1. Why a Pile of Clear Ice Crystals Looks White
Snow is made of many small ice crystal grains gathered together [2]. Between these grains, there is air. At the boundary where ice meets air, light bends or reflects [1]. Because the grains point in many different directions, light changes direction many times inside the snow and scatters out in all directions [2]. When light of all different colors is reflected back in almost the same way, the object looks white [1].
Sugar and salt are similar. Each grain is transparent, but when they are gathered together, they look white. On the other hand, ice that is packed tightly with no gaps lets most light pass through, so it looks transparent [2]. What decides the color of ice is not just what it is made of, but whether it is a collection of grains or a solid block.
2. How Ice Absorbs Red Light More Than Blue Light
Ice absorbs very little visible light, but it absorbs different colors by different amounts. According to one paper, the distance light travels in ice before it is absorbed (the absorption length) is about 2 m (about 6.5 feet) for red light (wavelength 700 nm) and about 200 m (about 656 feet) for blue-violet light (400 nm) [3]. This means blue-violet light can travel about 100 times farther than red light. These distances of 2 m and 200 m are lengths inside solid ice, not the thickness of snow.
The radius of a typical snow grain is about 0.1 mm (100 μm). The paper estimates that even if light refracts 1,000 times before returning to the snow surface, the total distance it travels inside the ice is only about 20 cm (about 8 inches) [3]. This is an estimate based on assumptions. At a distance of 20 cm, red light is not absorbed enough to stand out. That is why the surface of snow looks white. The report states that the reflectivity (albedo) of surface snow in Antarctica is 0.99 for blue-violet light and 0.96 for red light [3]. Both are close to 1, and the difference is small.
3. How the Color Changes as You Dig Deeper into Snow
An explanation from the University of Alaska describes the color changes in a clean, deep hole in snow. Near the entrance, it looks yellowish. As it gets deeper, it changes to yellow-green and blue-green, and at the very bottom, it becomes a vivid blue [5].
According to research published in the journal of the Meteorological Society of Japan, light inside the snow hits the grains and changes direction many times (multiple scattering). During this process, red and orange light, which are absorbed more easily, decrease first. Blue light, which is absorbed the least, remains relatively more. This is why the inside looks blue [4]. When light passes through ice, red is absorbed and blue passes through. The color of the snow hole changes depending on the distance light travels through the snow or ice. In shallow areas where the distance is short, it looks white or yellow. As the distance gets longer, blue becomes more noticeable [5].
Safety Note: The absorption lengths for red and blue-violet light differ by about 100 times. So, as the distance light travels increases, red decreases first, and blue remains relatively more.
4. Whether New Snow and Old Snow Look Equally Blue Inside
In the research mentioned above, calculations showed that snow with larger grains looks bluer inside. This means that old snow or granular snow with larger grains may look bluer inside than new snow with smaller grains [4]. However, this is the result of a calculation using a multiple scattering model. It is not a guarantee that actual snow will always look this way. Since it is a calculation, actual snow might not always be the same color.
This calculation tells us that "grain size" is a clue when studying the color of snow.
5. How the Blue of Glacier Ice Is Connected to Blue Snow
In glacier crevasses (deep cracks) and icebergs, the ice looks clearly blue. Sea ice with few bubbles can also look bluish [3]. The United States Geological Survey (USGS) explains that glacier blue ice is blue because red and yellow light are absorbed [6]. However, the USGS also explains that ice containing bubbles or sediment does not turn blue [6]. In ice with many bubbles, light scatters over short distances, making the blue color less noticeable.
The blue of snow and the blue of glaciers share the same reason: "ice absorbs red light." In snow, blue remains because light travels a long path while scattering off grains. In glacier ice, it is thought that light travels a long distance through ice with few bubbles, causing it to turn blue. The section heading in the paper was "Ice is blue, but snow is white" [3].
6. Why Snow That Reflects Light Well Can Cause Sunburn
According to the Japan Meteorological Agency's explanation of the UV Index, the reflectivity of ultraviolet rays on new snow reaches 80%. Grass and asphalt are below 10%, and sandy beaches are about 25% [7]. This 80% is the reflectivity of ultraviolet rays, not visible light. Because a large percentage of ultraviolet rays bounce off the snow surface, it is said that people are more likely to get sunburned in snowy places. These numbers show that white snow is a surface that reflects light well.
7. How to Test Why Things Look White Using Sugar in Your Kitchen
Try placing a little sugar on black paper and observe how it looks. Each grain of sugar should be transparent. What color does it look like when gathered together? Snow and sugar have different contents inside their grains. However, both snow and sugar look white because the collection of grains scatters light [2].
If you want to know more, you can read the explanation of glacier blue from the Institute of Low Temperature Science at Hokkaido University [2] or the Japan Meteorological Agency's explanation of the UV Index [7].