To most people, the sound of a tornado siren means it's time to head for the basement.
But it's music to Mike Nelson's ears.
Nelson, a 20-year-old college student at Minnesota State University in Mankato, watches the skies so he knows when to engage one of the world's loudest hobbies -- cranking up a siren.
He came to admire them during elementary school in Burnsville.
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"We had an older, like cold-war era siren right down at the school here," Nelson said. "And every Wednesday, I would not want to go out for recess because I knew it was going to go off. But after it quit going off, I would always just sneak outside and have a look at it. And it's just one of those things that's always been kind of neat. And big loud stuff is fun, anyways."
Meteorologists and storm chasers are well known players in the drama of severe weather. Nelson belongs to another small group of sky watchers who appreciate a different aspect of bad weather -- its distinctive sound. As tornado reports in Minnesota have led the nation so far this year, they've had more opportunity than ever to listen for the wail of sirens.
Today, Nelson has a Federal Signal Thunderbolt 1000 siren of his very own and it's mounted on a wheeled dolly in his back yard in Burnsville. It's considered the classic siren, with a big square horn that's about five-feet long that rotates as the siren sounds.
And boy, does it sound. Nelson wears both ear plugs and ear muffs when his siren cranks up.
"When I'm setting it off, and I'm sitting behind it, it rattles your chest," he said. "You can see your eyes vibrating around. I mean, it's crazy. It's amazing what sound can do at these volumes."
Those volumes run up to 130 decibels, as loud as a standing beside jet engine at takeoff. On all but one afternoon a month, when other sirens are tested nearby, that's too loud for Nelson's back yard.
But that's one advantage for siren watchers -- in many cases, their hobby involves a public service, and the sound comes to them.
Atop parks and schools, beside fire departments and on water towers all over the state, hundreds of sirens stand ready to sound the alarm for miles around.
No one actually knows how far -- or even how many sirens there are.
Outdoor sirens are a patchwork of local systems and equipment that aren't addressed by any state law or agency -- except for the area immediately surrounding the state's two nuclear plants.
That patchwork makes people like Rob Mast one of the leading authorities on the subject in Minnesota.
By day, he works prepping jobs at a print shop in Byron, near Rochester. But after work -- sometimes even over the lunch hour -- he looks for tornado sirens.
"It's pretty much like an Easter Egg hunt," Mast said. "You know, they're usually by water towers or schools or parks or that kind of thing."
He receives maps from local emergency managers, notes sightings on a nationwide mailing list and sometimes just looks for sirens as he's driving around. His online map is one of the best sources for finding the sirens in Minnesota, although it still only covers about a quarter of the state.
Mast said it's a little like bird watching -- looking for rarities perched on poles and bolted to towers.
"Usually, the kinds you find are the Federal Signal 2001s," he said.
Those are small, white sirens with a bowl-shaped horn manufactured for the last 30 years or so.
"But when you find a Thunderbolt or an Allertor, it's like, wow, you found something really cool."
He's has an online photo gallery of some of those, like an old Erick siren, from a Minnesota company that built sirens from about 1920 to 1950. Mast is thinking about someday going over to Gibbon, where spotters have reported another unusual find -- a rare dual-horned Soundmaster 125 on the city's water tower.
Other sirens have names like Superbanshee, Screamer and Tempest.
But siren watching isn't just a novelty.
Joe Mikkelson is another Minnesota enthusiast. He's a college student in White Bear Lake and a close observer of the sirens in the Twin Cities. He videotapes siren tests and posts them online -- and sometimes notices discrepancies, like a recent malfunction of a siren near the Maplewood Mall.
Mikkelson watched it make a just a quarter turn during a test -- silently.
He has a keen ear for the air sirens, which sound by turning an electric fan inside a cylinder with a row of holes on one end. The cylinder and holes are called a chopper. Some sirens have dual fans that blow through choppers with two sets of holes, or ports.
Mikkelson said they make a distinct sound, like the one near his house. He's identified it as a Thunderbolt 1003.
"When you have two rows of air that move through the siren chopper, what happens is that it produces like a minor 3rd or a major 5th of a tone," he said. "So if this siren sounds like maybe an E flat, different port ratios make the sound come out of the horn differently."
It's not exactly a song, but it's one small part of the soundtrack of summer for Mikkelson and siren enthusiasts like him.