A recent article in the local newspaper asks the question "Why does snow glow blue?"

The author gave one inspiration for the article as "..the way white snow glows turquoise in the holes left by boots or ski poles..."

The explanation given in the article is the same as that you can find elsewhere on the internet, and certainly applies to glacial ice: absorption of the red end of the solar spectrum upon passing through ice. Does it indeed apply to "holes left by boots or ski poles" in snow?

(The answer is "Not in fresh, light snow, though possible in more compressed or wetter snow." And for some quirk of this blog software, I cannot put the following plot in its proper location further down, so I place it here. Please ignore it for now and refer to it at the end of this post.)

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Most snow crystals are single crystals. Being single is an outcome of their growth process and small size. On the other hand, most larger ice formations are not single crystals. These latter types are called "polycrystals". A polycrystal usually appears the same as any other type of ice -- smooth, uniform, clear or white -- just as if it were also single crystal. But the poly nature can be revealed when the ice warms to the melting temperature. At melting temperature, the boundaries between the separate crystals become visible. As an example, note the white lines in the ice in the inset in the image below.

(This image is from the ice drip complex in the previous post.) We call each individual crystal a "grain", and the boundaries between grains as "grain boundaries". The grain boundaries show up because the region has disordered ice that will melt at a lower temperature than regular ice. Thus, the light, upon passing through a grain boundary, will scatter more, making a whiter region, as shown in the image.

The ice is thus weaker on a grain boundary and has a tendency to break along these boundaries. On the surface of ponds and lakes, the grains (individual crystals) can be several inches or more across. So, if you get a sheet of such ice, and let it warm up to melting, you will find it easy to break the ice sheet along a grain boundary and thus isolate a large single crystal.

The size and pattern of the grains affect the mechanical properties of the ice. So, glaciologists, who want to know how a given glacier or ice sheet moves, are very interested in such patterns. They call this pattern the "ice fabric".

-JN