As we gazed up at the mushroom cloud coming directly towards us, it looked as though the world was about to end. A thick, black, swirling mass of noise and destruction was heading our way. The wind started to pick up, the air temperature dropped 10 degrees. In about two minutes' time, all hell would break loose. Right on the spot where I standing.
To most people, getting caught in a storm like this constitutes an extreme case of being in the wrong place at the wrong time. Accor-ding to weather reports, the supercell - a giant thunderstorm around 15 miles across and containing a deep, rotating updraft of wind - that was almost on us contained hailstones the size of golf balls; anyone caught outside when it unloaded was risking serious injury. And yet, for the 15 storm-chasers I was with, it formed the highlight of their week.
I had joined them in Colorado at the start of a six-day storm-chasing tour, and travelled with them across Middle America as they searched for the ultimate prize: a tornado. Now, in Montana, on day five, this was the closest we had come yet. According to our tour leader, Roger Hill, the approaching storm had huge tornado potential: "Wow," he said, "this thing is an absolute monster."
A qualified meteorologist, Hill has spent the past 21 years chasing storms like this. During that time he has seen no fewer than 93 tornadoes. Following a career in the air force, he turned a childhood fascination into a thriving business by becoming a guide and partner in Silver Lining Tours, a company who run storm-chasing trips throughout the Midwest from May to July.
At this time of year, the area of Middle America known as "Tornado Alley" - which includes the states of Kansas, Nebraska, Colorado and the Dakotas - comes under attack from hundreds of storms on a daily basis. Many inflict huge damage on whatever lies in their path, but are not to be confused with hurricanes such as the ones that caused so much destruction in Florida and New Orleans last year.
A hurricane is characterised by ultra-high-speed winds that circulate around a centre of low pressure; this looks like a giant, comma-shaped cloud, with a point ("eye") in the middle.
Tornadoes, on the other hand, occur when the upflowing winds of supercells spiral out of control. Not every supercell ends up as a tornado, but nearly all contain massive hailstones, torrential rain, deafening thunder and spectacular lightning. Witnessing one is an awe-inspiring experience; getting caught in the centre of one is terrifying, as I was about to find out.
In a country recently battered by some of the most damaging storms on record, you would expect Americans to do anything in their power to avoid them. Yet storm-chasing is reportedly one of the fastest-growing tourist activities in the States.
As we sat around the table at the pre-tour briefing, I was surprised to find that none of the group fitted the adrenalin-junkie stereotype I had expected. For a start, most of them were middle-aged, such as Colorado resident Jane Reller, who was celebrating her 50th birthday while on the tour.
Ron Price, also in his fifties, had the most exper-ience of all of us about just how dangerous extreme weather can be; his home in Florida has been pummelled by three hurricanes, including Katrina last year.
Each day had begun with Roger briefing us as to what we could expect to see that day, and where we would have to drive to see it. Before many of us were out of bed he would be beavering away on his laptop. The area we wanted might be 700 miles away, calling for an epic drive across the Great Plains, along ruler-straight roads punctuated only by the odd single-street town.
On the road, Roger constantly surfed the web and watched the Weather Channel; it is not unusual for a storm to die out before you get there, or for a better one to develop in the opposite direction. In the course of an average storm-chasing tour you can expect to see between two and four super- cells. However, we had been lucky enough to see one almost every day, beginning on our first evening.
Roger had been monitoring a huge storm all after-noon, and it was an intimidating sight: a writhing mass of inky cloud thrashed about angrily, as the upcurrent sucked dust and tumbleweeds up into its belly like an angry weather god.
Extremely dangerous they may be, but storms are undeniably beautiful. In the midst of seemingly apocalyptic blackness I had seen a stunning patch of jade green; according to Roger, this is caused by light being diffused by the hail rattling round inside the storm.
The key to safe storm- chasing is knowing when to leave. While we gawped, pointed and took pictures, Roger had been keeping a beady eye on how close the storm was getting. As soon as he judged it was time to leave, there was no dilly-dallying; even a 30-second delay could mean the difference between a great photo and grave danger.
Throughout the rest of the week, we had seen a number of storms like this. However, none of them had come quite as close as the monster we were now facing in Montana. As the huge mushroom cloud descended on us, Roger ordered every-one into the cars. Our aim was to get around to the side of the storm, avoiding its destructive centre. But we had left it too late. While everyone else hurtled off, I got stuck behind a lorry in the Mini I was driving, and was forced to pull off the road as the centre of the supercell engulfed the car.
The hail arrived shortly after, driven by 90mph winds. The stones were the biggest I had ever seen, causing the windscreen to flex worryingly as they bounced off it.
Suddenly, the side window caved in, showering my passenger and I in glass. Hail cascaded into the car and we curled up into balls, protecting our faces. This was no longer an adventure; frankly I was terrified. The whole experience probably lasted only three minutes as the core of the storm passed directly overhead, yet it felt like a lifetime.
Afterwards, a survey of the damage to the Mini provided a indication of just how strong this storm had been. Every single panel was dented, bits of plastic had been ripped off and patches of bodywork were stripped down to the bare metal.
Hours later, it became clear we had had a lucky escape: after patching up the car, we limped on to the nearby town of Glendive, East Montana, passing by felled trees and overturned trucks. That night, the storm turned into a tornado that wrecked buildings and brought down power lines.
The next morning I met some of the locals whose property was damaged; they were not impressed when they found out why we were there. "I prefer to run when one of those storms comes. I can't understand why anyone would want to go looking for them - I think it's nuts," said one farmer. After my experience of the previous day, I couldn't help agreeing with him.
Contact Silver Lining Tours (001 281 759 4181; silverliningtours.com) for details of storm-chasing trips. British Airways (0870 850 9850; ba.com) fly from London Heathrow to Denver daily. Fares from £529 return from September onwards