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The Washington Post's Antarctic heat wave melted 20 percent of an island’s snow cover in days, caused melt ponds to proliferate includes the figure below of meltwater ponds on top of snow/ice.

The article is a good read, but my question is one of optics, namely what makes a pond of meltwater (essentially pure water) on top of something so white become so dark?

It increases sunlight absorption and heating, which in this case is "bad" but what's the physics or phenomena behind this that makes these so dark in the first place?

If I understand correctly, even shallow ponds or puddles of water can significantly reduce albedo, increase absorption and induce further melting, sometimes in a "runaway" manner. So I don't think it's bulk absorption by the water or contaminants, I think there's something more interesting going on.

Washington Post: Melt ponds on the Antarctic Peninsula’s George VI Ice Shelf on Jan. 19, 2020. (NASA)

Melt ponds on the Antarctic Peninsula’s George VI Ice Shelf on Jan. 19, 2020. (NASA)

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Water has lowest EM absorption in the blue part of light spectrum and increases rapidly towards both UV and red parts of spectrum.

enter image description here

As a result in visible light water is blue. Same goes for the ice as it has very similar absorption spectrum.

While there is a lot of white in the picture, all of it is a thin snow cover on top of blue ice. Once the snow melts you get a blue water on top of thick layer of transparent blue ice that combined absorb most of the light making ponds looks exceptionally dark. Water would be so much lighter in colour if, for example, instead of transparent ice, you'd have sand on the bottom.

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    $\begingroup$ in addition to the changes in heat absorbsion by ice and water the different pollutants that have acumulated on the ice and snow comes to the surface and changes the albedo,things like dust-ash and algae living in the ice and snow. $\endgroup$ – trond hansen Feb 25 '20 at 6:54
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    $\begingroup$ See also: Why does glacier ice look blue? earthscience.stackexchange.com/q/13240 $\endgroup$ – Jean-Marie Prival Feb 25 '20 at 8:33
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    $\begingroup$ So the answer is really: snow has high albedo, ice doesn't, and water on snow turns very quickly into water on ice? $\endgroup$ – user18801 Feb 25 '20 at 10:59
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    $\begingroup$ Regarding sand, take a look at the photo in the background of my twitter profile: twitter.com/ericlippert This is a freshwater limestone reef with limestone and sand on the bottom; the entire reef is under water. At the shallowest point it is about a meter deep and at the deepest, maybe six or eight meters. You can really see how the blueness of the water changes how the light brown limestone and sand looks at different depths. $\endgroup$ – Eric Lippert Feb 25 '20 at 18:27
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    $\begingroup$ It's probably worth noting that ice and snow (and water) get most of their reflectivity from the boundary between air and water -- and snow has far more boundaries to reflect from than either water or ice. $\endgroup$ – Mark Feb 25 '20 at 21:02
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"Transparent" is not the same as "white" : white bodies reflect most of the light while transparent bodies let the light though. Once the light enters into water, it may need to travel a long way before it has a chance to go out, and that long travel path provides more opportunities for absorption.

Water absorbs light by itself much more than snow, but contamination makes the situation even worse. If there's a dirt particle in the snow, it has to be located within the top few centimetres to get any significant energy from sunlight: only 20% of light penetrate snow deeper than 5 cm . In clear water, light penetrates much deeper (25% of energy at 10 m depth), so all dirt particles within that volume will readily dissipate heat when exposed to sunlight.

If you wonder why snow is white, consider this: dry snow is a mix of ice (refractive index 1.31) and air (refractive index 1), so there's a lot of possible angles at which light will be reflected from the boundary (partially or totally), and a lot of boundaries as well. Wet snow is a mix of ice (refractive index 1.31) and water (refractive index 1.33), so those boundaries barely affect passing light. Solid ice or water have a single boundary with air and no internal boundaries at all.

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    $\begingroup$ It may be worth adding that the light is coming in at an acute angle, so that it can traverse a significant distance even in a shallow pond. Also, that the base of the pond (which will be slush, I think) is less smooth than white snow around the pond. $\endgroup$ – M Juckes Feb 29 '20 at 13:00
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Water (solid or liquid) has some absorbtion. It is rather low for pure water for visible or near-visible light and that's why water it is considered transparent. But only to an extent - few meters of water look blue and few hundred meters look black, esp. if you are UNDER those few hundred meters.

Then we have snow. Snow has an abundance of optical boundaries in all directions, so it scatters the light well. The light passes only few milimeters water in total before it gets reflected or refracted out of the snow mass. You don't absorb much light in a few mm of water.

If you compress or melt the snow, optical boundaries disappear and the light is allowed to go deep in the water body where it gets chance to be absorbed.

p.s. Water is not the only substance that looks white when powdered. Most powdered substances tend to look white-ish, even if in their solid form they are pretty much not white and just partially transparent. See sulfur, copper sulfate and so on.

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    $\begingroup$ To illustrate your last paragraph: I've been milling some lavas, grey basalt yields grey powder, while black obsidian yields white(ish) powder! $\endgroup$ – Jean-Marie Prival Feb 26 '20 at 11:50
  • $\begingroup$ Is this why snow shadows are blue? $\endgroup$ – RedSonja Feb 26 '20 at 12:18
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    $\begingroup$ More or like the same thing as when you drill perspex - the filings will be white due to rough/textured surface.... $\endgroup$ – rackandboneman Feb 26 '20 at 15:42
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    $\begingroup$ @RedSonja probably not. Ad hoc: the shadows are lit only by the sky and not by the Sun directly. The clear sky is blue (atmosphere scatters short-wavelengths more and it has to be clear in order to have shadows in the first place). $\endgroup$ – fraxinus Feb 26 '20 at 15:48

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