Why is snow white?

by Harijan

Why is snow white?

Ripped from Wiki Creative Commons

Ripped from Wiki Creative Commons

As strange as it may sound, snow is white because of air.

But snow doesn’t necessarily have to be white. For instance, a ball of snow submerged in liquid nitrogen is would be clear. And if you were to take just one snow flake, which is a crystal, it would appear clear as well.

I am not playing a semantic game of sorts. Yes, snow is white in its most natural state as an accumulation of snow flakes which has collected after a night of winter storm. However, it is not the crystals themselves which gives snow the appearance of being white.

Now someone versed in optical physics might say, “well, snow is white because, as a system of snow crystal matrix, the sum of refraction through the crystals basically creates a diffuse reflection.”

Ripped from wiki creative commons

Ripped from wiki creative commons

That statement I would say is as true as if I were to say “Cars move because the engine turns the driveshaft and the axles.” But then what I would have done is to transfer the responsibility of movement from the car to the engine, without getting to the heart of the question, that which begs for the explanation that the chemical combustion process – oxidation of carbon fuel – is moving the engine, driveshaft, differential, axles, wheels, and ultimately, the car.

Of course, snow indeed is white because of the sum refraction of ice crystals. And what gives rise to the individual refraction on the ice crystals?

Well, if you remember the definition of refraction, it is the phenomenon of light “bending” at a surface boundary between two media of differing refractive index – say, between air (1.000277) and water (1.333).

Ripped from wiki creative commons

Ripped from wiki creative commons

The above statement about refraction then begs the question “what is a refractive index?” A refractive index is the speed through which light propagates through a given media – like water or air. The higher the refractive index, the slower the speed of light in the medium such that light moves through water at about 1/1.333 of the speed that it does in vacuum (which has an index of 1).

At this point, most people are ever so sorry for having asked the question “why is snow white” because they feel as though they have opened a can of worms. Does the answer really have to be this convoluted and complicated?

No, it really doesn’t. But that’s why I have decided to sit down and spend this Sunday morning explaining this. We’re almost at the end.

The answer to the question is in the nature of the thing we have called snow, and this is why “air” is a critical component of the definition of “snow”, which we perceive to be white when we look at it and crumbles beneath our shoes when we step on it.

The thing that allows snow to be white is the diffuse reflection. And this diffuse reflection is caused by all the millions and billions of refractions. Each of these refraction happens because of the different refractive indexes of ice (water crystals, snow flakes, etc) and air.

Thus we come to explain that snow not just with ice but ice and air.

If this is true, then can we take air out of snow and see it turn clear? Yes, of course!

If you take a wad of snow into your hands and squeeze them, what do you have? A snowball. And let’s say you were.. or rather, remember, in an epic childhood snowball fight.

Your team was losing ground and the enemy was coming up to your snow-fortress. Desperate situations require desperate response. So what did you do with that snowball? You squeezed it some more.

And what did you have in your hands?

It was hard and not so white anymore. It was the inevitable product of the escalation of arms race that was the winter wonderland conflict. And that inevitability was no longer white as snow but clear as the ice. That was to say the white was becoming clear as the snow was turning into ice, which was air escaping and ice crystals binding.

Left: snowball. Right: iceball.

Left: snowball. Right: iceball. Ripped from real world.

Someone might say… well couldn’t you have white snow that doesn’t have air?

Yes, snow can be white without air.

If we were to put a block of snow in a cold vacuum chamber, then you could take all the air out of it, and the snow would still be white. This is because the snow crystals would continue to retain all the refractive properties of the ice crystals which are now separated – not by air (index 1.000277) – but by vacuum with a refractive index of 1.

In the vacuum winter wonderland, the refraction would not have changed much from the real winter wonderland (1.000277 is practically 1). So it is possible to have white snow without air. It’s just that we began talking about snow in its natural environment. Out in the real world, we observe snow to be white, and the reason real world snow appears white is because of the air within.

Snow is white because it has air.