Why is snow white?
One of the reasons that so many people love snow is that it coats everything in a clean, "pure" white blanket. We even talk about snow in these terms -- weatherman say that we'll be getting "some of the white stuff" and every December you're likely to hear the song "White Christmas" over and over again. Snow wouldn't be snow if it wasn't white. But if you think about it, it seems weird that snow is white at all, since it's just a bunch of ice crystals stuck together. So where does it get its distinctive color?
To understand where the whiteness comes from, we need to back up and look at why different things have different colors in the first place. Visible light is made up of many different frequencies of light. Our eyes detect different frequencies as different colors. Different objects have different colors because the particular particles (atoms and molecules) that make up the object have different vibration frequencies. Basically, the electrons of the particle will vibrate a certain amount in response to energy, depending on the frequency of the energy. In the case of light energy, the molecules and atoms absorb a certain amount of light energy depending on the frequency of the light, and then emit this absorbed energy as heat. This means that objects absorb certain frequencies of light more than others.
A couple of different things can happen to the light frequencies that are not absorbed. In some material, when a particle re-emits the photons, they continue to pass through to the next particle. In this case, light travels all the way through the material, so the material is clear. In most solid material, the particles re-emit most of the unabsorbed photons out of the material, so no light, or very little light, passes through and the object is opaque. The color of an opaque object is just the combination of the light energy that the object's particles did not absorb.
So, since snow is frozen water, and we all know that frozen water is clear, why does snow have a distinctive color? To understand this, we need to back up and look at an individual piece of ice. Ice is not transparent; it's actually translucent. This means that the light photons don't pass right through the material in a direct path -- the material's particles change the light's direction. This happens because the distances between some atoms in the ice's molecular structure are close to the height of light wavelengths, which means the light photons will interact with the structures. The result is that the light photon's path is altered and it exits the ice in a different direction than it entered the ice.
Snow is a whole bunch of individual ice crystals arranged together. When a light photon enters a layer of snow, it goes through an ice crystal on the top, which changes its direction slightly and sends it on to a new ice crystal, which does the same thing. Basically, all the crystals bounce the light all around so that it comes right back out of the snow pile. It does the same thing to all the different light frequencies, so all colors of light are bounced back out. The "color" of all the frequencies in the visible spectrum combined in equal measure is white, so this is the color we see in snow, while it's not the color we see in the individual ice crystals that form snow.
For lots more information on snow, ice and all things cold, see the next page.