Despite the brutal heat of summer, snow exists during this season at elevations above 9,500 feet in Yosemite National Park, but its appearance may not be what people expect: instead of the powdery white goodness often seen on mountaintops, this snow has a startling pink hue. 

This strange phenomenon is caused by an algae found in melting snow that flourishes in frigid temperatures, according to a Facebook post from the park on August 4th. The snow algae, also known as Chlamydomonas nivalis, contains not only chlorophyll but also a vibrant red carotenoid pigment that acts as a sunscreen to protect the organism from harsh UV rays at high altitudes. 

“The pigment dyes the surrounding area a darker color, giving the effect of a pink or red snow field, and allows the snow to heat up faster and melt more quickly,” Yosemite National Park wrote in their post. 

Its unique color, as well as its faintly sweet smell, earned the algae-filled snow its tropical fruit nickname, according to Wayne’s Word, an online textbook of natural history from Palomar College. 

“Compressing the snow with your boot leaves a distinct footprint the color of watermelon pulp. The snow even has a fresh watermelon scent and is sometimes called watermelon snow,” the textbook states. 

Watermelon snow mostly appears during the summer, as the hot weather and abundant sunlight provides the most ideal environment for the algae to thrive. 

“The algae are present in the winter, but not actively growing, and their growth is inhibited by the fact that snowfalls keep covering them,” Dr. Thomas Artiss, an upper school biology teacher, said. “In the spring and summer, temperatures warm and the algae are exposed to sunlight, and they begin to grow actively. 

Sophomore Alexa Lowe, who saw the snow three years ago at Mt. Lassen, describes watermelon snow as a crimson shade whose color caught her off guard. 

“I was surprised at first because I had no idea what it was,” Alexa said. “It was hard to believe it was a living organism because it really looked like blood with its bright red color that stood out from the background.”

Although the sight may seem bizarre and rare, watermelon snow was discovered more than 2000 years ago when Aristotle first wrote about colored snow, according to Wayne’s Word. At first, humans attributed the odd discoloration to oxidation products from rocks or iron deposits from meteors. It was not until the nineteenth century that scientists, with the aid of more advanced microscopic technology, determined the cause of the pink snow to be green algae that secretes a red sunscreen, according to a study published in 2006.

Some scientists believe that the algae may be relevant to predicting the effects of climate change. The red pigment the algae produces causes the snow to melt faster, which then allows for more algae growth. 

“Imagine wearing a dark red T-shirt on a sunny day rather than a white one,” Dr. Artiss wrote. “It’s a classic example of a positive feedback loop, and since this algae can be found on snow all over the planet, it may well play a role in melting snow and glaciers faster than if it weren’t there.

However, unlike its namesake, watermelon snow may not be as tasty or as edible. 

“As a Canadian expat, we were taught the golden rule of Canadian winters was not to eat yellow snow. Apparently, this should be extended to pink snow as well,” Dr. Artiss wrote. “It may or may not taste like watermelon, but it is apparently mildly toxic, and anecdotally, a laxative.”