A summer hailstorm prompts inevitable questions on global warming

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.

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.)

Global warming? Not so much, apparently.

Nonetheless, it's worth noting that scientists have observed that as average temperatures have inched up in recent decades, the jet stream has shifted poleward at a rate of 1.25 miles per year, at least between 1979 and 2001.

It's impossible to say how this shift bears on Tuesday's storm. The jet stream isn't exactly straight. It dips and rises in sine-wave fashion as it travels west to east around the globe.

But there are other reasons to think that a warmer world could have more hail.

According to NOAA, hail forms inside a thunderstorm when updrafts carry water droplets upwards into cooler air. They freeze high up and then travel back down with downdrafts. Then they catch another updraft, adding yet another layer of ice as they rise again into the cold.

This can go on and on, the hail stone growing with each trip into the freezing zone. A cross section of a hailstone will reveal growth rings like a tree. (Take a look at NOAA's photos.)

More intense storms are a common prediction for a warmer world. And to the degree that more powerful storms increase the probability of that droplet going up and down and up again — gaining ice mass with each trip — it's probable that hailstorms may be more intense in the years to come.

That was the conclusion of a 2007 study in the Proceedings of the National Academy of Sciences. Particularly, the authors note, storm severity increases as the temperature and humidity differences between distinct air masses at different altitudes increases. The cooler the air above relative to that below, the faster the hot air will rise, and the more intense the storm.

Then there's the phenomenon of large chunks of ice falling from otherwise clear skies. Spain was battered with such falling ice in 2000. And in 2006, a microwave-oven-size ice-ball fell from skies over Douglasdale, a suburb of Johannesburg, South Africa, the first such recorded case in Africa.

These large ice balls are apparently in a different league than your average hailstone. And they have their own designation: "megacryometeors."

A Spanish scientist, Jesus Martinez-Frias, who co-coined the term, has a website entirely dedicated to tracking them.

On his site, he says: "Atmospheric megacryometeors could be a new type of fingerprint (geoindicator) of Climate Change. Tropospheric Global Warming (and mainly Stratospheric Cooling) might be making the tropopause colder, moister and more turbulent, creating conditions in which ice crystals could grow, forming, unusually and much more recurrently, large ice conglomerations."

The troposphere – the part of the atmosphere closest to Earth's surface – is where most weather happens. The stratosphere is above the troposphere; that's where jet airlines fly to avoid most weather, and it's generally cooler. And the tropopause lies between.

As it turns out, summer hailstorms in New York City are more common than one might imagine. The Times has reports of hail falling in the region stretching back to the late 19th century. And yes, witnesses always say they've never seen anything like it.