What makes snow curl off of the roof?

FARGO - Sometimes it seems like snow can defy gravity and it makes a pretty wicked view when conditions are just right.

To get these snow curls we need a wet snow to fall and start to stack up. The temperature has to warm up close to or slightly above freezing to help melt the bottom layer of the snow on a roof to eliminate friction and any cohesion between the roof and snow.

This slight melt at the base of the snow will lubricate the roof surface enough to allow the entire stack of snow to start sliding downward. If it’s warm enough the snow will slide completely off but if temperatures fall fast enough the slowly sliding snow will freeze in this juxtaposed position, pointed toward the ground but not reaching the ground.

This new layer of exposed and slightly melted snow now freezes and is colder than the bottom layer still touching the roof, which is radiating some heat.

This is where albedo comes into play as well, the white snow now hanging off the roof reflects most of the sun’s rays while the roof will absorb some of the radiation and continue to melt the bottom layer of snow and the process can continue. The new layer slides directly down with the help of gravity but the old layer is now frozen and so it curls under to make room for the newly sliding snow. But when temperatures do warm up, those curls and snow will eventually come sliding down, no matter their shape.