Thursday, March 7, 2013

Snow rollers, rare phenomenon when wind, precipitation, and temperature are just right

We drove to Portage from Girdwood today to visit a friend and see her place. It was snowing, yes, and there was a winter weather advisory, yes, but it was not windy in Girdwood. We thought we could get over there, it is only 10 miles away, and back before the storm became too strong. We all should know better, there is no second guessing the weather when mountains are involved. 

As we neared Portage, we could tell the wind was stronger. It was so fierce in Portage (Bear Valley) that this amazing snow phenomenon was being formed. Driving down the Portage Glacier road felt like semi-controlled hydro-planing and the water was making a more dull sound against the under carriage of the car than what I am used to hearing from rain. 

After our visit in Portage, we realized we needed to drive back to Glacier Valley before sunset as the visibility was decreasing in Portage. As the car made its way back to paved road, I noticed the road surface was textured, not just wet and not just snow. You might even think it looks like a choppy lake surface. These photos reveal what the car experienced on the way out of Portage Valley: snow rollers! 

Of course I did not identify this cold weather phenomenon until I returned home and did some research. I believe these are miniature versions of larger snow rollers that occur on mountain sides or in fields. Wind blows snow that may be sitting on a layer of ice or frost that the snow does not stick to thus ‘rolling’ up like a carpet or a cinnamon roll. There are loads of photos of large dry snow rollers on the internet. I believe the phenomenon I saw in Portage Valley could also be classified as snow rollers.

The conditions in Portage Valley the day these snow rollers were captured: snow mixed with rain, temperature ranging from 29° to 34°, wind blowing with gusts to 60 mph. There was a layer of ice and water on the roadway when the snow began. The rare combination of the wind at just the right speed as snow hit the ice or water on the level road enabled the snow to collect and move in little cylindrical shapes across the surface. 

You can see tire tracks in the surface of the precipitation for scale. I literally had to hold on to the car to capture these photos. Gusts of wind were so strong I could have been blown over. Although I did not measure these little cylinders, I estimate them to have been 15 - 25 mm in diameter. If you look closely, you can see little caps on some rollers, like little waves, where the wind is moving the snow forward to roll again. 

After seeing these little slush rolls, I could understand why the sound on the under carriage of the car was deeper than rain would have been. What a cool learning experience and a testament to the effects of mountains on weather. The wind was channeled in just the right way through Portage yet not the same in Glacier Valley. Cool.

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