Ask Sam: Why is Some Ice Black And Some Ice White?
Every other Friday on Morning Edition NHPR’s Sam Evans-Brown tracks down answers to questions about the environment and outdoors for our listeners in a segment we call “Ask Sam.” This edition of Ask Sam originally aired in January, 2020.
Ruth from Sandwich asks: “I just was skating on beautiful Squam Lake with black ice, and we could look down and see some other stuff. I was wondering if you could talk about ice forms, why there’s cracks, why there’s little ridges, why there’s bubbles… all that stuff.”
I’m going to demur on the question of why there are cracks in the ice, since it’s been answered masterfully elsewhere, and since I think the question of why there are bubbles and schmutz frozen into the ice is self-explanatory, so I’ll skip that one too. (Sorry Ruth.)
But there is some nifty science stuff embedded here, and I think we should start with a fundamental question: what color is water? (Yes, I know that the real answer is that its color is a phenomenon of human perception, but I've already started by being unhelpful to Ruth so I’m going to leave it aside.)
If you don’t overthink it, and answer exactly as if you were a middle schooler contemplating which crayon to grab in order to draw the ocean, you should answer that water is blue. This is not a trick question; that is the right answer. When light enters water, the molecules absorb more of the light that’s on the red or yellow end of the spectrum — the longer wavelengths — and reflect back the blue ones — the shorter waves.
Water looks clear in a glass because there’s just not enough of it. Think of a cup of tea on the counter: you look inside and it looks like tea, but if you spill a tiny bit on the counter it looks clear. The molecules in liquid water are not dense enough to reflect back enough blue for our eyes to perceive its blueness until it is rather deep. This is why shallow water can look transparent, but as it gets deeper that same water starts to look more and more blue.
Similarly, in its purest form ice is also blue. If you need proof of this, just do a google image search for glacier ice. Glacier ice starts as snow, and then becomes more and more compacted by the weight of new snow on top of it. The pressure forces out impurities until eventually it’s just very dense crystals of mostly water, and in this form you’re really able to see the water’s elemental (errr… molecular?) blueness. When the conditions are just right, like they were on Lake Michigan in 2016, that same thing can happen to lake ice as well.
OK, so if the real color of water is blue, why does ice look white and or even black?
Witchcraft? No. Crystals!
“People kind of can break things down into black ice and sometimes they can say white ice, and really it’s neither of those colors,” explains Lauren Farnsworth, a research scientist with the Army Corps of Engineers Cold Regions Research Lab in Hanover.
As ice forms, it shapes up into crystals, and if it’s windy or anything else that causes turbulence in the water like a stream flowing in, the turbulence interrupts the growth of those crystals. In other words, turbulent water equals more, smaller ice crystals, instead of fewer, bigger ice crystals.
“Basically if you have smaller crystals, you’re going to reflect light more easily and more often,” explains Farnsworth.
Just think of gemstones: sometimes you look at a diamond and you see a little glint... a little flash of white. That's the light being reflected off of the facet of a crystal. So called “white ice” is that same effect multiplied many many times down through the ice. Those rare “black ice” conditions happen when you have calm, still nights which give you big, uniform ice crystals. (There’s a deep rabbit-hole that you can vanish down about the shape and alignment of ice crystals, as well, but I’ll sum it up by simply saying that for ice to be transparent, the crystals need to be aligned, which also happens more often when the water freezes slowly and without being disturbed.)
To make this simple, “white ice” is ice with many crystals formed at odd-angles to each other, and the white is the glint of light being reflected back to your eye. “Black ice” is ice with fewer, large crystals formed all in alignment, so that the ice is transparent, but looks black because you’re able to see the lake beneath it, which between the water and the lake bottom absorb all the light and reflect very little back.
So, what color is ice? Well, what is color anyway? Given that our eyes are weak, and easily tricked, maybe the best thing to do is to not dwell too long on that question, and just enjoy the skating while it’s good!
Sam Evans-Brown, is host of NHPR’s Outside/In which you can subscribe to where-ever you get your podcasts. If you’d like to submit a question you can record it as a voice memo on your smartphone and send it to email@example.com, OR call the hotline, 1-844-GO-OTTER.