Since pure water is clear, and snow is frozen water, it might seem as though snow should be clear – not white. However, snow appears white – although on very rare occasions, it assumes other hues or appears blue when heavily compacted (the blue ice phenomenon in glaciers). Snow owes its appearance to the interplay of its molecular structure, light, and our perception of color.
◊ The molecular structure of snow ◊
Did you ever notice that when you crack a clear ice block, the crack appears to be white? It is because the change in the molecular structure creates a structural defect that impinges the transmission of light. Snow is comprised of hundreds of microscopic ice crystals. When water freezes, its molecules form a complex “crystal lattice” structure. This structure, and the presence of hundreds of ice crystals, makes snow translucent instead of transparent. Snow particles contain air in them, since frozen water has a higher volume. However, if snow is heavily compacted – such that air is compressed out of it – the blue ice phenomena observed in glaciers would be apparent.
◊ Translucency versus transparency ◊
Transparent objects allow light to pass through. Translucent materials also allow light to pass through, but not in a straight beam. Therefore, the light passing through snow does so diffusely. Light passing through objects is reflected, absorbed and refracted to varying degrees. In the case of an optically transparent material (like pure water), there is far greater absorption with insignificant degrees of refraction and reflection. Snow, with its different molecular structure, has a high degree of refraction and reflection. In fact, each ice crystal in that complex lattice structure refracts light, enhancing the brightness and whiteness of snow.
◊ White light and snow ◊
The reason snow appears white to us is also because of the light in which we view it. White light is essentially a combination of all the colors in the spectrum, as demonstrated by refracting a source of white light through a prism. Unsurprisingly, solar light is a major source of white light – which merely enhances the whiteness of snow caused by light refraction.
◊ Visual perception ◊
Color is not an objective phenomenon; much of it is dependent on our visual perception of it (both physical and psychological). Therefore, the generally white appearance of snow is not just a physical property of snow. The appearance of color is based on how our cone cells respond to light frequencies; different frequencies of light are perceived as different colors. Our visual perception is based on selective reflection and absorption by an object. Snow appears white because refraction does not alter the amalgamated light frequencies and reflects all the light frequencies back to our eyes.
It is important to note than snow is not necessarily always white. Sometimes, impurities in snow (algae, for e.g.) can cause it to appear tinged with other colors. However, the same reason snow appears white is the same reason that ice, with structural defects like fissures and cracks, appears partially white. Whether snow is on the ground or falling through the air, it constantly interacts with its physical environment and our visual perceptual system. Basically, the translucent nature of snow leads its white appearance, although the tiny ice crystals comprising it are transparent.
