Umas fotografias de um fenómeno curioso e bastante raro denominado de «Snow Rollers»:
Snow Rollers on the Camas Prairie
On the evening of March 31st, 2009, Tim Tevebaugh was driving home from work east of Craigmont in the southern Idaho Panhandle (see map below). Across the rolling hay fields, Tim saw a very usual phenonmena. The snow rollers that he took pictures of are extremely rare because of the unique combination of snow, wind, temperature and moisture needed to create them. They form with light but sticky snow and strong (but not too strong) winds. These snow rollers formed during the day as they weren't present in the morning on Tim's drive to work.
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, leaving what looks like a doughnut or Swiss roll. Snow rollers have been seen to grow as large as two feet in diameter.
The following conditions are needed for snow rollers to form:
* The ground must be covered by a layer of ice that snow will not stick to.
* The layer of ice must be covered by wet, loose snow with a temperature near the melting point of ice.
* The wind must be strong enough to move the snow rollers, but not strong enough to blow them too fast.
* Alternatively, gravity can move the snow rollers as when a snowball, such as those that will fall from a tree or cliff, lands on 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.
A rare treat from nature: Perfect snow doughnuts
No, it's not a promotion for Winchell's or Krispy Kreme.
"This is no joke. We did not build it," said Mike Stanford, an avalanche-control expert with the state Department of Transportation (WSDOT). "They are a natural occurrence in nature."
Stanford found frozen doughnuts of snow on the top of Washington Pass in the North Cascades this week when he was doing avalanche-control work.
At first he couldn't believe his eyes: Perfectly shaped doughnuts had rolled down the mountainside and frozen in place.
He said it's only the second time in his 30 years of working in the snow that he's seen anything like it.
The larger of the snow rollers, as they are commonly called, was about 24 inches tall, he said, large enough for him to put his head through the hole.
Stanford said snow rollers form when there is a hard layer on the snow, covered by several more inches of dense snow. "Then you add a steep slope and a trigger such as a clump of snow falling out of a tree or off of a rock face."
As gravity pulls a clump down, it usually rolls down the hill and collapses, creating what the WSDOT calls a pinwheel. Or it will not roll at all, and 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, Stanford said.
Strong, gusty winds also can be a factor, according to NOAA's National Weather Service office in central Illinois, where snow rollers have occurred.
As soon as the sun comes out and it warms up, the doughnuts would be gone, Stanford said Friday.
Don't think you can drive up to see them. They sit on Washington Pass, 14 miles east of where the highway is closed for the winter.
"No, there are not many of them," Stanford said of his discovery. "The temperature and snow conditions have to be just right."
Perfect snow and weather conditions combine to create "snow rollers" in a Vermont field. Photo by Allon G. Wildgust and courtesy of meteorologist Sharon Meyer, WCAX-TV Burlington, VT.
Weather has some strange ways of making really cool things. One example is snow rollers. They are fairly rare, but given the right conditions they do occur.
Snow rollers form when the snow has just the right combination of sticky snow that is light enough to be moved by wind. If the wind is strong enough, it picks up snow crystals and begins to tumble them over across "downstream" snow cover. Soon a little mini snowball forms. As it grows and increases surface area, the wind gusts push sheets of snow along.
As it grows and rolls along, it picks up more and more snow just like when you build a snowman in your yard. Since there is no compaction going on by pressure from hands, the center can remain open.
Snow rollers can occur anywhere snow and wind conditions are just right. As you can see from the photo, it helps to have a "downhill lie."
It may not be alien crop circles, but it must be pretty startling to wake up and look out in your field and see snow rollers. Behold the mysteries of weather!