Snow is usually the simplest colour in a mountain landscape: white against dark rock and green forest. But high in California’s Sierra Nevada, patches of snow can sometimes turn pink, red, or even reddish-orange. These oddities, called watermelon snow, come from microscopic creatures that live in the snow.
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| Pink and red patches of watermelon snow appear on a high-elevation snowfield in California's Sierra Nevada. |
Pink snow has been seen in the high country of Yosemite, at elevations above 10,000 feet, where lingering snowfields provide a suitable habitat for snow algae. One such pigment is astaxanthin, and it protects these organisms from the sun’s harsh rays.
What Is Watermelon Snow?
Watermelon snow is snow that has been visibly coloured by blooms of microscopic snow algae. It is also sometimes called 'red snow' or 'blood snow' because the patches can range from pale pink to deep red. The name “watermelon snow” comes mainly from the resemblance of the pink-red snow to the flesh of a watermelon. Some scientific literature also notes that disturbed red snow can have a faint watermelon-like odour, although the smell is not the defining feature of this phenomenon. Snow algae are not a single species. Modern research has identified several different algae associated with red and pink snow, so it is no longer accurate to describe every example simply as Chlamydomonas nivalis.
Why Does Snow Turn Pink or Red?
The unusual colour comes from pigments found in the algae. Astaxanthin is a red carotenoid and one of the best-known pigments. In snow algae, this pigment is located around the cells and imparts a distinct red or pink colour to high concentrations of algae. The pigment also serves an important biological purpose: it helps protect the organisms from intense sunlight and ultraviolet radiation in exposed snowfields.
Fresh snow reflects a lot of incoming sunlight. When pigmented algae become concentrated near the surface, the snow becomes darker and absorbs more solar radiation. This makes for an interesting relationship between the algae and the environment. More absorbed sunlight can contribute to localised snowmelt, while the resulting liquid water creates conditions that allow the algae to remain active.
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| Snow algae create striking red and pink patches across the mountain snow. |
The Role of Chlamydomonas nivalis
Chlamydomonas nivalis is one of the best-known organisms historically associated with watermelon snow. It is a microscopic green alga adapted to harsh environments. Researchers have found it in summer snowfields in alpine and high-latitude regions, including the Sierra Nevada of California. Studies have indicated that its red colouration is due to astaxanthin stored in the cells.
However, modern taxonomy has complicated the old picture. Research published on red and orange snow has identified several algal species and has shown that some organisms historically identified as Chlamydomonas nivalis belong to other groups, including the genus Sanguina. So the safest description is that watermelon snow is a snow-algae phenomenon involving several species, with Chlamydomonas nivalis among the best-known examples.
Watermelon Snow in California’s Sierra Nevada
The Sierra Nevada provides an ideal setting for unusual snow algae because high mountain areas can retain snow well into late spring and summer. Yosemite Conservancy reports that hikers in Yosemite’s high country can encounter pink or reddish snow at elevations above 10,000 feet, with May Lake mentioned as one location where the phenomenon may be seen early in the season.
The broader Sierra Nevada is a major mountain system with extensive high-elevation snowfields. In Yosemite, the National Park Service notes that elevations around 10,000 feet and above can remain snowy into mid-June or later, depending on conditions. That lingering snow is important. Watermelon snow is generally associated with seasonal snowfields and other cold environments where algae can grow during periods when liquid water becomes available.
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| Microscopic snow algae can give lingering mountain snow a vivid pink or reddish appearance. |
When Is Watermelon Snow Most Likely to Appear?
Watermelon snow is particularly associated with spring and summer, when temperatures rise enough for snow to begin melting, but substantial snow remains on the ground. Snow-algae blooms can become visible when cells multiply to high concentrations within the snow. A scientific review notes that blooms commonly occur during spring and summer in semi-permanent snowfields and glaciers in temperate, mountainous and polar regions.
This variability is why the phenomenon can seem surprising. The surrounding landscape may already be experiencing warmer weather while pockets of snow remain in high, shaded or otherwise suitable locations. In Yosemite, the higher elevations can retain snow well after lower elevations have begun to dry out.
Does Watermelon Snow Really Smell Like Watermelon?
Sometimes, apparently yes—but the description should be treated carefully. Scientific research has shown that red snow can produce a faint watermelon-like odour when disturbed, which helps explain its popular name. The smell is not necessarily strong, and not every patch will have an obvious watermelon scent. The more reliable visual clue is its pink, red, or reddish colouration.
The old RiteBook article described how people compress the snow, which causes the red colouration to appear more strongly. The basic observation is consistent with the fact that snow containing concentrated algae can show much stronger colouration when disturbed, but the updated article should avoid suggesting that every patch behaves identically.
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| High-elevation snowfields in the Sierra Nevada can develop colourful algae blooms during the warmer months. |
Watermelon Snow Is Found Around the World
Although the Sierra Nevada provides one of the most beautiful settings for seeing it, watermelon snow is not unique to California. Red and pink snow associated with snow algae has been documented in alpine and polar environments on several continents. Scientific reviews describe snow algae communities occurring under extreme conditions involving low temperatures, strong radiation, and limited nutrients.
The National Park Service has also documented “watermelon snow” in other mountainous environments, describing it as a community involving microorganisms, including microalgae and bacteria. That worldwide distribution makes the phenomenon more than a strange feature of one California mountain range.
Why Snow Algae Matter
Watermelon snow may look like a simple visual curiosity, but snow algae are part of surprisingly active ecosystems. Research has shown that dense populations of Chlamydomonas nivalis can take up carbon dioxide and remain photosynthetically active within summer snowfields. One study measured substantial carbon dioxide uptake in heavily colonised snow.
The algae can also influence the snow itself. The red pigments make the surface darker, enabling it to absorb more solar energy than clean white snow. This may lead to localised melting of the snow. The U.S. Geological Survey has pointed to this connection in studies of “watermelon snow”—pigmented snow that absorbs more solar radiation than plain white snow and thus melts faster.
This does not mean that watermelon snow is the main driver of mountain snowmelt. Snowpack depends on many factors, including temperature, sunlight, precipitation, wind, and terrain. But snow algae are an intriguing example of living organisms that interact directly with their physical environment.
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| Concentrations of pigmented snow algae produce the unusual red colour known as watermelon snow. |
The Long History of Red Snow
People have been noticing strangely coloured snow for centuries. Yosemite Conservancy notes that Aristotle described the phenomenon more than 2,300 years ago. For a long time, however, the cause remained mysterious. Some historical observers proposed mineral deposits or material from surrounding rocks as explanations.
By the 19th century, explorers were still documenting red snow in polar regions and debating its origin. Scientific investigation eventually established that microscopic organisms were responsible for at least many of these red and pink snow events.
Today, DNA-based research and improved microscopy have revealed that the story is even more complicated than scientists once thought. Several groups of algae can contribute to coloured snow, and older names do not always correspond neatly to modern species classifications.
Related RiteBook Posts
If you enjoy unusual natural phenomena and colourful landscapes, you may also like these RiteBook articles:
Atacama Desert Flowers — how unusual rainfall can transform one of the world's driest landscapes with wildflowers.
Blue Pond Hokkaido: Japan’s Famous Shirogane Blue Pond — another unusual natural landscape where water, minerals and light create an extraordinary colour.
Lake McDonald and its Colourful Stones — a look at the strikingly coloured rocks in Montana's Glacier National Park.
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| Pink or reddish snow has been observed in Yosemite's high country above 10,000 feet. |
Watermelon Snow FAQs
What is watermelon snow?
Watermelon snow is snow-coloured pink, red, or reddish-orange by blooms of microscopic snow algae. The phenomenon is particularly associated with alpine and polar snowfields.
Why is watermelon snow pink?
The colour is due to pigments in snow algae, particularly carotenoids such as astaxanthin. These pigments can make the algae and surrounding snow look pink or red.
Is watermelon snow caused by Chlamydomonas nivalis?
Chlamydomonas nivalis is one well-known snow alga associated with red snow, but modern research shows that several different algal species can produce red or pink snow.
Where can watermelon snow be found?
Watermelon snow occurs in alpine and polar environments around the world. In California, it has been documented in the high Sierra Nevada, including Yosemite's high country.
Does watermelon snow smell like watermelon?
Some scientific observations report a faint watermelon-like odour when red snow is disturbed. The scent is not necessarily strong or present in every patch.
At what altitude can watermelon snow be seen in Yosemite?
Yosemite Conservancy reports pink or reddish snow at elevations above 10,000 feet in the high country.
Does watermelon snow make the snow melt faster?
Clean white snow reflects more solar radiation than pigmented snow, and this may lead to localised faster melting. Snow algae are one factor among many affecting snowmelt.
Is watermelon snow unique to California?
No. Red and pink snow caused by snow-algae communities occurs in alpine and polar environments in many parts of the world.
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| A snowfield in the Sierra Nevada shows contrasting white snow and red algae-covered areas. Image credit: Ruth Hartnup |
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| Snow algae can darken the snow surface and increase absorption of sunlight. |
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| The red pigment astaxanthin is associated with the distinctive colour of many red snow algae. |
Conclusion
Watermelon snow is one of the Sierra Nevada's strangest natural sights—not because the snow itself has somehow changed into something exotic, but because microscopic life has transformed its appearance. Tiny snow algae can survive in an environment that seems almost completely hostile to life, producing red pigments that turn patches of white snow pink or crimson.
In California's high Sierra, particularly around the higher elevations of Yosemite, the sight can be startling. A snowfield that looks ordinary from a distance may reveal vivid pink or red patches when viewed more closely. Behind that unusual colour lies a surprisingly complex biological story involving algae, pigments, sunlight, meltwater, and snow.
The phenomenon also reminds us that mountain snow is not an empty frozen surface. It can support living communities that influence the very environment in which they survive. That combination of extreme biology and spectacular scenery is what makes watermelon snow such a memorable natural phenomenon.
References & Sources
Yosemite Conservancy — Winter Watermelon: local Sierra Nevada/Yosemite information, including observations above 10,000 feet and the biology of the phenomenon. Yosemite Conservancy: Winter WatermelonNational Park Service — Watermelon Snow: explains red pigmentation and snow algae in a U.S. national-park context. National Park Service: Glaciers and Watermelon Snow U.S. Geological Survey — Snow Algae and Snowmelt: explains how pigmented snow absorbs more solar radiation and can influence snowmelt. USGS: Remotely Sensing Snow Algae Blooms Journal of Phycology / PMC — Snow and Glacial Algae Review: a comprehensive scientific review of snow-algae diversity, pigments, and ecology. Peer-reviewed taxonomic study: documents multiple algae responsible for red and orange snow, including Sanguina species.
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