Yellow snow from urine is a source of winter humor, but snow comes in other colors, too. Here’s a look at the causes of colored snow and whether it’s safe to eat it.
Yellow snow near a tree or spelling out a word gets its color from urine. Urine, in turn, is yellow because it contains the pigment urobilin, which comes from the breakdown of hemoglobin from old red blood cells. Snow also turns yellow from pollen, algae, anthocyanins from leaves, dust, sand, and air pollution. Discolored falling snow happens when snow crystals form around particles in clouds. Patches of yellow snow come from adding a substance to snow after it falls or from microorganisms that grow under cold conditions.
Watermelon snow comes in shades of red and green, like a watermelon. Often, this snow even has a sweet, fruity scent. Any of a number of types of algae and cyanobacteria produce watermelon snow, including Chlamydomonas nivalis, Chlamydomonas alpina, Mesotaenium bregrenii, and Chlorooceae cyanobacterium. This type of snow is common in alpine and polar regions worldwide, where it may also be called blood snow or pink snow. The green color comes from chlorophyll, while the red hue comes from astaxanthin, a carotenoid pigment that also colors flamingos, crabs, and salmon that feed on algae. Watermelon snow has ecological importance, as it acts as a food source for many organisms and affects the rate of snow melt. The melted water is a water supply not just for the algae, but also for animals in winter.
Algae and cyanobacteria aren’t the only cause of green snow. Snow can pick up the color from vegetation beneath it, including chlorophyll from leaves and moss.
Red, Orange, and Brown Snow
Red, orange, brown, and rusty snow often comes from algae, but it can also result from airborne particles of sand, dust, and air pollution. The falling snow may even be a dramatic red color. Sometimes the dust comes from deserts or plains far from the site of snowfall. Iron-rich minerals contribute the rusty pigments. Because the colored snow isn’t pure water, it may carry an odor. The orange and yellow snow that fell over Siberia in 2007 had an oily texture and smelled rotten.
Gray and Black Snow
Gray and black snow is dirty snow. The “dirt” could be dust from a volcano, soot, ash, or motor vehicle exhaust. The snow may have a dusty or oily scent. Snow discolored by pure carbon might look bad, but isn’t dangerous. However, snow colored by petrochemicals is toxic.
White snow often appears blue. Partly this is because snow is water, which actually is blue in large quantities. Partly the blue color comes from the way snow crystals refract light. Snow most often appears blue in shadows.
This white snow that looks blue is clean and perfectly safe to touch and eat, really blue snow may be unhealthy. At best, the blue color might derive from non-toxic pollution, like the Easter-egg-blue snow that fell in St. Petersburg in 2017. Other chemicals that dye snow blue might contain cobalt, a potentially toxic metal.
Don’t Eat the Yellow Snow (or other color)
Aside from being gross, eating yellow snow can expose you to diseases. Eating other colors of snow also poses health risks. The exception is snow that’s white but appears blue because of shadows. Watermelon snow may taste sweet, but some times of algae release nasty toxins. Snow colored by dust, sand, or pollution may contain toxic metals. If you must eat snow or use it in snow ice cream, choose clean white snow. Use freshly fallen snow or else scrape away the top layer of older snow before collection.
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