But as well as data, stormchasing is about experience. "You must be able to read the sky," says Thorn.
If you can't handle driving 320 to 960km a day, this holiday isn't for you. We set off early on day one of the six-day tour, travelling north from Denver into Wyoming. This is what they call Big Sky country. We drove along empty, straight highways, passing through places with names such as Thunder Basin and Lightning Creek.
After 960km we pulled off the highway to the edge of a lake. The sky had darkened and the air was thick with humidity.
Storm chasing is like fishing. It's goal-orientated, but involves a lot of waiting. On this occasion, the storm threatened but failed to break. However, I found staring at the sky is strangely meditative nonetheless.
When we checked into a motel that night in Minot, North Dakota, we were told that a tornado watch had been issued. If sirens sounded, we were to proceed to the basement.
"If you get stuck," Todd advised, "jump into the bath and pull a mattress over you."
I collapsed exhausted into bed, but was woken in the night by lightning illuminating my room. I later found out that seven tornados had passed north of the town.
Each morning on our week-long odyssey, after breakfast of pancakes dripping in maple syrup, we would head off in search of storms, stopping for gas in small towns along the way. Tornados or not, this is a great way to see small-town America.
By midweek, the weather stabilised and the skies were empty. When this happens, the holiday becomes a little more conventional. We visited Mount Rushmore in South Dakota, and the much bigger Crazy Horse monument, 24km down the road. It's an unfinished gigantic rock sculpture of the Sioux leader which is in the process of being dynamited out of Thunderhead Mountain.
We explored the rugged Badlands, a prehistoric, roughly eroded ocean bed, and stayed on the Pine Ridge Native American Reservation next to it. Then - at last - the weather worsened again.
It was evening when bright lights began to flash in the cloudy sky. We couldn't hear thunder, so we knew the storm was far away.
We set off fast, driving towards it across the reservation. By the time we pulled over and got out of the car to watch the building storm, it was pitch dark.
Each explosion of light was like an enormous camera flash that lit the towering, rising clouds, which Todd had described as perfect conditions for a major storm.
Lightning cut violently across the horizon. Branches of bright silver electricity twisted through the sky. It was an incredible sight.
Then the thunder began to boom and echo as the storm moved closer.
We decided to head to the safety of our hotel. Horizontal hail was soon lashing the car, reducing visibility to three metres. Our awe turned to fear.
The colossal power of the storm took me by surprise, but the rush of adrenaline was thrilling. I saw how storm-chasing could easily become an addiction.
Further information: Storm Chasing Adventure Tours runs week-long trips in May, June and July, departing from Denver, Colorado or Amarillo, Texas.
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