Torn apart
It’s the before and after images that are the most shocking: a stereotypical picture of the US suburban Midwest, perhaps a car salesroom, clipped lawns and billboards promoting the long-standing family business. That’s before the tornado gets to work. Once the storm has come and gone, it’s as though a giant has simply crushed the building into oblivion. Shattered timber beams and steel girders are strewn all around.
That’s just the property. Human lives get shattered, too, and the spate of tornadoes that hit the USA in April and May brought with them death on a scale not seen for decades. More than 500 people were killed across 16 states, leaving devastated communities behind. This is all a long way from the perception that some of us may have had that tornadoes are thrilling to watch but never did much more than lift Dorothy and Toto up in the air
and drop them in Oz.
The first spate of at least 200 tornadoes arrived on 14 April, lasted for three days and killed 38 people, hitting the Great Plains first, then Mississippi, Alabama, North and South Carolina and Virginia. But that was just a prelude to the largest concentration of tornadoes in US recorded history.
During the evening of 27 April, a strong area of low pressure lifted through the Ohio valley, allowing a cold front to sweep through the lower Mississippi valley. Ahead of the front, increased moisture off the Gulf of Mexico combined with a potent upper-level system to produce widespread severe weather. A Tornado Watch was issued by the National Oceanic and Atmospheric Administration (NOAA), which designated the unfolding events as an oddly droll-sounding ‘PDS’, or Particularly Dangerous Situation. More than 360 tornadoes struck, including 312 in 24 hours in Mississippi, Alabama, Georgia and Tennessee.
In May, the deadliest single tornado since 1950 hit the city of Joplin, Missouri, killing more than 150 people. The tornado was rated EF-5 – the highest possible rating on one of the scales by which tornadoes are characterised, with winds in excess of 300km/h – and was more than a kilometre wide. Further devastating tornadoes tore through Oklahoma, Arkansas and Kansas, claiming 18 lives.
Tornado territory
The topography of the Midwest and Deep South of the USA has long been identified as fertile tornado territory. Tornadoes tend to form from convective storms called supercells: as the ground heats up, moist air rises and is hit by cool and dry air from different directions, making it rotate, exploding upwards and forming a storm cloud. As the two winds move around each other, a funnel of spinning air, known as a vortex, where wind speeds can reach 480km/h, forms and drops down to earth. And in the US Midwest, all of the ingredients are at hand: a warm subtropical sea, the Gulf of Mexico, lies to the south of the plains, while cold, dry air flows from the Rocky mountains to the northwest.
While these were extraordinary events, scientists suspect that the carnage tells us more about the way we live today than it does about climate change or extreme weather. ‘There’s no obvious reason for this at all, other than that, over a 100-year timescale, these severe events happen,’ says Dr Terence Meaden, deputy head of the UK-based Tornado and Storm Research Organisation (TORRO). ‘What happened were just fluctuations in the general extremes of weather.’
Instead, it seems that this spate of tornadoes attracted more attention because of the way they tracked into towns. ‘There are probably other tornadoes that go unnoticed because they don’t hit anything – they just move over desert and farmland,’ says Meaden. ‘There are severe tornadoes that never get reported. If these tornadoes had been just five miles [eight kilometres] away from where they hit, they may even have gone unnoticed.’
And while the damage to towns was exceptional, it’s risky to read too much more into the events. ‘Most tornadoes don’t hit too much on the Great Plains, so people may chase them and take dramatic video footage, but devastation is limited to a few houses or neighbourhoods,’ says Dr David M Schultz, reader at the University of Manchester’s School of Earth, Atmospheric and Environmental Sciences. ‘In this case, the big ones actually hit towns and cities right in their heart and destroyed them.’
Chief culprit
The latest data from the NOAA suggest that 2011 is running close with 2008 as the worst tornado year on record. That year, there were 1,692 recorded tornadoes, compared with an annual average of around 1,300. In a typical year, the annual death toll is usually about 60.
Yet the consensus among violent-storm experts is that natural variability, rather than climate change, is the chief culprit. ‘This year has been a very bad one for tornadoes, but not so unprecedented that it’s out of the range of what I would expect for natural variability,’ says Professor Joshua Wurman, president of the Colorado-based Center for Severe Weather Research. ‘The worst damage [to Joplin] was caused by a fairly common kind of tornado outbreak.’
Schultz, points out that one or two days of an unusually high number of tornadoes can skew the statistics. ‘A weather pattern favourable for tornadic storms over a large area set up for several days in April, and again in May,’ he explains. ‘That would have been noteworthy to meteorologists. What became newsworthy to the world was that, of the tornadoes that were produced, the most violent ones tended to hit major towns and destroyed them.’
That is not to say that observers aren’t fascinated with what a warmer planet might mean for tornadoes. ‘With a warming climate, you might have more thunderstorms in the USA,’ explains Wurman. ‘But the jet stream – which is critical for forming supercells – may be pushed further north – so that might make for fewer tornadoes. We just can’t connect the dots with climate change.’
Schultz also advises caution. ‘It might be that a warmer planet means more convective storms, but that doesn’t necessarily equate to more supercells or more tornadoes,’ he says. ‘One of the issues with supercell and tornado forecasting is, what is happening to the winds with height [vertical wind shear]? Are they increasing? If so, how much? It isn’t clear what the wind shear will do in a warmer climate.
‘Being sure that any changes in the tornado record indicate a substantive change in the climate would require eliminating the changes in the number of tornado reports received and addressing the large year-to-year variability,’ he continues. ‘Those are difficult nuts to crack.’
Worst-case scenario
One thing is certain, however: the ever-expanding cities of the Midwest and Deep South should ensure we hear a lot more about tornadoes. ‘A hundred years ago, you got a lot of tornadoes, but they were chewing up open pas-ture rather than urban sprawl,’ says Wurman. ‘Unfortunately, what happened in Joplin isn’t the worst-case scenario. What if a tornado hit a densely populated part of suburban Chicago, Atlanta or Dallas? As cities expand, there’s a greater area of Tornado Alley to be hit.’
Increasingly precise forecasting is as tantalising a prospect for storm experts as it is for seismologists or volcanologists, and, despite the uncertainty, there seems to be a good deal that meteorologists can take from the spring outbreak. NOAA’s National Severe Storms Laboratory operated a fleet of mobile and fixed Doppler stations that at times gave unprecedented real-time information on the nature and track of the tornadoes. Some of the storms were tracked street by street (although you imagine it would take some nerve to be in situ at a computer tracking a tornado and trying to gauge if it will take your house out).
Meteorologists in Oklahoma were even given the unexpected opportunity to scrutinise a tornado more closely than they might have preferred. ‘Visiting forecasters at the NOAA Hazardous Weather Testbed 2011 Spring Experiment found it interesting to be under the threat of tornadoes and then to be in the forecast path of them,’ says a NOAA spokesman, dryly.
‘The forecasts issued by the National Weather Service were really quite good,’ argues Schultz. ‘Meteorologists did their job for the most part.’ Instead, more attention, he argues, must be paid to reinforcing the structural strength and design of property. ‘You won’t be able to protect your town from a violent tornado, but you can save your life. The best way to improve your chances of surviving is to be underground, so more houses need to be built with basements or storm shelters.’
Difficult lessons
Meteorologists also have some difficult lessons to learn from this year’s tornado season, argues Wurman. ‘A lot of us, myself included, probably got a little complacent,’ he says. ‘We had gone a couple of decades without seeing too much damage from tornadoes. Maybe these events aren’t as unlikely as one had hoped.’
For now, Wurman believes that the key to better forecasting isn’t new technology but a greater understanding of how tornadoes form. ‘People come out with all sorts of technology, but it’s premature – we still don’t know the fundamentals of how tornadoes form,’ he says. ‘It’s like medicine – once we know the cause of a disease, we’re better able to test for it. We need to know the precursors of what makes a storm into a tornado.’
The most frustrating thing, Wurman suggests, is that improving forecasting times by even a minute or two is problematic. ‘You have to think that in some of these cases, if there had been just five or ten minutes more warning, then some people might have survived,’ he says. ‘We need to keep improving the forecasts. We’re not studying a fringe phenomenon.
‘Looking back a few decades, the advance notice time we could give for a tornado was three minutes,’ he continues. ‘Today it’s 13 minutes. The false-alarm rate is still 70 per cent. We need to decrease the false-alarm rate, so people will take warnings more seriously, and get the warning period up to 30 minutes. That might just give people time to make it to the community shelter rather than hunker down in their bathtub.’
Chasing the storms
Even scientists in pursuit of sober, dispassionate data admit that they’re susceptible to the lure of the tornado. ‘I’m interested in the science of how storms develop, but I’m also interested in them because tornadoes and storms are fascinating phenomena,’ says Dr Lindsay Bennett, a postdoctoral research fellow at the University of Leeds.
Bennett was able to combine both elements of her interest when she participated in VORTEX2, a two-year collaboration of international scientists that’s seeking to understand the fundamentals of tornadoes, with the hope of one day predicting exactly when a tornado will form, where it will touch down, how long it will last and how intense it will be.
The two-year ‘field studies’ component of VORTEX2 concluded last year and involved up to 40 vehicles and ten mobile radar stations criss-crossing the Midwest in pursuit of storms, in order to monitor how tornadoes evolved and manifested themselves. ‘We picked a storm and headed towards it. The risk was managed,’ says Bennett. ‘If you are silly and get too close, then yes, it can be dangerous. Tornadoes meander a little bit, but they track with the storm, so generally people stay at a safe distance.’
Radars with different frequencies observed different parts of the storm, some were positioned 16 kilometres away, while others got close enough to monitor the rotating updraft, or mesocyclone. Bennett deployed tornado pods – circular steel structures with a pole in the middle that were placed on the ground to monitor low-level wind effects. ‘The pods had some close encounters but never a direct hit,’ she says. ‘The thinking was that they would record quite a lot of interesting data before they were destroyed.’
The terabytes of data collected by VORTEX2 are now being unpicked by scientists across the world and, perhaps surprisingly, any revelations may be relevant to the UK. ‘We may not have many tornadoes in the UK, but we can get significant ones,’ says Bennett, pointing to the 2005 tornado that left 80 houses in south Birmingham with dangerous structural damage and injured 19 people. The UK annual average is 30 tornadoes, which, according to the Tornado and Storm Research Organisation, means the UK experiences more tornadoes for its land area than any other country in the world. ‘But it’s clear there’s a greater urgency in the US,’ Bennett says.
September 2011
That’s just the property. Human lives get shattered, too, and the spate of tornadoes that hit the USA in April and May brought with them death on a scale not seen for decades. More than 500 people were killed across 16 states, leaving devastated communities behind. This is all a long way from the perception that some of us may have had that tornadoes are thrilling to watch but never did much more than lift Dorothy and Toto up in the air
and drop them in Oz.
The first spate of at least 200 tornadoes arrived on 14 April, lasted for three days and killed 38 people, hitting the Great Plains first, then Mississippi, Alabama, North and South Carolina and Virginia. But that was just a prelude to the largest concentration of tornadoes in US recorded history.
During the evening of 27 April, a strong area of low pressure lifted through the Ohio valley, allowing a cold front to sweep through the lower Mississippi valley. Ahead of the front, increased moisture off the Gulf of Mexico combined with a potent upper-level system to produce widespread severe weather. A Tornado Watch was issued by the National Oceanic and Atmospheric Administration (NOAA), which designated the unfolding events as an oddly droll-sounding ‘PDS’, or Particularly Dangerous Situation. More than 360 tornadoes struck, including 312 in 24 hours in Mississippi, Alabama, Georgia and Tennessee.
In May, the deadliest single tornado since 1950 hit the city of Joplin, Missouri, killing more than 150 people. The tornado was rated EF-5 – the highest possible rating on one of the scales by which tornadoes are characterised, with winds in excess of 300km/h – and was more than a kilometre wide. Further devastating tornadoes tore through Oklahoma, Arkansas and Kansas, claiming 18 lives.
Tornado territory
The topography of the Midwest and Deep South of the USA has long been identified as fertile tornado territory. Tornadoes tend to form from convective storms called supercells: as the ground heats up, moist air rises and is hit by cool and dry air from different directions, making it rotate, exploding upwards and forming a storm cloud. As the two winds move around each other, a funnel of spinning air, known as a vortex, where wind speeds can reach 480km/h, forms and drops down to earth. And in the US Midwest, all of the ingredients are at hand: a warm subtropical sea, the Gulf of Mexico, lies to the south of the plains, while cold, dry air flows from the Rocky mountains to the northwest.
While these were extraordinary events, scientists suspect that the carnage tells us more about the way we live today than it does about climate change or extreme weather. ‘There’s no obvious reason for this at all, other than that, over a 100-year timescale, these severe events happen,’ says Dr Terence Meaden, deputy head of the UK-based Tornado and Storm Research Organisation (TORRO). ‘What happened were just fluctuations in the general extremes of weather.’
Instead, it seems that this spate of tornadoes attracted more attention because of the way they tracked into towns. ‘There are probably other tornadoes that go unnoticed because they don’t hit anything – they just move over desert and farmland,’ says Meaden. ‘There are severe tornadoes that never get reported. If these tornadoes had been just five miles [eight kilometres] away from where they hit, they may even have gone unnoticed.’
And while the damage to towns was exceptional, it’s risky to read too much more into the events. ‘Most tornadoes don’t hit too much on the Great Plains, so people may chase them and take dramatic video footage, but devastation is limited to a few houses or neighbourhoods,’ says Dr David M Schultz, reader at the University of Manchester’s School of Earth, Atmospheric and Environmental Sciences. ‘In this case, the big ones actually hit towns and cities right in their heart and destroyed them.’
Chief culprit
The latest data from the NOAA suggest that 2011 is running close with 2008 as the worst tornado year on record. That year, there were 1,692 recorded tornadoes, compared with an annual average of around 1,300. In a typical year, the annual death toll is usually about 60.
Yet the consensus among violent-storm experts is that natural variability, rather than climate change, is the chief culprit. ‘This year has been a very bad one for tornadoes, but not so unprecedented that it’s out of the range of what I would expect for natural variability,’ says Professor Joshua Wurman, president of the Colorado-based Center for Severe Weather Research. ‘The worst damage [to Joplin] was caused by a fairly common kind of tornado outbreak.’
Schultz, points out that one or two days of an unusually high number of tornadoes can skew the statistics. ‘A weather pattern favourable for tornadic storms over a large area set up for several days in April, and again in May,’ he explains. ‘That would have been noteworthy to meteorologists. What became newsworthy to the world was that, of the tornadoes that were produced, the most violent ones tended to hit major towns and destroyed them.’
That is not to say that observers aren’t fascinated with what a warmer planet might mean for tornadoes. ‘With a warming climate, you might have more thunderstorms in the USA,’ explains Wurman. ‘But the jet stream – which is critical for forming supercells – may be pushed further north – so that might make for fewer tornadoes. We just can’t connect the dots with climate change.’
Schultz also advises caution. ‘It might be that a warmer planet means more convective storms, but that doesn’t necessarily equate to more supercells or more tornadoes,’ he says. ‘One of the issues with supercell and tornado forecasting is, what is happening to the winds with height [vertical wind shear]? Are they increasing? If so, how much? It isn’t clear what the wind shear will do in a warmer climate.
‘Being sure that any changes in the tornado record indicate a substantive change in the climate would require eliminating the changes in the number of tornado reports received and addressing the large year-to-year variability,’ he continues. ‘Those are difficult nuts to crack.’
Worst-case scenario
One thing is certain, however: the ever-expanding cities of the Midwest and Deep South should ensure we hear a lot more about tornadoes. ‘A hundred years ago, you got a lot of tornadoes, but they were chewing up open pas-ture rather than urban sprawl,’ says Wurman. ‘Unfortunately, what happened in Joplin isn’t the worst-case scenario. What if a tornado hit a densely populated part of suburban Chicago, Atlanta or Dallas? As cities expand, there’s a greater area of Tornado Alley to be hit.’
Increasingly precise forecasting is as tantalising a prospect for storm experts as it is for seismologists or volcanologists, and, despite the uncertainty, there seems to be a good deal that meteorologists can take from the spring outbreak. NOAA’s National Severe Storms Laboratory operated a fleet of mobile and fixed Doppler stations that at times gave unprecedented real-time information on the nature and track of the tornadoes. Some of the storms were tracked street by street (although you imagine it would take some nerve to be in situ at a computer tracking a tornado and trying to gauge if it will take your house out).
Meteorologists in Oklahoma were even given the unexpected opportunity to scrutinise a tornado more closely than they might have preferred. ‘Visiting forecasters at the NOAA Hazardous Weather Testbed 2011 Spring Experiment found it interesting to be under the threat of tornadoes and then to be in the forecast path of them,’ says a NOAA spokesman, dryly.
‘The forecasts issued by the National Weather Service were really quite good,’ argues Schultz. ‘Meteorologists did their job for the most part.’ Instead, more attention, he argues, must be paid to reinforcing the structural strength and design of property. ‘You won’t be able to protect your town from a violent tornado, but you can save your life. The best way to improve your chances of surviving is to be underground, so more houses need to be built with basements or storm shelters.’
Difficult lessons
Meteorologists also have some difficult lessons to learn from this year’s tornado season, argues Wurman. ‘A lot of us, myself included, probably got a little complacent,’ he says. ‘We had gone a couple of decades without seeing too much damage from tornadoes. Maybe these events aren’t as unlikely as one had hoped.’
For now, Wurman believes that the key to better forecasting isn’t new technology but a greater understanding of how tornadoes form. ‘People come out with all sorts of technology, but it’s premature – we still don’t know the fundamentals of how tornadoes form,’ he says. ‘It’s like medicine – once we know the cause of a disease, we’re better able to test for it. We need to know the precursors of what makes a storm into a tornado.’
The most frustrating thing, Wurman suggests, is that improving forecasting times by even a minute or two is problematic. ‘You have to think that in some of these cases, if there had been just five or ten minutes more warning, then some people might have survived,’ he says. ‘We need to keep improving the forecasts. We’re not studying a fringe phenomenon.
‘Looking back a few decades, the advance notice time we could give for a tornado was three minutes,’ he continues. ‘Today it’s 13 minutes. The false-alarm rate is still 70 per cent. We need to decrease the false-alarm rate, so people will take warnings more seriously, and get the warning period up to 30 minutes. That might just give people time to make it to the community shelter rather than hunker down in their bathtub.’
Chasing the storms
Even scientists in pursuit of sober, dispassionate data admit that they’re susceptible to the lure of the tornado. ‘I’m interested in the science of how storms develop, but I’m also interested in them because tornadoes and storms are fascinating phenomena,’ says Dr Lindsay Bennett, a postdoctoral research fellow at the University of Leeds.
Bennett was able to combine both elements of her interest when she participated in VORTEX2, a two-year collaboration of international scientists that’s seeking to understand the fundamentals of tornadoes, with the hope of one day predicting exactly when a tornado will form, where it will touch down, how long it will last and how intense it will be.
The two-year ‘field studies’ component of VORTEX2 concluded last year and involved up to 40 vehicles and ten mobile radar stations criss-crossing the Midwest in pursuit of storms, in order to monitor how tornadoes evolved and manifested themselves. ‘We picked a storm and headed towards it. The risk was managed,’ says Bennett. ‘If you are silly and get too close, then yes, it can be dangerous. Tornadoes meander a little bit, but they track with the storm, so generally people stay at a safe distance.’
Radars with different frequencies observed different parts of the storm, some were positioned 16 kilometres away, while others got close enough to monitor the rotating updraft, or mesocyclone. Bennett deployed tornado pods – circular steel structures with a pole in the middle that were placed on the ground to monitor low-level wind effects. ‘The pods had some close encounters but never a direct hit,’ she says. ‘The thinking was that they would record quite a lot of interesting data before they were destroyed.’
The terabytes of data collected by VORTEX2 are now being unpicked by scientists across the world and, perhaps surprisingly, any revelations may be relevant to the UK. ‘We may not have many tornadoes in the UK, but we can get significant ones,’ says Bennett, pointing to the 2005 tornado that left 80 houses in south Birmingham with dangerous structural damage and injured 19 people. The UK annual average is 30 tornadoes, which, according to the Tornado and Storm Research Organisation, means the UK experiences more tornadoes for its land area than any other country in the world. ‘But it’s clear there’s a greater urgency in the US,’ Bennett says.
September 2011
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