, near Harshaw, Wis., where I live. Because if you were to break through, drowning might not be your most pressing concern. Hypothermia likely would be.

Odds are that if you broke through you would find yourself holding onto the “shelf” of unbroken ice around the hole you made. And the very cold water would go to work on you immediately. If the water you fell into were between 32 and 40 degrees – a likely scenario on our winter lakes – you would have 15 to 30 minutes before becoming exhausted or losing consciousness. That’s according to the United States Search and Rescue Task Force (USSRTF). Here is how the USSRTF describes the effect of falling into frigid water:

“The first hazards to contend with are panic and shock. The initial shock can place severe strain on the body, producing instant cardiac arrest...Survivors of cold-water accidents have reported the breath driven from them on first impact with the water. Should your face be in the water during that first involuntary gasp for breath, it may well be water rather than air. Total disorientation may occur after cold-water immersion. Persons have reported “thrashing helplessly in the water” for 30 seconds or more until they were able to get their bearings.

“Immersion in cold water can quickly numb the extremities to the point of uselessness...  Within minutes, severe pain clouds rational thought...finally, hypothermia (exposure) sets in, and without rescue and proper first aid treatment, unconsciousness and death. Normal body temperature of course, is 98.6 [degrees F]. Shivering and the sensation of cold can begin when the body temperature lowers to approximately 96.5. Amnesia can begin to set in at approximately 94, unconsciousness at 86 and death at approximately 79 degrees.”

So, don’t kid yourself that you could easily survive a trip down through your lake’s ice. Be extremely careful out there. In the next post I’ll pass along some experts’ tips on what to do if you were to break through – or how to help a companion who has that misfortune.

The ice cometh: A few facts and figures

To get to know the ice on your lake, it helps to know its physical properties. Here are a few worth noting.

•         Density: 0.916 grams per cubic centimeter at 32 degrees F. For comparison, the density of water is one gram per cubic centimeter.
•        Stiffness modulus: This is a measure of how stiff a material is. The stiffness modulus of ice is about 1.5 million pounds per square inch – or about three times as stiff as a hard plastic.
•         Heat of melting (or heat of fusion): 80 calories per gram. This is the amount of heat required to melt a gram of ice that starts at a temperature of 32 degrees F.
•         Heat capacity: This is the energy required to raise the temperature of a gram of ice one degree Celsius: 0.50 calories per gram. Interestingly, this is only half the heat capacity of liquid water.
•         Fracture toughness: This measures how easily a crack spread through a substance. Ice is about one-tenth as tough as glass in this respect.
•         Thermal conductivity: Ice conducts heat almost four times as well as water.
•         Refractive index: Light travels more slowly through a solid material than through air or a vacuum. The speed of light in ice is about 75 percent of its speed in a vacuum.    

The ice cometh: Why won’t moving water freeze?

Yesterday strong winds blew across Birch Lake; as I stood on our frontage it smacked me straight in the face, the temperature in the single digits, the wind chill factor surely below zero. The near-shore water lay frozen; a few yards out, slabs of ice, like pieces of a crude jigsaw puzzle, rose and fell on waves. Other than that our lobe of the lake was open, choppy in the wind. Looking left toward the narrows leading to the other, more wind-protected lobe, I saw that it was frozen, covered in snow.

All it would take now, I thought, would be a cold, still evening, and the lake would freeze clear across. This morning confirmed my belief. Looking out from our hill as the morning brightened, I saw an unbroken sheet of white, a skin of ice dusted by the night’s light snow.

And I wondered: Why did it take a still night to make the lake freeze? Why wouldn’t it freeze despite the wind and waves? After all, 32 degrees is 32 degrees, and 8 degrees is 8 degrees. What difference should wind make in whether water gives up its requisite 80 degrees of heat energy per gram and freezes?

I searched all over the Internet without finding a satisfactory answer. One place said the motion of the water meant the molecular motion could not slow down enough to lower the temperature. I rejected this on the grounds it seems to confuse the macroscopic motion of the water with the sub-microscopic motion of molecules that correlates with temperature -- are we to believe that the mere fact of physical motion imparts heat? Another source said the mechanical motion of the water continuously fractures any ice crystals that form. I don’t have a technical argument against this; it just doesn’t seem plausible.

So I came up with a theory of my own, which is that the wave action creates a stirring effect, bringing up warmer, denser water from below and sustaining the surface temperature above freezing. Remember that, as earlier posts have said, water is the most dense at about 4 degrees C (39 degrees F). So the temperature gradient in a lake in winter runs from near zero degrees C at the surface (32 degrees F), gradually down to 4 degrees C. If the surface is heavily disturbed, as it was yesterday, then the agitation would be constantly bringing up warmer (or less cold) water from below, so that the surface water could not reach the freezing temperature. Then, once the wind calmed down and the water lay smooth, the surface water would readily give up its heat to the far-below-freezing air above and turn to ice.

Well, then, a critic might say, what keeps a river or creek flowing all winter, no matter how cold it gets, even if the river is rather slow-flowing and there is no wave action to speak of? Here we need to remember that a river or creek is not a closed system. In winter its source is likely, to a great extent, groundwater, which in northern latitudes is going to bubble out of the earth at temperatures in the low 40s F. Water at that temperature will enter continuously at numerous points along the stream’s route, so there is no real possibility of freezing. Of course many rivers do freeze on the surface, although the water beneath keeps flowing.

Now, as for Birch Lake, which has only a very small creek running in and out, and so is to a large extent a closed system in winter (absent snow melt and runoff) – what if the temperature remained at, say, 10 degrees indefinitely, and strong winds continued for weeks on end? Would the lake’s surface ultimately freeze?

That’s a good question. Does anyone have an answer? And would anyone care to confirm or shoot down my theories as to why rivers, streams and wind-blown lakes won’t freeze despite very cold air above them? I am all ears.

The ice cometh: Hear it boom

Ice is slowly taking hold here on Birch Lake, near Harshaw, Wis. Actually the lake had a skin of ice over more than half its surface late last week, but that was before winds kicked up. Last night the temperature was in single digits, where it remains this mourning, yet our entire lobe of the lake – more than half the total surface – is wide open. A 20 mph winds is keeping it so. If things calm down and the weather stays cold, we are in for a very fast freeze.

As I wrote earlier, this ice is a fascinating substance. One of its properties is its expansion. Most compounds contract when they cool and expand when they warm up. At the level of chemical structure, that means the molecules pack closer together when the substance is cold and spread out when it is warm.

Water and ice break that pattern. In liquid water (H₂O), each molecule – two hydrogen atoms (H) and one oxygen atom (O) – is bonded to three or four others. In ice, each molecule is bound to four others. That means more open space between the molecules, and so an expanded – thus less dense – substance.

As kids my brother and I found our about the expansion of ice when we left a few quart bottles of pop on the enclosed but unheated porch one very cold night. In the morning the necks of the bottles were blown clean off (or parents were not amused). Actually expanding ice is a nearly irresistible force. Water that creeps into a crevice in a rock can break that rock apart when it freezes. The expansion if ice is responsible for much of the natural weathering that takes place in northern landscapes.

How much does ice expand? About 9 percent by volume. Stated another way, ice is about nine-tenths as dense as water. This is why you may hear it said that only one about one-tenth of a floating iceberg is above the water.

One of the joys of winter on your lake is the sound if ice expanding. It is an eerie sound, a bit disconcerting if you happen to be on the lake at the time. Often while skating I have heard and seen an expansion crack sizzle right past me and off into the distance. If you can take having a window open on a very cold, still night, try lying in bed and listening to the ice boom. I guess it’s winter’s version of listening to the loons call.

The Ice Cometh: How much weight can it hold?

When I was a kid, I went with a brother and a couple of friends down to our local river on a cold December Sunday morning. We found the river (actually at that place an estuary, its water level determined by Lake Michigan) frozen clear as glass.

We first thought of skating – but was the ice thick enough? We dropped a couple softball-sized stones from the bridge and, instead of breaking through, each landed with a solid thock! That sent us scrambling down the embankment to the water. One by one, we inched out from shore, the water under the ice less than a foot deep. We kept inching farther, staying well apart, still over shallow water. A few stress cracks showed the ice to be about two inches thick. Mustering our courage, we each tried jumping and landing hard. The ice held firm. Then we moved together into a circle and jumped in unison. Still no sign of weakness.

We ended up skating (with parents’ permission) for hours that day, on ice so clear we could see every detail on the bottom, two to four feet down. But were we really safe? Just how strong is ice? How thick does it have to be to walk on, skate on, drive a car on? I grew up with an old saying, allegedly from someone’s grandfather, that “Two inches of ice will hold a team of horses.” Every authoritative source I’ve read lately contradicts that.

Any discussion of when ice is “safe” must account for the possibility of springs, flowing water underneath (as on a river), snow on top, objects like logs or rocks protruding, recent temperature changes (have there been any freezing/thawing cycles?) and the condition of the ice itself (late-winter ice that is honeycombed is notoriously hazardous). Most experts will tell you there is no such thing as “safe” ice – venturing out is always at your own risk.

Even when we’re talking about clear blue ice – the kind formed from calm, very cold nights – authorities disagree on how much ice is acceptable. Some say to stay off ice three inches or less in thickness. Others say less than two inches will do in some circumstances. I have skated on river ice barely two inches thick. It was creaky, but I didn’t worry too much because I stayed near the bank, over water no more than a foot or two deep. If the ice broke I would at worst end up with cold and very wet feet and lower legs, and the river was right behind my house, so a warming place was nearby.

I won’t be so casual venturing out onto Birch Lake, where the depth just a few dozen yards from our shore drops to eight feet and more. If you’re looking for a little guidance on ice safety, here are a couple of perspectives from knowledgeable sources. The Lincoln (Neb.) Fire and Rescue dive and water rescue team gives this advice:

·        Less than 3 inches: Keep off

·        4-5 inches: Ice fishing and skating (person up to 250 pounds)

·        6-7 inches: Snowmobile or ATV (1,100 pounds)

·        8-11 inches: Light car or truck (3,500 pounds)

·        12-14 inches: Medium truck (8,000 pounds)

The U.S. Army Corps of Engineers breaks it down this way:

·        Less than 1.75 inches: Keep off

·        1.75 inches: One person on skis

·        2 inches: One person on foot or skates

·        3 inches: One snowmobile or a group of people walking single file

·        7 inches: Automobile

·        8 inches: 2.5-ton truck

·        9 inches: 3.5-ton truck

·        10 inches: 7- to 8-ton truck

This guidance applies to clear, blue, sound ice. So the old folk wisdom about the team of horses is not to be trusted. Here is an old saying you can live by:

Thick and blue, tried and true.

Thin and crispy, way too risky.

When in doubt, don't go out. 

The ice cometh: Why is it slippery?

One of my great winter joys has been ice skating – on a lake or river rather than a rink (especially an indoor rink). Nothing beat the first venture with blades onto brand new ice, barely thick enough to be safe, often clear as glass. And I’m not just talking about childhood here. As recently as 10 years ago we lived on a dammed section of river that by Thanksgiving usually had frozen solid. I would skate on it as often as I could, winding upstream through marsh and woods, until snow came and covered the ice. I am hoping one of these years our Birch Lake will freeze and stay clear of snow for at least a few days. It is fascinating to look down into a lake through sheet of ice.

Do you skate on your lake, conditions permitting? Have you ever wondered why you’re able to skate – that is, why ice is slippery? After all, the solid phases of most compounds are not. Perhaps you don’t wonder because you think you know the answer. If you’re like me, you were taught in grade school that we can skate because the pressure of the blades lowers the melting temperature (32 degrees F) at the ice surface, creating a thin film of water on which we glide. But if that’s true, why are we able to slide across ice while wearing flat-soled shoes, which exert much less intense pressure?

It turns out science has pretty well rejected the pressure explanation. In an article in the New York Times (http://www.nytimes.com/2006/02/21/science/21ice.html?pagewanted=all&_r=0) Robert Rosenberg, an emeritus professor of chemistry at Lawrence University in Appleton, Wis., explained why: “The explanation fails, he said, because the pressure-melting effect is small. A 150-pound person standing on ice wearing a pair of ice skates exerts a pressure of only 50 pounds per square inch on the ice. (A typical blade edge, which is not razor sharp, is about one-eighth of an inch wide and about 12 inches long, yielding a surface area of 1.5 square inches each or 3 square inches for two blades.) That amount of pressure lowers the melting temperature only a small amount, from 32 degrees to 31.97 degrees. Yet ice skaters can easily slip and fall at temperatures much colder.”

There are now two other explanations – apparently not mutually exclusive. One is that friction from the skate blade (or shoe) is what heats and melts the ice and creates the slipperiness. The other is that the ice surface is inherently slippery. “This argument holds that water molecules at the ice surface vibrate more, because there are no molecules above them to help hold them in place, and they thus remain an unfrozen liquid even at temperatures far below freezing,” the Times article said.

Among scientists who believe in inherent slipperiness is Dr. Gabor Somorjai at Lawrence Berkeley National Laboratory. His sophisticated tests support the theory and so does a basic observation: A person standing motionless on ice is not creating friction – yet still may easily slip and fall.

One who disputes the importance of the liquid layer is Dr. Miquel Salmeron, also of Lawrence Berkeley. The Times reports that he and colleagues dragged a device similar to a tiny phonograph needle across ice and found the friction to be “very high.” And high friction of course would generate meaningful heat.

“Dr. Salmeron said this finding indicates that while the top layer of ice may be liquid, it is too thin to contribute much to slipperiness except near the melting temperature. In his view, friction is the primary reason ice is slippery,” the Times article said.

Which theory is right? Maybe one or the other, maybe both. Scientists disagree. For all the advances in scientific knowledge, ice remains a mysterious substance. So, let’s not worry too much about the explanation. Let’s get out on the ice and slide around!

The ice cometh. Time to get to know it.

Is there any ice on your lake yet? There’s some on Birch Lake near Harshaw, Wis., where I live. It’s creeping out from the shore in ovoid shapes around the shallow inlet we call Indian Bay. I haven’t seen any ice on the main body of the lake yet, but then we haven’t seen temperatures down into the low 20s and teens, not even the overnight lows. Just give it time I guess.

Ice is a fascinating substance, and I’ll be writing about it in the next few posts. Right now the water in your lake is cooled down close to the freezing point of 32 degrees Fahrenheit (zero degrees Celsius). Or I should say, that’s the temperature at and near your lake’s the lake surface. It’s a little warmer deeper down.

Water is a peculiar compound in that it doesn’t continue getting denser as it cools. It reaches its greatest density at about 4 degrees Celsius (39 degrees Fahrenheit). And of course denser water sinks. So the water at the bottom of your lake is at around 4 degrees C, and the water at the surface is about ready to freeze. A very cold night or two will get the job done, forming a skin of ice over the surface that will thicken with more cold weather.

Here’s an interesting thought: What would happen if water and ice behaved the way most compounds do with falling temperature? That is, what if the coldest water were the densest and ice were denser still? Well, if you were to take a jar full of melted wax, and drop in a chunk of wax, that solid piece would drop to the bottom.

If water behaved that same way, ice forming at your lake’s surface from exposure to very cold air would sink. As more and more ice formed, it would continue dropping to the bottom, until after a long winter the lake most likely would be frozen solid from top to bottom. Nothing would survive the winter. In fact, deeper lakes probably would not thaw completely even through the summer.

As it is, ice floats. And the ice sheet that forms on your lake provides insulation that allows most of the water below to remain in a liquid state. So the fish, clams, turtles, frogs, snails and all sorts of macro- and microscopic life get through the winter just fine.

This anomalous density pattern is just one of many interesting properties of ice. In the next post we’ll explore why ice is slippery. You probably think you know the answer – I thought I did – but the latest scientific consensus says that answer is wrong.