A snow roller is a rare meteorological phenomenon in which large snowballs are formed naturally as chunks of snow are blown along the ground by wind, picking up material along the way, in much the same way that the large snowballs used in snowmen are made.
Unlike snowballs made by people, snow rollers are typically cylindrical in shape, and are often hollow since the inner layers, which are the first layers to form, are weak and thin compared to the outer layers and can easily be blown away. Snow rollers can grow as large as a foot in diameter.
The following conditions are needed for snow rollers to form:
Because of this last condition, snow rollers are more common in hilly areas. However, the precise nature of the conditions required makes them a very rare phenomenon.
This phenomenon is especially common during the summer months in the Sierra Nevada of California where snow has lingered from winter storms, mainly at altitudes of 10,000 to 12,000 feet. 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." Walking in pink snow often results in a temporary discoloration of your clothing, such as bright red soles and pinkish pant cuffs. It was not until the end of the nineteenth century that the unusual phenomenon was finally attributed to high concentrations or "blooms" of microscopic algae. In the high mountain ranges of the western United States at least 60 different species of snow algae have been identified, but only a few kinds have been reported from the Sierra Nevada. One of the most common species of snow algae in California, and the one responsible for pink snow, is Chlamydomonas nivalis. This unicellular organism is a member of the diverse green algae Division Chlorophyta (Order Volvocales), and contains a bright red carotenoid pigment in addition to chlorophyll. Unlike most species of fresh-water algae, it is cryophilic (cold-loving) and thrives in freezing water. Its scientific surname, nivalis, is from Latin and refers to snow. Compressing pink snow with your boot increases the density of the red cells and heightens the color.
The term ground blizzard refers to a weather condition where loose snow or ice on the ground is lifted and blown by strong winds. The primary difference between a ground blizzard as opposed to a regular blizzard is that in a ground blizzard no precipitation is produced at the time, but rather all the precipitation is already present in the form of snow or ice at the surface of the earth.