In the high alpine region, at altitudes of 10,000 to 12,000 feet, sometimes patches of pink or red appear on snow. The phenomenon is commonly seen during the summer months in the Sierra Nevada of California where temperatures remain low enough for snow from winter storms to hang around until summer. Compressing the snow with your boot leaves a distinct footprint the color of watermelon pulp, and the color can stain your boots or pant cuffs. The snow even has a fresh watermelon scent and is therefore commonly called "watermelon snow."

The mysterious snow had puzzled climbers, explorers and naturalists for thousands of years, including the ancient Greek philosopher Aristotle who left the first-known written accounts of watermelon snow. At that time, no explanations were given. Centuries later some people began to speculate that the reddish color was geological in nature, either being caused by mineral deposits on the snow or chemicals leaching from rocks.

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In May 1818, four ships sailed from England to search for the Northwest Passage and chart the Arctic coastline of North America, but severe weather made them turn around. While passing back by the northwest coast of Greenland, Captain Ross noted many blood-like red streams coming down snowy slopes. He had several crewmembers bring back samples of the red snow with them to England. By the time the samples reached England, the snow had melted into a dark red liquid resembling red port wine.

The liquid underwent extensive tests, as was possible during those times, but scientists couldn't arrive at a conclusion. It was then suggested that the color might be derived from the soil on which the snow fell. Finally, it was erroneously concluded that the reddish color was caused by meteor deposits.

Captain Ross published a full account of his expedition by the end of 1818, and within the documentation he included a botanical appendix written by famed Scottish botanist Robert Brown. In that appendix, Brown speculated the reddish-colored snow was caused by a type of algae. It would not be until the late 1800s before scientists would correctly determine that watermelon snow was actually caused by high concentrations of microscopic algae, known as snow algae, but it was Brown's assessment that provided the foundation for the scientific discovery.

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Snow algae, or chlamydomonas nivalis, is actually a green algae but also contains a bright red carotenoid pigment, which protects the algae from the sun’s ultraviolet radiation. The red color also absorbs heat, which provides the alga with liquid water as the snow melts around it. This can cause "sun cups" which are shallow depressions in the snow, just like cryoconite holes on glaciers.

Unlike most species of algae, snow algae thrives in the cold. During the winter months, when snow covers them, the algae become dormant. Then, during spring and summer, increasing amounts of light, snowmelt, and nutrients triggers the germination process of the snow algae. Once they germinate, the algae rise towards the surface of the snow where they appear as pink patches. The algal blooms may extend to a depth of 25 cm, with each cell measuring about 20 to 30 micrometers in diameter, about four times the diameter of a human red blood cell.

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Sources: Wikipedia / Wayne's World / Summit Post