Blue Light From Deep Snow
December 22, 2003
After a big snow storm, when you're shoveling your walkway or just measuring how much snow fell, have you ever noticed a turquoise or robins egg blue color? The color isn't due to blue snowflakes nor is a result of the same process responsible for making the sky look blue. Rather it results from the optics of large, non spherical particles. If new fallen snow is sufficiently deep (more than about 10 inches or 25 cm), the myriad snow crystals both scatter and absorb sunlight (even on an overcast day). The longer wavelengths (red and yellow) of visible light are more readily absorbed than are the shorter wavelengths (greens and blues). Eventually, the red light is completely absorbed, and only blue light emerges when a hole is poked into the snow. If the hole is deep enough, the blue color completely disappears since all of the light will be absorbed.
The photo above was taken last February in Silver Spring, Maryland the day after a powerful nor'easter buried portions of the Middle Atlantic region and New England under nearly 2 feet (60 cm) of snow. Anytime this thick mantle of snow was jabbed with a yard-stick, blue light was liberated. Here's hoping that you have a nice Christmas and that no blue snowflakes start falling or blue memories come calling.