6/24/2010 Celia Elliott
A rare form of arcus cloud rolled through Urbana on Tuesday afternoon. Called a "roll cloud," the unusual formation attracted the attention of physicists in Nigel Goldenfeld's group.
Written by Celia Elliott
UPDATE-June 28, 2010
Richard Lewis (http://xtremestormx.blogspot.com) has graciously provided more information about the rare arcus roll cloud that Nigel Goldenfeld and Patricio Jeraldo photographed over the Main Library on Tuesday, June 22.
Lewis observed the formation as a shelf cloud in Decatur about 12:30 p.m. CDT. At that time, the storm system was moving west to east.
Lewis explained in an email to Goldenfeld: "A shelf can turn into a roll cloud; here is how: When that system of storms was near Quincy, it had a severe warning. Because we had a heat advisory out as well, the air was very humid with 80-percent humidity. The dew point temp was in the upper 70s. So the air was quite warm and humid, and when the rain-cooled outflow from this thunderstorm hit the warm air, it developed that beautiful shelf cloud.
"But shelf clouds are always attached to the parent base cloud, which is typically the thunderstorm. The actual thunderstorm this day dissolved as it approached Champaign. As the storm was dying out, the parent thunderstorm cloud that the shelf was attached to began to also dissolve and break apart , essentially cutting off the shelf cloud. However, because so much cooled outflow had built up, there was still plenty of downdraft from this decaying storm to evolve that shelf cloud into a roll cloud. With the parent thunderstorm cloud or base gone, the shelf became detached and developed into the roll cloud you saw."
on Tuesday, June 22, 2010, near Decatur, Illinois
Photo by Richard Lewis
June 24, 2010--Urbana
Some of us have watched the thunderstorms roll through town the last few days—calculating how long it will be before the rains come, wondering where we left our umbrellas, and hoping for cooler temperatures and lower humidity. Others look at the roiling skies as giant experiments in fundamental physics.
Physicists in Nigel Goldenfeld's group were rewarded with a glimpse of a rare arcus "roll" cloud on Tuesday afternoon as it barreled across the Morrow Plots on the south campus. Physics graduate student Patricio Jeraldo, who works on numerical and theoretical representations of complex flows in quantum liquids, captured the unusual evolving meteorological phenomenon in a series of remarkable photographs taken from the Institute for Genomic Biology, looking west toward the Main Library. Goldenfeld took the photo shown at the beginning of the story.
"A roll cloud has been described as a soliton—a stable, isolated nonlinear wave that maintains its shape while traveling at a constant speed," said Goldenfeld.
"I was working in my office in the IGB when I looked out of the window and saw this propagating front approaching. It was quite spectacular and took about 3 minutes to pass over the building."
Arcus clouds are low, horizontal cloud formations that are associated with the leading edge of cold outflow air from thunderstorm activity (occluding fronts) or coastal sea breezes. The more common "shelf cloud" is wedge shaped and attached to the base of the parent cloud.
The rarer roll cloud is completely detached from other cloud formations and appears as a long, horizontal tube, rotating around a central axis.
