Red snow, also known as watermelon snow, pink snow, or blood snow, is a natural phenomenon in which patches of snow acquire a pink‑to‑red coloration. The discoloration is caused by blooms of specialized cryophilic (cold‑loving) algae that inhabit meltwater on snow surfaces. The primary organism responsible is Chlamydomonas nivalis—recent taxonomic work places many red‑snow populations in the newly described genus Sanguina (species S. nivaloides for red snow and S. aurantia for orange snow). These microscopic green algae contain the red carotenoid pigment astaxanthin in addition to chlorophyll, giving the snow its characteristic hue.
Distribution
Red‑snow events are reported worldwide in alpine and polar regions during the melt season. Notable occurrences include the Sierra Nevada mountains of California, the Uinta Mountains of Utah, the Scottish Highlands, the Cairngorms, and Arctic coastal areas such as Greenland’s Baffin Bay. The phenomenon is most common at elevations where snow persists through the summer, typically between 3 000 m and 3 600 m above sea level.
Ecology and Impact
The algae thrive in liquid water films on snow, using the pigment astaxanthin as a sunscreen to protect against intense ultraviolet radiation. Their growth can substantially darken the snow surface, reducing albedo (reflectivity) and thereby accelerating melt rates. Consequently, red snow can influence glacial and snowpack dynamics, with implications for downstream water resources.
Historical Observations
References to red snow appear in the writings of Aristotle. Systematic scientific documentation began in the early 19th century when Captain John Ross observed crimson‑streaked snow along Greenland’s coast in 1818 and sent samples to England for analysis. Initial speculation attributed the coloration to mineral deposits or meteoric iron, but later botanical investigations identified an algal origin.
Taxonomy
While historically attributed to Chlamydomonas nivalis, molecular studies have re‑classified many red‑snow algae into the genus Sanguina. This revision reflects distinct genetic lineages separate from the traditional Chlamydomonas genus.
Significance
Red snow serves as an indicator of microbial activity in extreme cold environments and provides a natural laboratory for studying psychrophilic (cold‑adapted) life forms. Its role in modifying surface albedo also makes it relevant to climate‑related research on snow and glacier melt.
