Snow Rollers and Other Rare Finds

SNOW ROLLERS
How do you like your doughnuts? How about glazed in ice? While these interesting features look like giant doughnuts, the circular snowballs are better known as snow rollers.

Snow rollers form under very specific weather conditions. One requirement is a crusty layer of snow on the ground to which new snow will not stick. A light accumulation of loose, wet snow on top of the crusty snow can then roll up when blown by gusty winds that serve to scoop it up off the crusty layer.

As gravity pulls a clump down, it usually rolls down the hill and collapses. Sometimes it will not roll at all, and will come down in an avalanche of snow. But if the snow is the perfect density and temperature, it rolls down leaving a hole in the center.

The weather conditions have to be utterly perfect for the rollers to form. In fact, most go their whole life without ever seeing one.

The National Weather Service office in Lincoln, Ill., put together a nice webpage on snow rollers, with a nice formation explanation as well as photos of a snow roller event that occurred in Feb. 2003.

SUN and MOON HALOS
Halos are a group of optical phenomenon due to the reflection or refraction of solar light on ice crystals suspended in the atmosphere (cirrus clouds, thin snow, icy fog, blown snow). The basic halos take the form of circles around the sun, at 22° (more common and brighter) and 46°.

The ring around the Moon is caused by the refraction of Moonlight (which of course is reflected sunlight) from ice crystals in the upper atmosphere. The shape of the ice crystals results in a focusing of the light into a ring. Since the ice crystals typically have the same shape, namely a hexagonal shape, the Moon ring is almost always the same size. Less typical are the halos that may be produced by different angles in the crystals. They can create halos with an angle of 46 degrees.

MOON RING FOLKLORE Folklore has it that a ring around the moon signifies bad weather is coming, and in many cases this may be true. So how can rings around the moon be a predictor of weather to come? The ice crystals that cover the halo signify high altitude, thin cirrus clouds that normally precede a warm front by one or two days. Ancient folklore says the number of stars within a moon halo indicate the number days before bad weather will arrive. I've also heard, if snow is coming, the number of stars within the ring will indicate the amount of snow (in inches) we'll get. Give it a try the next time you observe a moon halo. Let me know how it turns out. --Meteorologist Brent Watts