Step outside after a winter storm and the contrast is striking. Long icicles hang from the roofline, so clear you can see right through them. Below, the snow is piled up in drifts so white it almost hurts to look at. Both are nothing but frozen water. So why do they look so different?
Here's the answer: it's all about shape, not substance. A single piece of ice is clear because light passes straight through it. But snow is made of countless tiny ice crystals with many small faces, edges, and hollows. Light bounces off all those surfaces in every direction, and when all the colors of sunlight are scattered back to your eyes together, you see white.
One Big Window vs. a Million Tiny Mirrors
Think of an icicle as one smooth, solid chunk. Light enters, travels through, and comes out the other side with little interference. That's what makes it see-through, a lot like a windowpane.
A snowbank is a jumble of crystals with air between them. Each time light hits a crystal face, some of it reflects and some of it bends in a new direction. After bouncing around among millions of these surfaces, the light comes back out scattered in all directions. Scientists call this diffuse reflection. Because it treats all colors of light about the same, the result looks white.
You've probably seen the same effect in your kitchen. A clear ice cube looks transparent, but a scoop of crushed ice looks frosty and white. Same water, same freezer, but a very different look once it's broken into lots of little pieces.
Why Some Ice Turns Cloudy
Ice isn't always crystal clear. Depending on impurities like soil particles or trapped air bubbles, it can range from transparent to an opaque bluish white. Those tiny air bubbles act like little scattering surfaces, much like the gaps in snow.
Frost shows this too. Soft rime, the feathery frost that coats branches on cold, foggy days, has a lot of trapped air, which makes it look white. Hard rime is denser and more transparent.
The Hidden Blue in Ice
Here's a bonus twist. Ice isn't perfectly colorless. Like water, it absorbs a bit more light from the red end of the rainbow, so very thick ice takes on a blue tint, slightly greener than liquid water. In a thin icicle, you can barely notice it.
In a glacier, you can't miss it. Glacier ice is squeezed so hard that air bubbles are pushed out, and light travels a long way through solid ice. With the white-making bubbles gone and lots of red light absorbed, glaciers glow a deep blue.
The Beauty of a Single Flake
Snowflakes owe their famous six-sided shape to the hexagonal structure of ice crystals. Surprisingly, studies suggest fewer than 0.1 percent of snowflakes have the perfect six-fold symmetry we see on holiday cards.
Starting in 1885, a man named Wilson Bentley photographed thousands of snowflakes through a microscope, revealing their incredible variety. Each flake on its own is a clear little piece of ice. Pile enough of them together, and they turn the whole world white.
So the next time you see snow and icicles side by side, you're looking at the same water dressed two different ways. One lets the light walk right through. The other throws it back at you from a million tiny angles.
