There snow it is a form of atmospheric precipitation composed of a mix of ice crystals and air. It is precisely its particular structure that gives snow its characteristic white colour: when light passes through a snowflake, the rays corresponding to the individual colors are deflected and diffused in all directions, effectively bringing to our eyes a mixture of all colors, which our brain perceives very close to pure white. This happens even when ice is transparent: the latter has a more compact and homogeneous structure, so the light does not refract much and appears transparent.
Because the snow is white
We know that if we see colors it is thanks to light, which is made up of different wavelengths. When light hits an object, part of these wavelengths is absorbed while the rest is reflected. Our eyes collect reflected light thanks to special cells in the retina that send messages to the brain.
Basically, the way light is dispersed and transmitted through the structure of snow and ice it determines the color difference between the two.
The ice crystals that make up snow are randomly organized and are mixed with air, thus forming a highly porous structure. Because of this distribution, when it hits the snow, the light it spreads, it refracts and yes reflects in the ice crystals over and over and in all directions. This particular phenomenon, where light hits an object and comes widespread within it a very high number of times, it is called multiple scattering and that’s why we see snow white.
Indeed, this dispersion of light within the structure of the snow involves all wavelengths of visible light, and because they are reflected in similar proportions, the reflected light we see is overall white. The smaller the crystals, the more likely it is that light will spread through the snow. Usually the average crystal size varies from a few micrometers up to millimeters in the largest flakes.
Furthermore it must be said that snow has a surface reflective power (the so-called albedo, i.e. a measurement of how much light reflects on the surface of a specific material) is very high and for this reason it reflects 90% of the incident visible light, which is why we see very white snow. This property makes it one of the most reflective natural materials. When, at higher temperatures, the snow begins to melt, the presence of liquid water between the crystals reduces the light diffusion effect, decreasing the albedo.
Because ice is transparent
There ice structurehowever, it is more compact and less porous compared to snow, with an extremely regular crystal lattice and very few disturbing interactions and, usually, does not include bubbles or impurities. For this reason light passes through this type of crystalline structure without being significantly widespread or dispersed. In this way the light rays can pass through the ice in a direct and more linear way, without being disturbed on their journey, which is why we see it transparent.
In some light conditions, ice appears bluerather than fully transparent, because it selectively absorbs the longer wavelengths of visible light (red and yellow), while better transmitting the shorter ones, such as blue. This effect is particularly noticeable in very large blocks of icewhere the path of light within the ice amplifies this selective absorption.
The natural color of snow or ice can be altered from organic matter, dust and salts: for example, soot particles reduce the reflective power by absorbing more light.
Because it is useful to know this difference
Know the structural differences between snow and ice, their optical properties and their visibility, is crucial to calculate the balance between the energy that the Earth receives from the Sun and the energy that the Earth radiates into outer space (terrestrial radiation budget), useful for climate change monitoringunderstanding the Earth’s climate and developing climate models.
Knowing this parameter allows us to study the interaction between atmosphere and surface, to monitor temporal and spatial variations, and therefore also to optimize environmental policies. Also, understand the differences between the reflective powers of these two different surfaces, allows you to use the remote sensing and the satellite images For monitor the extent of the ice sheets.
