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A summer hailstorm prompts inevitable questions on global warming

By Moises Velasquez-Manoff / August 20, 2009

People take photos of downed trees in New York's Central Park, after a powerful storm with strong winds lightning, and hail struck Tuesday night, Aug. 18.

REUTERS/Shannon Stapleton

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Earlier this week — Tuesday, Aug. 18, to be exact — New York City experienced an unlikely late-summer weather event. The high that day was 88 degrees F., just slightly above the average of 82 degrees F., but thankfully lower than the record 94 degrees F.

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But just before 10 p.m., the wind suddenly picked up, lightning flashed (one New Yorker caught what appears to be a lightning strike on camera), thunder clapped, and the most remarkable sound followed: that of hailstones clattering against cars and – more noticeably to those cowering inside – the outside portions of the city's many air-conditioning units.

On a typical hot and muggy summer day, New Yorkers suddenly found ice falling from the sky.

Having spent much time in the high-altitude deserts of the Southwest, I am no stranger to hail. There, it falls often enough throughout spring, summer, and fall. And it often arrives suddenly during otherwise sunny days.

But weather in the Southwest is different from that in the Northeast. Summer storms in the Southwest almost always arrive suddenly and catastrophically, drenching and flooding — or, as the case may be, shredding with hailstones — and then just as quickly, they're gone and the sun returns.

Indeed, it's not uncommon to have the impression that it's raining in the backyard while it's still sunny in the front.

By comparison, New York City weather seems much less temperamental, much more committed. When it rains, it rains. When it snows, it snows.

Which is why Tuesday's sudden, rip-roaring hailstorm seemed so out of sorts.

Others thought so, too. On Wednesday, we found out that 70 m,p.h. winds had felled hundreds of century-old trees in Central Park. “Central Park has been devastated,” Adrian Benepe, the city parks commissioner, told the New York Times. “It created more damage than I’ve seen in 30 years of working in the parks.”

He added: "To me it looked like pictures I had seen of war zones, of trees I had seen that had been hit with artillery shells. Some were split in half, halfway up their trunk, others completely uprooted. If you love trees, as we do, it’s emotionally upsetting."

But the first question to occur to a science reporter was: A hailstorm in the middle of summer – how unusual is that? What does the science on global warming say about hail?

And more to the point, when it's as hot and muggy as it was — humidity that day hovered somewhere above 90 percent — how big does a hail stone need to be in order to not melt on its way down?

The truth is, New York City has had a relatively cool summer this year. According to the Times, this was the second, or perhaps third, coolest June and July on record.

Our first 90-degree day came on Aug. 10. And we had had other hailstorms nearby, too. In July, several inches of hail fell in Yonkers. Then, Joe Pollina, a meteorologist with the National Weather Service, told the Times that the stormy weather was the result of a polar jet stream that had, uncharacteristically, stayed on a southerly course. (The jet stream forms at the boundary between cooler northern air and warmer southern air, and that's where storms often start.)

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