AP Photo/Bebeto Matthews

Yesterday, New York City officials closed

portions of the West Side Highway as large chunks of ice pummeled the pavement below One World Trade Center. It wasn't the first report this winter of ice falling from the sky. In a city packed with skyscrapers, the danger of ice aggregating on rooftops and tumbling onto passersby is a routine and terrifying winter reality.

"When a thin layer of ice forms on a rooftop, it often falls like a sharp blade," says JJ Lou, a professor at New York University's Department of Civil and Urban Engineering. "It can be very dangerous, especially if someone is unlucky enough to be in its way."

Lou estimates that a golf ball-sized block of ice would reach about 70 miles per hour at terminal velocity during its 15-second freefall from the top of One World Trade Center. For the sake of comparison, skydivers reach just about twice that speed their descents. Ice slabs as big as softballs from the 1776-foot One World Trade Center yesterday, and Lou says that those chunks of ice likely approached 100 miles per hour before smashing into the street below.

Can we design against the danger of falling ice? In snowier parts of the world, enterprising architects learned to contend with icy conditions centuries ago. "The Russian roof system is rather ingenious," Lou says. "We see irregular, onion-shaped roofs in Russia because it's very hard for snow and ice to accumulate on that kind of surface."

Closer to home, Lou tells the apocryphal tale of how Leslie Robertson, the famed structural engineer behind the original World Trade Center, once used an electric blanket to temporarily thaw one of its frozen antennae. Lou himself developed a similar system over a decade ago, which involved using deicing cables to warm skyscraper ledges and forestall icy buildup. Unfortunately, "it was a novelty type of thing, and not many buildings incorporated it," he says.

Instead, modern skyscrapers often employ snow guards, or irregular rooftop barriers that prevent ice from impaling pedestrians. One World Trade Center is still under construction, but it looks as though it will not feature deicing cables or special snow guards to prevent ice accretion.

This content is imported from Twitter. You may be able to find the same content in another format, or you may be able to find more information, at their web site.

Heads up! That's a 6-by-4 feet chunk of ice falling 1,776 feet from One World Trade Center this morning. #yikes pic.twitter.com/IbBgfXWpWH — amNewYork (@amNewYork) February 19, 2014

Nonetheless, "safety is our first priority," says Anthony Hayes, a spokesperson for the Port Authority of New York and New Jersey. A construction crew at One World Trade Center occasionally clears ice from the skyscraper's rooftop by hand, Hayes says, and a temporary sidewalk shed offers pedestrians some cover from the falling ice.

"World Trade Center construction and Port Authority police monitor ice accumulation throughout the winter months," he says. "Given the extreme weather that we've been having, ice accumulation is not unique to One World Trade Center."

In addition, he says, One WTC is not yet heated because construction is ongoing; the building should see less ice buildup in the years to come.

Ice lurks around every skyscraper's ledge, laying in wait for a strong wind to send it hurtling at 70 miles per hour onto the street below. Fortunately, death by icicle is not terribly likely. "Falling ice will often disintegrate into smaller pieces, which is not that bad," Lou says. "The probability of being hit is very low."

This content is created and maintained by a third party, and imported onto this page to help users provide their email addresses. You may be able to find more information about this and similar content at piano.io