Snow roller in New Hampshire
EarthSky community member Patricia Evans wrote:
The freshly fallen snow on the hillside in my backyard and the unique set of weather conditions we just had created a perfect combination of conditions for snow rollers to form. I spotted this one in my backyard on March 5, just as I was leaving to go to work. Of course, I had to pause to take a few photos of this rare phenomenon. Snow rollers also have the power to make you late for work!
Since many of us at EarthSky live in Texas and rarely experience snow, we’d never heard of snow rollers. Patty anticipated that, and she also pointed us to an entry about snow rollers in Wikipedia:
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. They can be as small as a tennis ball, but they can also be bigger than a car. Most snow rollers are a few inches/centimeters wide.
The following conditions are needed for snow rollers to form:
There must be a relatively thin surface layer of wet, loose snow, with a temperature near the melting point of ice.
Under this thin layer of wet snow there must be a substrate to which the thin surface layer of wet snow will not stick, such as ice or powder snow.
The wind must be strong enough to move the snow rollers, but not strong enough to blow them apart.
Alternatively, gravity can move the snow rollers as when a snowball, such as those that will fall from a tree or cliff, lands on a steep hill and begins to roll down the hill.
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.
Thank you, Patty!
Bottom line: Photo of a snow roller in New Hampshire in March 2019.