Wind Driven Snow Rollers

My tips for witnessing unusual, interesting or simply beautiful weather phenomena have always included spending as much time as possible outdoors and keeping your eyes up, as in looking up at the sky often. A couple days ago I found some unusual weather down at my feet. Snow rollers! My other advice is to always have your camera with you, which I did not, but it was close and I was able to fetch it before the mid afternoon dusk turned to complete darkness. Between the duskiness, the falling snow and the flat light, my photos turned out pretty rough, but by the next morning, after a few hours of rain, the snow rollers were history…you would not have suspected a thing. (Please click on the photos to see larger versions.)

Snow rollers form when the snow possesses a certain layered tackiness that allows the top layer to peel off the underlying layer (or sometimes the ground) and stay together as it gets rolled into a ball or tube. The rolling can be bone by either gravity or the wind. No elves or gremlins are needed. If by gravity, a pretty steep slope is usually needed. If wind, then you can guess that a pretty strong wind is needed.

The rollers in this photo were traveling UP a slight incline, so it had to have been the wind, and some strong gusts to boot. Notice the tracks left by the rollers show that the wind was coming from slightly different directions at different times, consistent with periodic gusts strong enough to move the rollers interspersed with weaker wind. At the very top of the photo you can see the fence around our Haines Little League field. The whole area between the foreground of this photo and the ball field, the entire ball field, and a gravel pit to the right was littered with thousands of these roller ranging up to the size of a large rolled sleeping bag. Here’s a couple photo at the ball field. By then using a flash was the only hope of a photo and the small bright spots are snowflakes caught in the flash.

What weather conditions lead to snow rollers?

There are several ways the layered tackiness needed for snow rollers can come about. One way is a rapid warming of a dry snow pack such as can happen when a strong storm moves into an area. Downslope (Chinook type) winds in mountainous areas usually bring both rapid warming and strong winds. This was the case with once incident I published a photo of in a past Alaska Weather Calendar in the eastern Chugach Mountains in which the photographer actually saw the rollers being formed. In the recent Haines case, wet snow fell on top of a hard frozen crust as a front brought a surge of warmth and moisture. Here are the weather observations for the Haines Airport (about 1.5 miles or 2 km west of where I found these snow rollers). The photos were taken between 1600 and 1630 AST on the 15th. (The times shown on these observations have been converted to ADT – Alaska Standard Time). The wind had been gusting to around 30 kts (35 mph or 15 m/s) since morning, and the had been snow coming down pretty heavy and at a temperature right at, or slightly above, the melt/freeze point…the perfect temperature for a nice cohesive kind of snow. (The temperatures shown on these observations have been converted to Fahrenheit). Remember, the snowpack up to that time had been a hard rain crust, hard enough to jump up and down on (and according to one report, hard enough to drive a small truck on top of!) The new snow was not going to bond to that very easily.

Site M/A Day Time Sky Conditions   VIS Weather Temp DP Wind(kt)  Alt  RH  Chill Peak

PAHN  AA 15 0954  OVC008             1 S-F      32  31 11012     987  96%  22 
PAHN  AP 15 1019  OVC008           1/2 SF       32  30 09011G30  986  92%  22  30 
PAHN  AA 15 1054  OVC008           3/4 S-F      32  31 10013G24  984  96%  22  30
PAHN  AP 15 1111  OVC008           1/4 S+F      32  30 11013G22  983  92%  22
PAHN  AA 15 1154  OVC006           1/4 S+F      32  32 11014G27  981 100%  21  28
PAHN  AP 15 1211  OVC006           1/2 SF       32  30 09015G29  981  92%  21  29
PAHN  AP 15 1219  OVC006           1/4 S+F      32  32 09015G28  980 100%  21  29
PAHN  AP 15 1230  OVC008           1/2 SF       34  32 09020G30  980  92%  22  30
PAHN  AA 15 1254  BKN008 OVC013    1/2 SF       33  32 09015G29  979  96%  22  30
PAHN  AP 15 1342  OVC008             1 S-F      34  32 11012G26  977  92%  25  32
PAHN  AA 15 1354  BKN008 OVC012    1 1/2 S-F    33  32 09013G24  975  96%  23  32
PAHN  AA 15 1454  BKN008 OVC014    1 3/4 S-F    33  32 09013G26  974  96%  23  26
PAHN  AP 15 1510  BKN010 OVC014        2 S-F    34  32 10012G28  975  92%  25  28
PAHN  AP 15 1520  BKN008 OVC014    2 1/2 S-F    34  32 10014G29  975  92%  24  29
PAHN  AP 15 1525  OVC010               3 S-F    34  32 10014G29  974  92%  24  29
PAHN  AP 15 1534  OVC009               4 S-F    34  32 11014G27  973  92%  24  29
PAHN  AA 15 1554  BKN009 OVC013        4 S-F    34  32 11016G27  973  92%  23  29
PAHN  AP 15 1621  OVC011               4 S-F    34  34 11016G31  972 100%  23  31
PAHN  AA 15 1654  BKN011 OVC015        5 R-F    35  33 10020G33  973  92%  23  33
PAHN  AA 15 1754  OVC013               7 R-     35  33 11012G25  976  92%  26  31
PAHN  AA 15 1854  OVC015              10 R-     36  34 11011G25  980  92%  28  29
PAHN  AP 15 1911  OVC013              10 R-     36  34 11009G20  981  92%  29
PAHN  AA 15 1954  BKN015 OVC065       10 R-     36  34 10009G22  984  92%  29
PAHN  AA 15 2054  OVC014              10 R-     36  34 11010G21  986  92%  28
PAHN  AP 15 2114  BKN016 OVC022       10        37  34 11011G19  987  89%  29

It is quite likely there were many more snow rollers in our area that day. There are miles of river flats nearby that get this sort of wind or stronger. I would love to hear of anyone who saw snow rollers on December 15th in the Chilkat Valley. Or any time or anywhere for that matter. Snow rollers are pretty uncommon, but not “once-in-a-lifetime” rare as some websites like to spout. Again, it is a matter of being aware, being outside and keeping your eyes open. Of course it does not hurt to live in a snowy area.

Fair weather rollers

Another way the snow can reach the right consistency for snow rollers is the warming of a cold snowpack by the sun. In this case gravity is likely going to be the motive force to form snow rollers since strong winds and warming sun don’t often come together. Sometime snow falling off trees get the ball rolling but sometimes they start on their own. Here’s a beautiful example of some gravity snow rollers taken in April along the Alcan Highway in the Yukon Territory, courtesy of Alan Sorum of Valdez.

There are many more photos of snow roller on the Internet: Here are some good ones with good information. You may notice most of them are from the plains states…wide open fields, snow, quick changing weather and plenty of wind.

http://www.wrh.noaa.gov/otx/photo_gallery/snow_rollers.php

http://www.crh.noaa.gov/ilx/events/roller/roller.php

http://webecoist.momtastic.com/2011/01/25/snow-rollers-tumblin-tumbleweeds-of-icy-white-delight/

So be outdoors, and look up… and down. Let me know what you see.