The tornadoes that recently swept across parts of the South killing more than 50 people exposed an uncomfortable truth: Tornado deaths and injuries are the predictable result of poorly conceived construction patterns that threaten to reverse the benefits that have resulted from advanced storm warning and forecasting capabilities.
Preliminary reports suggest a significant number of those who died during the terrible tragedy were living in manufactured or mobile homes that are highly vulnerable to the destructive forces of tornados. In the past 30 years, there has been a decreasing trend in deaths caused by tornados, yet the fraction of tornado deaths among people living in manufactured or mobile homes has more than doubled.
Statistics from the National Oceanic and Atmospheric Administration (the federal department that runs the weather service) show that more than 50 percent of the current deaths during tornadoes occur in manufactured or mobile homes. Yet, according to the U.S. Census, these types of homes constitute less than 7 percent of the housing stock in the United States.
In the aftermath of Hurricane Katrina and the sweeping wildfires in the West, much of this story is now familiar. Large populations are moving to locations that are especially vulnerable to natural hazards either because of personal preference or economic necessity. Coastal communities — which tend to be scenic, frequently affluent and rapidly growing — are susceptible to hurricanes, flooding and rising sea levels. Other hazardous locations, such as grassy hilltops in California or the 9th ward in New Orleans, have provided affordable housing in otherwise expensive real estate markets.
Manufactured housing creates a big problem for tornado risks because it is a rapidly growing fraction of the housing stock across the South and Midwest and it provides the only affordable housing for many people.
Today, it constitutes more than 20 percent of the housing stock in parts of the South, while the South and Midwest are the strongest growing markets for manufactured housing — more than 18 percent per year according to a recent industry report.
From an engineering perspective, the experience with manufactured homes during tornadoes is not surprising given that the structures provide only minimal resistance to winds and they are typically sited without basements. As a result, residents are often defenseless with no place to hide when a tornado sweeps through their community. Moving to a manufactured home increases the chance of dying in a tornado by more than a factor of 10.
These vulnerabilities of manufactured housing are especially sobering when one considers the huge advances that have occurred in tornado warnings over the past 75 years and how these have dramatically reduced the death and injury rate. In 1925, almost 800 Americans were killed in tornadoes, with 700 in just one storm. Since this time, the per-capita death rate from tornadoes has been reduced by more than a factor of 10, largely because of the development of advanced weather radar coupled to rapid real-time warning systems. However, this trend appears to be reversing, according to NOAA researchers, all because of the increased prevalence of manufactured homes.
The fact is that warning systems only work if people have some place to take shelter after they receive the information. In recent years, there has been a large effort in the engineering community to design low-cost concrete closets that have been incorporated as a shelter in newly constructed homes throughout the South and Midwest.
Unfortunately, similar safety features are not a part of manufactured and mobile homes. While there have been proposals to require shelters in larger manufactured home communities, these do not address the vulnerability of individual homes or the economic realities behind this problem.
Given that people live in manufactured homes out of economic necessity, there is a need to identify new financing mechanisms to fund construction of personal shelters for these vulnerable people. The costs of such safety structures would be modest — these closets are simple concrete structures — yet the benefits could be huge when one considers the trauma from tornado deaths and injuries.
Charles Meade is a senior physical scientist at the RAND Corporation, a nonprofit research organization.
This commentary originally appeared in Tennessean on February 23, 2008. Commentary gives RAND researchers a platform to convey insights based on their professional expertise and often on their peer-reviewed research and analysis.