Water’s wacky density leads to strange effects that researchers are still uncovering.
Typically, liquids become denser the more they cool. But freshwater is densest at 4° Celsius. As it cools below that temperature, the water becomes less dense and rises. As a result, ice columns submerged in liquid water can melt into three different shapes, depending on the water’s temperature, researchers report in the Jan. 28 Physical Review Letters.
“Almost everything” about the findings was surprising, says mathematician Leif Ristroph of New York University.
Ristroph and colleagues anchored ultrapure ice cylinders up to 30 centimeters long in place and submerged them in tanks of water at temperatures from 2° to 10° C.
The ice melted into smooth, downward-pointing spikes if placed in water lower than about 5° C. Simulations showed “a strange thing — that the cold liquid water near the ice is actually buoyant” due to being less dense than the rest of the water in the tank, Ristroph says. So that upward flow draws warmer water closer to the ice’s base, causing it to melt faster than the top.
The opposite occurred above about 7° C; the ice formed an upward-pointing spike. That’s because colder water near the ice is denser than the surrounding water and sinks, pulling in warmer water at the top of the ice and causing it to melt faster than the bottom, simulations showed. This matches “what your intuition would expect,” Ristroph says.
Between about 5° to 7° C, the ice melted into scalloped columns. “Basically, the water is confused,” Ristroph says, so it forms different layers, some of which tend to rise and others which tend to sink, depending on their density. Ultimately, the water organizes into “swirls or vortices of fluid that carve the weird ripples into the ice.”
More work is needed to understand the complex interplay of factors that may generate these and other shapes on ice melting in nature
A sustained, warm, strong wind can make large areas of an ice sheet quite rough for skating, sailing and even walking. Wind Scallops are typically 3 to 6" across and 1/2 to 1+" deep. The formation mechanism appears to be from eddies that form on the downwind side of small ridges (or grooves?) in the ice. Once started, they make their own ridges. They can be mostly separated from each other or they can overlap when the conditions are intense and sustained enough. They appear to get bigger at higher wind speeds. The wind speeds in this case were 20-30 mph sustained with higher gusts up to 40 mph. The air temperatures held in the upper 50's for about 1-1/2 days.
Ice loss in scallop formation is due to a combination of melting and evaporation. Melt water can drain through pores in the well thawed ice sheet. Evaporation may be the more significant factor given the high wind speeds and air temperature. The scallops are similar to scallops found on hard rocks in very strong rapids.
They typically are more frequent and larger away from a windward shore. They tend to be more developed in areas where the wind is intensified such as around points of land. Very large areas can be affected so it is wise to not expect things to get better if you just go far enough (an approach that often works for cobblestone ice). The best method for avoiding scalloped ice is stay on windward shores that are often protected somewhat from the strongest winds coming off the land. The downwind side of a pressure ridge might offer a smoother path although keep in mind that it is likely that, in these conditions, ridges may be completely melted away making them uncrossable and not offering much of a wind break for the ice sheet. In general, conditions that are warm and windy enough to scallop the surface are also likely to create many other problems with the ice sheet.
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