Why trailer parks suffer tornado touchdowns: Researchers say 'transition zones' on the edge of town are most at risk
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Trailer parks are often left devastated by a tornado touchdown - and researchers have finally worked out why.
They say the key is the location of the parks - often of the edge of town, where land has not been developed.
Researchers who analysed 60 years of weather data say these 'transition zones' have a higher likelihood of severe weather and tornado touchdowns.
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A trailer park in Benton, Louisiana left devastated by a tornado in 1999
WHAT CAUSES A TORNADO?
Researchers say severe storms and tornados can be formed by increased "surface roughness" - an abrupt change in the height of land surface features.
This can stretch or squash a column of air, increasing the air's rate of spin, which could contribute to the formation of severe storms.
The researchers say the 'microclimates' created by the heat from large cities could affect their weather.
Purdue University researchers analysed more than 60 years of Indiana tornado climatology data from the National Weather Service's Storm Prediction Center.
They found the majority of tornado touchdowns occurred near areas where dramatically different landscapes meet - for example, where a city fades into farmland or a forest meets a plain.
Forecasters and city planners may need to pay closer attention to these 'transition zones' to better understand tornado risks, said Olivia Kellner, who led the study.
'There are still many unanswered questions about tornado climatology, but what we're finding is that there may be a relationship between the Earth's surface and the atmosphere that contributes to where tornadoes tend to touch down,' Kellner said.
An analysis of locations where tornadoes touched down between 1950 and 2012 revealed that 61 percent of tornado touchdowns occurred within 1 kilometer (about 0.62 mile) of urban areas while 43 percent of touchdowns fell within 1 kilometer of forest.
Some tornadoes touched down in close proximity to both cities and forests.
The analysis showed a large percentage of touchdowns also occurred in low-population regions with significant changes in surface features.
A tornado in a Kansas Wheat Field: Researchers say that areas where the landscape shifts from urban to rural, such as trailer parks, have biggest risk of touchdowns
Kellner said the percentages suggest that certain locations may enhance the likelihood of tornado touchdowns. Increased 'surface roughness' - an abrupt change in the height of land surface features - can stretch or squash a column of air, increasing the air's rate of spin, which could contribute to the formation of severe storms.
Dev Niyogi, Indiana's state climatologist and co-author of the study, said the possibility that land surface could affect the development of severe weather deserves further scrutiny.
WHAT IS A TORNADO
A tornado, or 'twister', is a violently rotating column of air that is in contact with both the surface of the earth and a cloud.
Tornadoes come in many shapes and sizes, but they are typically in the form of a visible condensation funnel, whose narrow end touches Earth and is often encircled by a cloud of debris and dust.
The most extreme tornadoes can attain wind speeds of more than 300mph (483 kph), stretch more than two miles (3.2km) across and stay on the ground for dozens of miles, or more than 100km.
Tornadoes have been observed on every continent except Antarctica.
However, the vast majority of them occur in the Tornado Alley region of the U.S. - they can occur nearly anywhere in North America.
An F5 tornado, the strongest category, rips buildings off their foundations and can deform large skyscrapers.
'Forecasting and preparing for severe weather risks such as tornadoes are difficult and societally important tasks,' he said.
'We might need to pay more attention to areas where land surface features transition from rough to smooth, flat to sloped, or wet to dry.
'These changes in landscape may provide triggers for severe weather.'
A car marked by searchers sits in what is left of a trailer park after a tornado hit the small town of Lone Grove, Oklahoma in 2009
WHEN IS TORNADO SEASON?
According to the study, Indiana has a distinct spring tornado season with a majority of tornadoes occurring in June, May and July, respectively.
Strong tornadoes with estimated wind speeds of more than 158 miles per hour occur most frequently in April and June.
The total number of tornado days per year - days on which at least one tornado report is made - has not increased over time.
The study also found that tornado touchdowns in urban areas tend to occur at about 1 and 10 miles from the city center.
Kellner said these 'rings' of increased tornado activity could be related to how cities are developed.
'Cities impact the surrounding climate in terms of regional airflow and temperature,' she said.
'The size of cities, what they're made of and the heat they produce are factors that could affect the microclimate.
'Perhaps we have the potential to engineer cities to be more resilient to severe weather by thinking holistically about the way cities can be developed and how they affect local climate conditions.'
According to the study, Indiana has a distinct spring tornado season with a majority of tornadoes occurring in June, May and July, respectively.
Mobile homes are torn apart in a trailer park north of Port Charlotte, Florida. Saturday in 2004
Strong tornadoes with estimated wind speeds of more than 158 miles per hour occur most frequently in April and June.
The total number of tornado days per year - days on which at least one tornado report is made - has not increased over time.
The study also found that drought conditions and climate variations such as El Niño have some impact on Indiana tornado climatology.
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