You may have heard that snow can be found in other colors besides white. It's true! Red snow, green snow, and brown snow are relatively common. Really, snow can occur in just about any color. Here's a look at some common causes of colored snow.
Watermelon Snow or Snow Algae
The most common cause of colored snow is the growth of algae. One type of algae, Chlamydomonas nivalis
, is associated with a red or green snow that may be called watermelon snow. Watermelon snow is common in the alpine regions worldwide, in the polar regions or at altitudes of 10,000 to 12,000 feet (3,000–3,600 m). This snow may be green or red and has a sweet scent reminiscent of a watermelon. The cold-thriving algae contains photosynthetic chlorophyll
, which is green, but also has a secondary red carotenoid pigment, astaxanthin, which protects the algae from ultraviolet light and aborbs energy to melt snow and provide the algae with liquid water.
Other Colors of Algae Snow
In addition to green and red, algae may color snow blue, yellow, or brown. Snow that has been colored by algae acquires its color after it has fallen.
Red, Orange and Brown Snow
While watermelon snow and other algae snow falls white and becomes colored as the algae grows on it, you may see snow that falls red, orange or brown due the the presence of dust, sand, or pollutants in the air. One famous example of this is the orange and yellow snow
that fell over Siberia in 2007.
Gray and Black Snow
Gray or black snow can result from precipitation through soot or petroleum-based contaminants. The snow may be oily and smelly. This type of snow tends to be seen early in the snowfall of a heavily polluted area or one which has experienced a recent spill or accident. Any chemical in the air may become incorporated into snow, causing it to become colored.
If you see yellow snow, chances are it is caused by urine. Other causes of yellow snow could be leaching of plant pigments (e.g., from fallen leaves) up into the snow or the growth of yellow-colored algae.
Snow usually appears white because each snowflake has many light-reflective surfaces. However, snow is made from water. Large amounts of frozen water really are pale blue, so a lot of snow, particularly in a shadowed location, will show this blue color.