What color is your snow? In an urban environment, it’s best not to think about what happens to a beautiful snowfall, but in mountain and other wilderness areas where deep snows last well into the spring, the snow may be tinted red, green, orange, or, um, yellow.
Susan Milius begins her profile of microbiologist Brian Duval with the inevitable snickers about yellow snow, a fact of life for Duval. But the snow stains he studies are actually created by at least 350 species of algae, which thrive in “near-freezing, nutrient-poor, acidic, sun-blasted slush” to stain melting snow a rainbow of colors.
The blooming algae are the basis of a microscopic habitat, complete with tardigrades, micro-animals also known as waterbears and moss piglets. They are eight-legged, segmented creatures who can survive an incredible range of temperatures and last for as long as a decade without food or water. This is all a good reminder how adaptable and resilient life is. There are also some interesting implications here for the possibility of life forms on other planets: the “little green men” of science fiction may be quite small indeed.
Meanwhile, the blue of deep snow and ice, particularly glaciers, has another source. Glaciers are created by the compression of old snow, pressed down by more snow for decades, even centuries: ice-cores in Antarctica date back 800,000 years. Ivars Peterson looks into the resulting blue, and sometimes blue-green, of snow and ice in his short profile of meteorologist Craig F. Bohren.
Basically, snow reflects the entire spectrum of visible light, meaning it looks white to us. But when it’s piled high and deep, the density of ice crystals absorb more red light than blue. The scattering blue is what hits our eyes. Bohren notes that this is different from the blue of the sky: the sky is blue because molecules and fine dust particles scatter more blue than red light, except in the direction of the sun—particularly at the horizon, which is why sunrise and sunset can be so gloriously red.