What causes "watermelon snow"?

SDStaff Jillgat replies:

I’ve seen pink and red snow while climbing up around 12,000 feet in the Sierra mountains, and it always served as a not-so-subtle reminder to me of what my path might look like, should I lose purchase with my ice ax and go tumbling down the snow slope with the ax flailing and crampons on my boots (i.e., rolling again and again over hard sharp metal things). Eek. Never knew what actually did cause it, though, and didn’t dig around to see what might be under it. There are good pictures of pinkish/reddish snow on this site, and also some good history on how it was first identified.

Turns out pink snow is caused by an algae called chlamydomonas nivalis, which sounds like something you might have treated at the local public health clinic after a weekend of partying. There are actually more than 350 kinds of algae that survive in very cold temperatures. These algae can turn the snow black, brown or yellow (disregarding for the moment the obvious cause of yellow snow), too. Chlamydomonas nivalis tends to flourish when the weather warms up a little after the darkest, coldest part of winter. It starts out green, then turns pink or reddish as the weather brightens. The cells have a gelatinous sheath that protect them from the strong ultraviolet radiation at high altitudes, and this sheath is what produces the pink color.

Some kinds of algae that thrive in snow can be toxic, so it’s smart not to eat colored snow. The Website I included above reports, too, that this snow can have a distinctive watermelon scent, and says, “There are unconfirmed reports that consuming ‘generous quantities’ of pink snow may cause diarrhea, a rather distressing situation above timberline.” I’ll say. I ate a whole bag of dried apricots once while backpacking, and … anyhoo. Several sources I read theorized that consuming pink snow would have a laxative effect, but no one seemed to want to try it. Be my guest.

There is also a lawn fungus known to gardeners as “Pink Snow Mold” (Michrodochium nivale), but this probably isn’t what you’re describing. Pink snow mold grows on grass occasionally under cool, damp conditions with or without snow cover.

Another theory, from an eminently reputable source; “The Cat in The Hat Comes Back” by Dr. Seuss, describes pink snow resulting from a desperate attempt to remove cake frosting from the bathtub, then mother’s dress, then the wall, father’s shoes, the rug, the bed, and finally leaving the snow all around the house pink. To turn pink snow white again, you need VOOM, carried in the hat of Little Cat Z.

SDStaff Jillgat, Straight Dope Science Advisory Board

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