What’s Blue on the Inside and White on the Outside?

February 04, 2013

Igloo (2)

Photographer: Rick Faber
Summary Authors: Rick and Kathy Faber; Jim Foster

It may take a second or two to identify the perspective of the above photo. Click here to see a view with sunlight shining upon it. Yes, the picture shows the interior of an igloo -- in Siskiyou National Forest, near Mt. Ashland, Oregon.

The color of the snow on the inside of this igloo isn't due to a blue colored light bulb, to an amazing fall of blue colored snowflakes nor is it due to the same mechanism that makes the sky look blue. Snow looks white to our eyes because snow crystals scatter the colors of sunlight that compose the visible spectrum about the same. Very little sunlight is absorbed by a snowpack -- most (upwards of 80 percent if the snow is clean) is reflected.

If newly fallen snow, or in this case packed snow, is sufficiently deep (more than about 10 inches or 25 cm), the myriad snow crystals both scatter and absorb sunlight -- even on cloudy days. The longer wavelengths (red and yellow) of visible light are more readily absorbed than are the shorter wavelengths (greens and blues). Eventually, with enough snow, the red light is completely absorbed. Note that if the snow is really deep or the igloo blocks substantially thick (perhaps 10 ft or 3 m, depending on the snow density), the blue color would disappear as well -- all of the light would be absorbed.

Photo details: Camera Model: Canon PowerShot SD1000; Focal Length: 5.8mm; Aperture: f/2.8; Exposure Time: 0.0063 s (1/160); ISO equiv: 80.