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Chicago Tribune, January 18, 2010

Chicago Tribune, January 22, 2009

After weeks of gray, snowy days, the sun slanted through a clear sky Wednesday and set the city's rooftops and ledges to sparkling.

Warmth gently applied to piles of snow has turned Chicago into a city of icicles, adding a touch of wintry poetry to the landscapeóbut also a dose of heightened danger.

Icicles have formed in recent days as heat from houses and buildings melted snow and sent it dripping into frozen waterfalls from gutters and eaves. With ample sunlight further warming rooftops, millions more of the tiny trickles were set into motion Wednesday, and the icicles grew longer.

Catching the sun on neighborhood streets, the icy decorations inspired delight and fascination as they gleamed at passersby. But it's worth remembering that icicles are also a Damocletian hazard for pedestrians and a worrisome sign for homeowners.

"It might look pretty, but I'm more concerned about public safety and structural integrity than I am about aesthetics right now," said Michael Cornicelli, executive vice president of the Building Owners and Managers Association of Chicago. A lawyer and a former building manager, he has dealt for years with the injuries, lawsuits and damage that are the dark side of falling ice.

Icicles are more common on homes than on downtown buildings, whose steeper sides, slicker surfaces and lack of ledges discourage icicle formation. When traffic cones and signs sprout on the sidewalks of the Loop, it's usually to warn people about falling sheets or chunks of ice, not icicles.

Homeowners who are seeing lots of icicles may want to consider beefing up their attic insulation, said Richard Stone of the University of Minnesota Extension. Escaping heat turns rooftop snow into water, and when it refreezes the resulting ice dams can damage roofs and sometimes the interiors of homes.

In the short term, he said, removing snow from the roof or chipping out channels in the ice dam can help, but people should go about it with extreme caution.

The formation of icicles requires a ready supply of snow for melting and a precarious combination of heat and cold, said Charles Knight of the National Center for Atmospheric Research in Boulder, Colo.

Chicago on Wednesday offered ideal conditions: ample snow, bright sun to warm rooftops, afternoon temperatures just under freezing, low humidity that encouraged evaporation and not too much wind.

Icicles have a simple, recognizable shape, but scientists have recently come to realize that the way they form is surprisingly involved, said Raymond E. Goldstein, the Schlumberger professor of complex physical systems at the University of Cambridge in England.

With two other researchers, the applied mathematician recently studied why icicles form into their distinctive points.

As a film of water runs down an icicle, it releases heat into the surrounding air. That blanket of warmer air, a few millimeters thick and slightly warmer than the frigid air around it, rises along the icicle, Goldstein said.

The rising warmer air is thicker near the top of the icicle, which limits freezing there, and thinner down by the tip. That means the dripping water is likeliest to freeze at the end of the icicle, making it the part that grows fastest and giving icicles their distinctive shape.

Once an icicle has formed, Goldstein said, the process tends to reinforce itself, building up the icicle's size.

But when outside temperatures warm too much, gravity starts to take over and the icicles can break and fall, especially if a wind kicks up.

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