The resemblance between certain types of snow and Styrofoam is a common observation. The explanation lies in the shared physical characteristic of both materials: a composition dominated by trapped air. This visual comparison involves the physical structure of frozen water, the optical physics behind its color, and the changes in its texture.
Snow’s Composition: Ice and Trapped Air
Snow is not a solid mass of ice but rather a highly porous composite material. Its physical structure is a delicate lattice of ice grains with numerous interconnecting pore spaces of air. The immense volume of air trapped within the snowpack gives it its characteristic lightness. In very cold, fluffy powder, the snow-to-liquid ratio can be as high as 30-to-1, meaning the snow is only about three percent ice and 97 percent air. This high porosity is the fundamental similarity to Styrofoam, which is polystyrene expanded into a foam containing mostly trapped air bubbles.
The Optics of White: How Light Scattering Creates Opacity
The whiteness of snow, which contributes to the Styrofoam resemblance, is explained by the physics of light scattering. While a single ice crystal is clear and translucent, a mass of them appears opaque and purely white.
When sunlight strikes the snow surface, it encounters countless boundaries between the ice crystals and the trapped air pockets. At these interfaces, the light is scattered through refraction and reflection, a process known as diffuse reflection.
Because the ice grains are irregularly shaped, the light is scattered repeatedly and randomly in every direction. This continuous, multiple scattering prevents any single wavelength of visible light from being absorbed. Since all visible wavelengths are scattered back to the observer equally, the snow appears as a uniform, bright white.
Snow Metamorphosis: Why Snow Becomes Granular
The resemblance to Styrofoam often extends beyond color to the material’s texture, particularly the appearance of small, granular beads or pellets. This texture is the result of snow metamorphism, the process of snow crystals changing their shape and structure after they have fallen. This transformation can happen quickly, driven by temperature and vapor pressure differences within the snowpack.
One specific type of precipitation that strongly resembles Styrofoam pellets is graupel, also known as snow pellets. Graupel forms when a snowflake falls through a layer of supercooled water droplets, which rapidly freeze onto the snowflake in a process called rime accretion. This creates an opaque, soft, and granular ball that is often mistaken for hail.
Even without graupel, the delicate, dendritic arms of fresh snowflakes begin to break down and round off due to destructive metamorphism. The ice attempts to reduce its overall surface area, causing water vapor to sublimate from sharp points and deposit in depressions. Over time, this process rounds the intricate crystals into denser, clumpier grains, giving the snow a distinctly granular texture similar to the small, packed beads of foamed plastic.