Why Is So Much Ice Falling Off One World Trade Center?

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."

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