Saturday, December 19, 2009

Snow and entropy

Snowfall not only evokes the predictable platitudinous admonitions to "only travel if it's absolutely necessary", but from a more recondite perspective, also partially preserves the entropy of light.

Snow, of course, is white in colour, so it reflects all the colours of the spectrum. In contrast, the normal landscape, consisting of greens and browns and greys, absorbs all the other colours of the spectrum, and in each case reflects just one visible component of the sunlight impinging upon it (the absorbed energy is re-radiated in the infrared). Snow therefore preserves the spectral information in sunlight which is otherwise destroyed by the natural landscape concealed beneath.

Moreover, snow which has frozen overnight will sparkle in sunlight the next day, consisting as it does of numerous ice crystals. In effect, the surface of the snow consists of millions of tiny mirrors, each oriented at a different angle. Each tiny mirror reflects sunlight in a different direction, from a unique position, so the snow sparkles as the observer moves about and the line of sight intersects the reflected sunlight from differently positioned and oriented crystal facets.

A mirrored surface is distinguished from a white, non-mirrored surface simply by virtue of the smoothness of the former. A smooth surface permits the specular reflection of light, in which the directional information in the light is preserved, whilst an irregular surface results in diffuse reflection, which destroys the directional information. A mirrored surface therefore prevents the entropy of light from increasing by virtue of preserving both the spectral information and the directional information in the light. The blanket of sparkling white snow which covers a landscape, therefore operates as a mechanism to reduce the increasing entropy of light.

But do wrap up warm, because the wind will make it feel even colder.

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