Their study of the massive hailstorm, published in the Bulletin of the American Meteorological Society, employs photogrammetry — a mathematical technique to gauge the size or velocity of objects from multiple perspectives — to estimate the size of the stones that clattered out of the sky.
The undertaking was led by Matthew Kumjian, an associate professor of meteorology at Penn State. He and his team were sent to Argentina in late 2018 as part of a larger project called Relampago, a National Science Foundation-funded effort aimed at unlocking the dynamics behind severe weather in South America.
Kumjian’s team encountered a lack of storms to study in late 2018 but made the most of their time in Argentina by visiting Villa Carlos Paz, the site of February’s extreme hail event.
“We were kind of fortuitously stationed [there],” Kumjian said. “I had seen that video [of the hail] on social media and figured we could try to measure [objects in the video] and see what we could do with it.”
Among the footage Kumjian’s team studied was that of one particular hailstone as it crashes down atop a metal awning before bouncing off and smashing down onto a busy downtown street.
“Fortunately, the initial impact with the awning slowed the hailstone and kept it more-or-less intact until it impacted the pavement, allowing it to be clearly viewed in a sequence of video frames,” the study states.
While noting “numerous sources of uncertainty and error,” Kumjian’s team estimated this stone at 7.4 to 9.3 inches in diameter. This size rivals the current world-record hailstone, an eight-inch whopper, which pummeled Vivian, S.D., on July 23, 2010. That stone weighed nearly two pounds.
Because the size of this Argentine hailstone was estimated and not directly measured, it will not displace the Vivian hailstone as the world-record holder.
Widespread reports of jumbo hail
A number of hailstones were sampled in the Argentina storm, adding to the scientists’ confidence in the extreme nature of the event. Researchers were able to scan in 3-D a 4.48-inch stone preserved in a freezer and also reviewed photographic data of a stone confirmed at 7.1 inches.
The 7.1-inch stone happens to be one that The Washington Post’s Capital Weather Gang wrote about the day after the storm in 2018; that stone was retrieved by Victoria Druetta, a teenager living in Villa Carlos Paz. It was thought to be the largest observed in the Southern Hemisphere.
“We were able to meet Victoria and her parents,” Kumjian said. “We also met another woman named Maria who had a hailstone in her freezer.”
Druetta had told CWG that hail began falling around 4:30 p.m. and at first was “tiny and fun.” Before long, the stones reached the size of tennis balls — and then grew to mammoth proportions over the course of 20 minutes. Druetta hurriedly snagged the largest one she could find.
“It hit and then exploded and then melted some,” she told CWG. “It was probably even bigger.”
The Penn State paper reported that Druetta had run outside with a motorcycle helmet on to retrieve the stone but was unable to find appendages that broke off it. It weighed nearly a pound.
The Vivian, S.D., hailstone weighed roughly twice that of Druetta’s. While the Vivian stone was rounder, the diameter of Druetta’s was inflated somewhat by oblong protrusions.
“These large protuberances greatly increase the maximum dimension of the particle,” read the paper.
The need for a new category: ‘Gargantuan’ hail
While hailstones of this magnitude are far from common, they may not be quite as rare as originally thought. The team of Penn State researchers coined a new term they feel adequately describes hail more than six inches in diameter: gargantuan.
“I think these giant-hail events definitely happen more often than what we’re seeing out there with the data,” said Rachel Gutierrez, a graduate student at Penn State.
“They don’t happen every day. But first we need someone who finds the hailstone, then reports it and then gives us an accurate measurement. We’d really appreciate anyone who finds hailstones to slap down a ruler.”
Gutierrez also emphasized the importance of preserving any significant hail.
Challenges for hail research
Researchers used a similar technique to estimate the size of hailstones that fell during the infamous El Reno, Okla., supercell on May 31, 2013. It was estimated that a number of hailstones may have achieved diameters of six to eight inches, among the “gargantuan” ranks of the Vivian and Villa Carlos Paz storms.
However, one of the issues with gargantuan-hail data is that the largest stones often go unreported — because they typically fall during the most powerful thunderstorms that often yield the threat of tornadoes. That can — and rightfully should — deter the general public from venturing outdoors to collect hail samples.
Uniqueness of Argentine thunderstorms
It’s long since been known that Argentina has some of the tallest thunderstorms in the world. But most aren’t rotating supercell thunderstorms because of paltry low-level wind shear, or a change in wind speed and/or direction with height.
While some storms initially fire as supercells, they often merge quickly. That, coupled with the high cloud bases associated with frequently “elevated” thunderstorms, limits the tornado risk.
“The supercell on this particular day seemed to be long enough lived,” Kumjian said. “It didn’t look particularly special on radar though.”
Atmospheric parameters at the time did favor severe weather, but Kumjian says there was nothing obvious in the forecast models or data that would have pointed toward potential world-record hail.
Giant hail a rare breed
Gutierrez, who partnered with Kumjian on this project, has documented about a dozen cases of gargantuan hail — all, save for the February 2018 storm, in the United States. Interestingly enough, it’s been found that storms that produce the most prolific hail often are not tornadic.
“Tornadic supercells tend to not have as pronounced or as persistent of large hail signatures,” Kumjian said. “We think we’re seeing some indication that may not be just anecdotal. There may be some reasons for that.”
The subject of hail has been a hot topic in recent years; large hail can often yield more- widespread damage than tornadoes, quickly becoming a billion-dollar disaster if it strikes a highly populated area. Kumjian and his team seek to learn more about hail, but they say that begins with soliciting more ground-based reports.
“There’s this sort of inherent bias when you have this big tornado outbreak that hail is underreported,” Kumjian said. “But hail can cause major economic losses. Insurance companies are very interested.”