Tuesday, August 07, 2018
The Coolest Substance on Earth
Under such circumstances, there is only one thing for me to do. I want to do what I can to help everyone cool down. To this end, here is an article about a very cool substance, which I hope will make you feel cool—in more ways than one!
The very cool substance I want to discuss is ice. It is a huge subject, and some aspects of it get more discussion than others. In particular, Arctic sea ice, glaciers, especially those in Greenland and Antarctica, and icebergs get a lot of coverage while numerous other sorts of ice, especially those that form on canals, rivers and lakes and along shorelines, hardly get discussed at all. And the topic I find most interesting is that of the challenges and opportunities provided by watery landscapes that are either partially or completely ice-bound.
As far as us carbon-based life forms are concerned, the most important substance on Earth is water: it is what allows life to exist. The centrality of water in our lives is reflected in the Celsius temperature scale that’s used everywhere in the world except for Bahamas, Belize, Cayman Islands, Liberia, Palau and some other backward states which still use Fahrenheit. 0ºC is the freezing point of (fresh) water, 100ºC is its boiling point (at sea level). For an entirely unknown reason, Fahrenheit sets these at 32ºF and 212ºF. We live at 36.6ºC while most life happens between 0ºC and 41ºC—the temperature at which proteins start to break down. Luckily, over most of the planet, temperatures fluctuate between –40ºC and +40ºC, and us warm-blooded, large-bodied organisms can generally thrive within this entire range, its lower extreme by shivering and its upper extreme by sweating.
The fact that large swathes of the planet’s surface straddle the 0ºC boundary between liquid and solid water demands some specific adaptations, which become increasingly important the further north we move. While in the temperate zones ice and snow are usually regarded as little more than nuisances, in the north they provide numerous amenities. Here are a few of them.
• Snow as insulation for shelter and for crops
• Snow and ice as roadway for skis, skates and sleds (which can be wind-driven)
• Ice as road surface for transportation
• Summer refrigeration—by harvesting ice in wintertime and packing it in hay or sawdust
• Source of fresh water where liquid water is salty or brackish
• Springtime irrigation—by trapping snow during winter
• Winter dockage for boats and ships—to ice shelves, where the shoreline is boggy or shallow
• Summer leads in shore ice provide sheltered access to deepwater for small boats
• Partial dry dock for ships, created by freezing ships in place, then sawing out ice around the parts that need maintenance, such as rudders and props
• Surface of sea ice as a source of sea salt
• Ice as construction material for temporary rafts or barges
To make good use of snow and ice, it is essential to understand their properties and their lifecycle. Although 0ºC is the highest temperature at which ice and snow can persist for arbitrary periods of time, they do not form at that temperature but at a lower one. The essential ingredients for ice formation are supercooled water and either turbulence or impurities. Perfectly pure water in a glass container shielded from vibration can be gradually supercooled to –20ºC. But if you then drop in a particle of ice or a grain of sand, ice will start to form around it rather suddenly, and go on forming until the entire volume of water has turned to ice, at which point the glass vessel will explode. If you supercool a bucket of water to just –0.1ºC and drop in a particle, it will form a few kilograms of ice, but then ice formation will stop once the heat of crystallization raises the temperature of the water to 0ºC.
Salt water ice is a rather different substance than fresh water ice. To start with, it forms at a lower temperature, depending on the water’s salinity. Ocean water, with its typical salinity of around 3.5%, will start to freeze at around –1.01ºC. Salt water ice is a composite: ice crystals force out the salt into vertical capillaries that contain liquid brine, which then drains down, raising the salinity of the water at the lower surface of the ice (where ice forms), in turn further lowering the temperature at which it freezes.
Sea ice slowly loses its salt over time, and multiyear pack ice (which is less and less prevalent due to global warming) is almost entirely salt-free. Under the usual set of circumstances, the brine, as it drains down, leaves behind hollow tubes that riddle the ice. In the springtime, these tubes fill with meltwater, which quickly undermines the structure of the ice, making it treacherous. It may look perfectly thick and solid, but is in fact completely rotten from within, full of liquid water, and will crumble to slush as soon as you step on it. But if water and air temperatures are low enough, the hollow tubes may gradually “heal” by filling up with solid ice. In the process, salt gets squeezed to the surface, coating it in a thin layer of brine.
The simplest process of ice formation can be seen in surface ice on clear, calm, fresh water on a lake or a pond. It starts with crystals of ice that form below the surface, then float up and grow into ice needles. These are typically 2–3 cm in length, 0.5–1 cm in width and only 0.5–1 mm in thickness. The needles are made up of interlocked flat hexagonal ice crystals—the shape most easily seen on frosted-over window-glass. For mysterious reasons, ice crystals refuse to form ideal honeycombs and instead grow in fractal patterns. The ice then starts growing thicker by adding more flat layers of needles at the bottom. These then interlock by forming crystalline prisms and pyramids. This type of ice is called glass ice, and it is just one of many other kinds.
Surface ice can set up rather quickly, because the process starts with supercooled water, but once the heat of crystallization warms it to 0ºC the process slows down. Even if the air temperature above the ice is mostly negative, the layer of ice acts to insulate the water from the air. As the ice grows thicker, it further and further restricts the heat flow from its bottom surface to its top surface. Ice thickness can be looked up on a chart: the inputs are initial ice thickness (determined by drilling out and measuring a core sample) and the degree-days since (add up average daily temperatures). If the degree-days number is positive, the ice will grow thinner; if negative—thicker. The overall curve is a parabola, owing to the insulating ability of the ice, which increases with thickness.
Snow tends to complicate this picture. If snow starts falling after the surface ice sets up, it will not affect the planar crystalline structure of the ice needles. At first, it may accelerate ice formation, because the snowflakes are often significantly colder than the air at the surface of the ice and can absorb the heat of crystallization. Thereafter, the snow will slow down the process of ice formation because it is an excellent insulating material. The surface of the snow can be cooled to as little as -40ºC while the ice below remains barely below 0ºC, largely trapping the heat of crystallization below the ice. At that point, the thickness of the ice asymptotically approaches its maximum.
If snow starts falling before glass ice has a chance to set up, the result is different. Each individual snowflake is a planar, hexagonal ice crystal, similar in its fractal structure to the ice needles that form on the surface of the water, but symmetric, since it crystallizes out of a water droplet falling through the air. It is the perfect nucleation point to start ice formation. But the orientation in which it falls into the water is arbitrary, resulting in a mad jumble of ice crystals rather than in interlocking flat sheets of ice. This jumble becomes more and more chaotic as it grows, resulting in grainy ice that is almost entirely opaque and somewhat weaker.
Fast-moving streams sometimes freeze not from the surface, where water flows smoothly, but from the bottom, where obstacles create turbulence and stirred-up sediment provides nucleation points. Ice crystals first form on objects embedded in the bottom—most readily on heat-conducting materials such as on metal or stone, and not at all on thermal insulators such as wood or plastic. A row of steel rebar driven into the bottom across a stream and protruding above its surface will form an ice dam.
Bottom ice tends to be amorphous and spongelike, and often incorporates a lot of silt and sunken organic matter that weighs it down. After a sufficiently large chunk of ice has formed, it may detach from the bottom and float to the surface, where it will start to form surface ice. But large chunks of ice may also adhere to the bottom for long periods of time, then float up suddenly, creating hazards to navigation. Once surface ice forms, bottom ice formation will typically stop. This is because the surface ice will serve as a layer of insulation, preventing the heat of crystallization of bottom ice from escaping to the atmosphere.
Once surface ice reaches 5 cm in thickness it may be safe enough to walk on, although this is not for the feint of heart: the ice will flex under your feet. The sheet of ice is at once fragile, and will shatter if struck, but will also flex under a temporary vertical load, and will slowly submerge under a more permanent one. Sea ice, formed on salt water, is somewhat weaker then freshwater ice, but is also more flexible. As ice forms along a shore, it is often possible to see waves reach the edge of the ice sheet and then cause it to undulate as they travel under the ice toward the shore, gradually decreasing in height.
Once ice thickness reaches 10 cm, it becomes useable as a road surface. For this purpose, it needs to be cleared of snow. But this brings up a problem. If the top surface of the ice is exposed to the cold air, it is at air temperature, while its bottom surface is at around 0ºC. But if it is covered with snow, the temperature gradient across the ice is much smaller. As snow is removed, the temperature gradient increases, the ice closer to the surface shrinks as it cools, and cracks appear on the surface of the ice. These cracks do not go all the way through the ice, but they may under load. Cracks across the path of travel are less dangerous than the ones that run along it; the latter can open up like a zipper under a moving load.
The width of the “channel” to be cleared of snow is generally 40-50 meters wide. If any cracks appear, especially lengthwise ones, and especially ones that are 5 cm wide or more, they need to be repaired by filling them with water. If the ice road is to put to heavy use, then it may make sense to build up the ice thickness to half a meter by watering it. At 0.5 meters, the ice thickness is generally considered sufficient for most transportation purposes. Then courses across the ice can be plotted and marked (fir boughs frozen to the surface make good markers) and automotive traffic can be sent through.
There are certain traffic rules that must be obeyed. First, the loads have to be tightly regulated based on the temperature, thickness and composition of the ice. For example, sea ice with salinity of 0.4-0.6%, 40 cm thick and without cracks will safely hold up 5 tonnes at -2ºC and 6 tonnes at -8ºC. If it were freshwater ice, it would hold up 12 and 16 tonnes, respectively. And if the sea ice were opaque rather than transparent (because it formed out of snow) then the load numbers would be halved.
Second, strict speed limit rules need to be obeyed, both for minimum and maximum speeds. There can be no passing, and traffic has to spaced out by 3 minutes or so. Passing can create intersecting waves that have local amplitudes in excess of what the ice can sustain without cracking. You might think it a good idea to pull right up to a stranded vehicle and offer help, but you must stop something like 20 meters away from it and walk the distance because the ice may not bear twice the load. Maximum speeds are determined based on conditions, but are generally around 20 km/h. Parallel tracks (in same or opposite directions) must be spaced 150-200 meters apart. In spite of all possible safety precautions, ice roads become treacherous as springtime approaches and temperatures rise.
Although ice and snow have some measure of immunity to increasing sunlight, being as good a sunlight reflector as open water is a sunlight absorber, there are some nuances. Just a day with surface air temperature above 0ºC is enough to cause the top surface of the snow to melt, then refreeze, forming a transparent crust. The melting causes the air pollution that has accumulated on the surface of the snow—be it from dust blown in from faraway arid regions, automotive exhaust or soot from coal burning—all of them being heavier than snow, to sink down into it.
And now we have a sort of local greenhouse effect: a transparent, glass-like layer above a darker, pigmented substrate to catch the sunlight to accumulate the heat and conduct it to the ice below. The snow may still look normal, but if it’s crusted over then it may be sitting on a layer of slush rather than hard ice and the ice road, though still appearing serviceable, may turn out to be surrounded by not much more than water.
Since ice roads are far more economical than any other conceivable roadbuilding experiment, there is usually a great deal of economic pressure to keep them open for as long as possible—and that’s usually a bit longer than would be perfectly safe. But at some point the cost of vehicles falling through the ice and of then rescuing the people exceeds the economic benefit of keeping them running, and the ice road must be closed to traffic. In all, the subject is complicated and technical enough to demand a higher level of organization than just letting people get out on the ice.
The ability of ice to provide roads where there aren’t any, and where any roadbuilding experiment beyond bulldozing through a simple dirt track (impassable much of the year) would be subeconomic, becomes first very valuable, then absolutely essential as one moves further north toward the Arctic.
Equally important is the ability of ice—shore ice, which forms as a skirt around islands—to provide dockage where there are no deepwater harbors and no dockage facilities of any sort.
A ship simply punches a channel through the ice, and vehicles can travel to it over the ice from settlements on land to load and unload cargoes, deliver mail, and take on and discharge passengers. The only investments needed to make this dockage facility operational are those needed to construct and maintain a relatively short ice road.
Ice and snow—ice that is smooth and hard, snow that is not too deep—have a myriad of uses. But many of their manifestations are either a nuisance, downright dangerous, or simply useless. They can also be quite beautiful.
When snow freezes upon contact with waves, it can be rolled into balls of sludge ice that is viscous, relatively uncompressible, completely unstable and therefore impassable: a no-go zone for anything other than an intrepid explorer wearing a float suit. (These are waterproof overalls that incorporate both flotation and insulation throughout, making it very difficult to get wet enough or cold enough.)
Another useless but curious manifestation is pancake ice: chunks of ice that wave action causes to rub against each other and form into consistent round or oval shapes.
Wave action can cause solid, clear ice to break apart into ragged chunks called nilas. It is particularly dangerous to boats, especially fiberglass boats, because its edges are hard and sharp, and can scrape and cut through hulls.
Most boat hulls are very badly suited to moving through waters that have any amount of ice in them because they are made to cut through water. A sharp, nearly vertical stem at the bow is the exact opposite of what’s needed to deal with ice because the ice is quite incompressible in the horizontal direction. What’s needed is an icebreaker bow, which is designed to move over the ice, cracking off and submerging it piece by piece, then sweeping the pieces aside.
Here is an older Russian icebreaker that, judging from the scars on its bow, has clearly seen a lot of action. Ships of this type have been essential in keeping the northern sea routes open.
Icebreaker technology has moved on since then. Here’s one of Russia’s latest, “50 Years of Victory”.
And the last step in the evolution has made the icebreaker unnecessary. Here is the liquefied natural gas tanker “Christophe de Margerie”, which delivers LNG from Russia’s newest Arctic city, Sabetta, to places around the world (the very first shipment was to Boston). It is an LNG tanker and an icebreaker rolled into one.
It’s easy to understand how most people see ice merely as a nuisance. But the further north you move, the more you realize how useful these solid forms of water really are. There, ice and snow are not dreaded but celebrated: at first snowfall, Russian schoolchildren become jubilant and can’t be stopped from running outside to throw snowballs and build snowmen.
Where ice is in forms that are of no conceivable benefit to anyone, they are often beautiful to explore and to contemplate.
Clearly, there is much more to say on this subject, but I hope that what I have said will help you appreciate just how cool a substance ice is, and will make you feel cooler for knowing it—in more ways than one.