The Physics of Falling Ice

The Physics of Falling Ice

The signs are all over North Michigan Avenue.

Waist high black and white ones, cone-shaped yellow ones. On a recent afternoon, most pedestrians seemed to ignore them…no looking up at the sky or shifting course.

Rick and Mary VanHorne didn’t look worried at all as they strolled toward two big signs advising CAUTION FALLING ICE CAUTION.

TAPE: We see it all the time.

She sees them ALL THE TIME… but what does she do?

TAPE: You look, you stop, kind of look around, go around them. I don’t know which way to go though. Is it falling from this way or is it falling from that way.

Exactly. The signs are all warning, no instructions. Brenda Thomas works downtown, and she’s devised her own response.

TAPE: I always swing out closer to the curb. NOW HOW DO YOU KNOW THAT’S THE RIGHT THING TO DO? Well I would say it’s the best thing to do instead of staying closer to the buildings.

Thomas and I were BY THE CURB—a good distance from the tall building above.


Ok, it was just a tiny piece that bounced off my shoulder. But as those signs suggest, falling chunks of ice or snow CAN be dangerous.

The city of Chicago doesn’t keep track of ice injuries. An ER doctor at Northwestern Hospital estimates falling ice brings in a handful of patients each winter, usually with mild concussions and scalp lacerations.

Last winter, the Chicago Sun-Times reported that one woman needed 12 staples to close the gash in her head.

In 1994, a man was killed by a piece of ice reportedly the size of a microwave oven.

Police close off streets when falling ice is especially bad. But otherwise it’s up to property managers to warn people.

Michael Cornicelli is with the Building Owners and Managers Association of Chicago. He has an answer for what to do when you see those signs.

TAPE: Move a little closer to the street. If you’re a little further away from the building, you’re probably less likely to encounter anything falling from the structure. Usually this material falls straight down. Unless there are high winds, in which case it becomes pretty unpredictable.

But aren’t there always high winds in Chicago? I decided to test Cornicelli’s answer with someone else, someone with a deeper understand of how this works.

TAPE: Terminal velocity is the

Clive Halliwell is a physicist from the University of Illinois at Chicago. He specializes in subnuclear physics—and for the past 15 years, he says he’s been creating miniature versions of the big bang.

He met me on Michigan Avenue holding a page of equations he’d written to calculate out how fast falling ice is coming at you:

TAPE: Basically what it comes down to is this…

A 2-inch piece could fall at about 60 miles per hour.

TAPE: So when it hits you, it’s going to be pretty nasty.

And the shape of ice matters. The man from the Building Owners group says most falling ice is small and flat—the size of a dollar bill, say, a quarter of an inch thick. Could that do much damage?

TAPE: It depends how it comes down. I mean if it comes down flat, it’ll be going extremely slowly, it’s like a snowflake. But if it then tips on it’s side, you’ve got a nice little cutting machine.

Especially dangerous are pointed shapes. And that’s just what we saw hanging down from the roof of the Nordstrom store building on Michigan Avenue.

TAPE: There you are! Look at those things at the end there! BIG ICICLES! Yeah, they’re icicles yeah. THEY’RE DRIPPING IS WHAT THEY’RE DOING. Thank God. You see, you’re not looking at a very big thing, it must be half a pound or a pound, but it’s spiked! So the pressure when it hits your head!

But avoiding falling icicles is common sense. The question here is, according to SCIENCE…

TAPE: WHAT’S YOUR BEST CHANCES OF AVOIDING THE ICE? Certainly away from the building if there’s no wind. But if the wind is blowing, it may be worse then. If there’s a wind then it’s anybody’s guess, it depends on which way the wind is blowing, out, round, gusting. Terrible. SO ITS REALLY KIND OF A CRAPSHOOT. Yes, I think so. WHAT DO THESE SIGNS REALLY DO? Nothing from the point of view of helping people, it’s probably is for insurance, I suppose. WHAT WOULD YOU DO IF YOU SAW THESE SIGNS? Well I would get a helmet I think.

Short of wearing a helmet, Halliwell recommends you look up for falling ice when you’re near tall buildings. And the signs at least remind people not to hang around.

Although that’s exactly what someone was doing…smoking a cigarette three feet from a caution sign, right underneath those Nordstrom icicles, looking up once in a while to check for falling ice.