Inside a giant test chamber, IBHS, an industry-funded group, recently conducted the first full-scale indoor hailstorm. A dozen multibarreled air cannons pelted a small model residence with more than 9,000 homemade, handcrafted hailstones. The 4-minute bombardment punctured shingles, dinged up metal roofing, and dented the gutters. It was part of IBHS's attempt to learn more about the effects of hail on building materials, and in turn help their clients better gauge insurance premiums based on materials homeowners elect to use. The test required about half a million dollars and years of prep work to create, among other things, a custom compressed-air firing system.
"This was something no one had ever done before on such a scale," says Tanya Brown, a research engineer at IBHS. "We spent years coming up with the firing system, developing the ice stones . . . this was not easy. It took a lot of trial and error."
For example, there's more to creating simulated hail than just freezing balls of water. Last year two IBHS teams traveled into the field and gathered more than 200 hailstones from nine storm sites. They measured the stones' predominant dimensions, mass, and hardness using an in-house compressive force instrument. Armed with this information and data from scientific literature, IBHS staff then set about concocting the right recipe for hailstones in the lab.
"What we found from the literature is that the density of real hailstones is much more variable and smaller than the density of pure ice," Brown says. The researchers figured if they could trap bubbles within the ice, air would take up some of the volume and thus lower the density. After some tinkering, the team found that a mix of 80 percent seltzer to 20 percent ordinary tap water created a good stand-in for real hail.
Then, as if making popsicles, IBHS staffers injected this mixture into molds and loaded them into freezers overnight. They made hailstones with diameters of 1 inch, 1 1/2 inches, and 2 inches?about the size of a quarter, a Ping-Pong ball, and a lime, respectively. Kim Elmore, a research meteorologist at the University of Oklahoma's Cooperative Institute for Mesoscale Meteorological Studies and NOAA's National Severe Storms Laboratory, said the researchers picked these sizes because of their ability to wreak havoc.
"Once you get to hail that's much bigger than an inch, it starts breaking things," Elmore says. "That's why we put the threshold for hail severity starting at 1 inch."
The firing system is a potato gun?like cannon made of PVC and powered by compressed air. A computer program and control box automate the firing sequence, but the hailstones have to be loaded manually through a gravity-fed hopper setup. Brown directed the teams that hand-loaded hailstones into the air cannons on the catwalk during the test run. "It was very, very loud up there," she says.
IBHS says it succeeded in getting the hailstones to zip along at the correct terminal velocity for each size stone?meaning, the velocity at which a stone could be expected to strike an object on the ground in real life. For a 1-inch hailstone, this works out to 51 mph, while the big bruisers?the 2-inchers?can hit a freeway speed of 76 mph.
The model home measured about 20 x 20 feet and rotated slowly on a platform during the simulation. To compare how different surfaces survive a hail assault, IBHS put asphalt shingles installed on an ordinary plywood deck over half the roof, and standing-seam metal over the other half. Two of the exterior walls featured fiber cement siding, while the remaining two had standard vinyl siding. The windows got a similar this-or-that treatment with both vinyl and aluminum frames. Rounding out this simulated home were a car, outdoor table, chairs, umbrella, and even a kid's toy truck. All bore the icy barrage.
As in real life, the hailstones in the simulation pounded the roof. Impact-resistant shingles suffered mostly cosmetic damage, while many of the standard shingles were so tattered they'd need to be replaced. Hail did not puncture the metal roofing, but did cause bigger, deeper dents to the metal placed over shingles. "We were able to look at damage patterns across the different roofing materials and create very realistic-looking damage," Brown says. "That let us qualitatively compare the performance of various materials."
To unleash even more realistic indoor hailstorms in the future, IBHS plans to develop an automated method for cranking out hailstones and to figure out how to tweak the hardness for a simulated hailstone, which for now cannot be controlled. Hardness is a major factor in the amount of destruction a hailstone ultimately leaves behind. "Ask anyone who has been around a lot of hail?you can easily tell the difference between the hard and the soft ones," Brown says.
To NOAA's Elmore, efforts like these by IBHS are probably the best approach to learning more about hail damage. "The only reason we do things in the lab is to have a controlled environment. We can't exercise control in nature?it just hands you what it's got."
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