Everyone knows that snow doesn't always stay white, but a dune yellowed by the neighbor's dog is actually the least interesting example of colored snow. Since snow is frozen water that accumulates on tiny particles of dust, the color of the particle can play a large role in determining the color of the snowflake while it is forming. If these particles happen to be made of something other than dust, the fallen snow’s color could be a range of different colors.

Snow that falls as red, orange, or brown could be caused by sand or various pollutants, while snow that falls black or gray can be caused by black carbon emissions.

Dark ice in Greenland taken by Joseph Box (2014).

Another cause of colored snow is algae. These tiny organisms grow and spread on snow after it has fallen and collected on the ground and can create a wide variety of colors, including red (known as watermelon snow), green, blue, and yellow.

Red colored “watermelon snow.”

Green snow algae in Cape Crozier, Antarctica by Paul Broady (1984-1985).

So next time New York City sidewalk slush seems like the most polluted and microorganism-filled snow in the world, don't worry because it can always get more extreme!