Tornado vulnerability depends on the incidence of and societal exposure to tornadoes for a particular location. This study assesses the vulnerability of Texas counties to tornadoes using tornado incidence and societal exposure composite scores. Three different assessment methods are used to quantify tornado vulnerability and a geographical information system is used for visualization. Using multiple assessment methods facilitates different ways of viewing tornado vulnerability. Even though the three tornado vulnerability maps produced in this study are spatially diverse, some counties were repeatedly identified as highly vulnerable. The most highly vulnerable counties were located within the northern and northeastern portions of the state, specifically in the northeastern corner within the Shreveport, Louisiana, county warning area.
The extreme and hazardous nature of tornadoes has long impacted society. The first verified tornado-related death in the United States was recorded in 1680 (Grazulis 2001). Over time, population growth and economic development have placed more people and property at risk. Tornadoes were responsible for an average of 56 fatalities per year over the period 1981–2010 (NWS 2010). Also, over the period 1949–2006, tornado catastrophes (i.e., tornadoes that cause >$1 million in losses) were responsible for an average annual loss of $982 million in 2006 dollars (Changnon 2009). Even with increased understanding and advances in technology, which have increased our ability to spot tornado signatures and have led to improved forecasting and warning (National Academy of Sciences 2002), tornado outbreaks such as those experienced in April and May 2011 serve as stark reminders of the potential for tornadoes to produce extensive economic and human loss.
Tornado impacts have the potential to be experienced by communities all across the United States; however, their incidence is greatest in certain regions of the central United States. A region known as Tornado Alley that stretches from northwest Texas northeastward through central Minnesota and North Dakota and from central Colorado and Wyoming eastward to northwest Iowa and east Kansas and Oklahoma (Brooks et al. 2003) is traditionally considered as having the greatest tornado incidence. In addition to the traditional Tornado Alley region, locations in the south and southeastern United States also are relatively tornado prone (Broyles and Crosbie 2004; Ashley 2007; Dixon et al. 2011). Recent research (Dixon et al. 2011) reported that there is not a statistically significant spatial separation between tornado-prone regions in the south and southeastern United States and those in the central United States, thus concluding that if it were not for terrain differences Tornado Alley would possibly stretch from the Great Plains through the Corn Belt and Deep South.
All communities located within tornado-prone regions have some level of tornado vulnerability. The level of vulnerability to natural hazards such as tornadoes, however, varies from location to location (Borden et al. 2007), often with the societal structure of the locations. Communities in the south have been shown to be especially vulnerable to tornado-related damages and fatalities (Brooks and Doswell 2001; Boruff et al. 2003; Ashley 2007; Simmons and Sutter 2011). For instance, Ashley (2007) reported that Texas had the greatest number of tornado-related deaths between 1880 and 2005, with 1812 deaths, and Changnon (2009) showed that Texas experienced the greatest number of tornado catastrophes over the period 1949–2006. Adding to the problem, population trends suggest that the Gulf Coast region, including Texas, is expected to become more densely populated (Crossett et al. 2004). This increase in population, as well as the additional infrastructure and development, is likely to increase the vulnerability of this region to natural hazards such as tornadoes.
This study assesses the spatial distribution of tornado vulnerability within Texas. The methodology used in this study to quantify the tornado vulnerability of Texas counties is adapted from Pielke and Pielke’s (1997) framework in which the vulnerability of a location to a particular hazard is a function of the societal exposure to and incidence of that particular hazard. Thus, as defined in this study, tornado vulnerability is the potential for tornado-related loss and is a function of the incidence of and societal exposure to tornadoes. Societal exposure is determined through an assessment of the people and property exposed to a particular hazard and incidence is determined by some measure of the frequency, location, and intensity of a particular hazard. Concurrent examination and mapping of tornado incidence and societal exposure allows for the identification of vulnerable counties and thus facilitates the documentation and visualization of the spatial variability of tornado vulnerability in Texas. In turn, this information can be utilized to improve pre-event mitigation and postevent response strategies, especially in the most vulnerable counties.
2. Literature review
A large number of studies have focused on the climatology and physical incidence of tornadoes (e.g., Bluestein 1999; Boruff et al. 2003; Brooks et al. 2003; Broyles and Crosbie 2004; Doswell and Burgess 1988; Feuerstein et al. 2005; Grazulis 2001; Verbout et al. 2006). An important outcome of this research has been the identification of high incidence regions.
Other studies have examined tornado-related losses. For instance, Brooks and Doswell (2001) examined economic loss associated with tornadoes. They concluded that, although the most damaging tornadoes are not necessarily producing more economic damages over time when adjusted for wealth, the expected tornado-related economic damages should increase with inflation and accumulation of wealth in the United States. Ashley (2007) examined tornado fatalities and found that the south-central and southeastern United States experiences the greatest number of fatalities and the greatest number of killer events. Boruff et al. (2003) examined tornado hazards, which they defined as tornadoes that cause human and/or economic losses, in the United States. Their results indicate high levels of tornado hazards clustered in the Midwest, Florida, lower Mississippi valley, and Gulf Coast regions.
Studies have also explored factors that increase a location’s vulnerability to tornadoes and thus lead to increased tornado-related losses. Boruff et al. (2003) studied the effect of population density and found little correlation with tornado-related losses and thus suggested that high incidence regions be examined with respect to other social and demographic characteristics that may increase a region’s vulnerability to tornadoes. One of the most commonly reported factors is a high mobile home percentage (Ashley 2007; Brooks and Doswell 2002; Eidson et al. 1990; Schmidlin and King 1995; Sutter and Simmons 2010; Simmons and Sutter 2011). Brooks and Doswell (2002) reported that the likelihood of fatality is 20 times greater in mobile homes than in other structures. Sutter and Simmons (2010) reported that 43.2% of tornado-related fatalities from 1985 to 2007 occurred in mobile homes, and Simmons and Sutter (2011) reported that tornado-related fatalities are 15 times more likely in mobile homes. Other reported factors that increase a location’s susceptibility to tornadoes include a large elderly population (Carter et al. 1989; Eidson et al. 1990; Schmidlin and King 1995; Cutter et al. 2003; Simmons and Sutter 2011), a large percentage of citizens with a disability (Cutter et al. 2003), and a large percentage of non-English speaking citizens (CDC 1988). Elderly citizens are generally less mobile and often have pre-existing health issues (Cutter et al. 2000; Kilijanek and Drabek 1979) and have also been reported to be less likely to seek shelter during tornado events (CDC 1992). Citizens that are unable to speak English may not comprehend warnings or understand how to react to them (CDC 1988). In addition to these factors, locations with a relatively high percentage of its citizens below the poverty level are less able to recover from natural hazard–related losses (Burton et al. 1993; Cutter et al. 2000; Anbarci et al. 2005; Kahn 2005).
There have been numerous studies of the incidence of tornadoes and their associated losses and of socioeconomic characteristics that increase social vulnerability of a location. However, there have only been a few studies to synthesize the incidence of tornadoes and the socioeconomic characteristics of a location into an overall measure of vulnerability. Ashley et al. (2008) examined the human vulnerability to nocturnal tornadoes in terms of tornado-related losses. Hout et al. (2010) developed a method to assess vulnerability trends of counties within Oklahoma and northern Texas. Neither of these studies, however, incorporated any socioeconomic variables. Simmons and Sutter (2011) provided a more comprehensive assessment of tornado impacts. With the use of regression models, they determined that mobile homes, the occurrence of nocturnal tornadoes, and the occurrence of winter-season tornadoes are all statistically significant determinants of tornado-related casualties. This study will complement these few studies by combining tornado incidence with social and economic factors that increase exposure to tornadoes in an attempt to quantify the overall tornado vulnerability of Texas counties.
3. Data sources, conceptual model, and vulnerability assessment methods
a. Data sources
This study utilizes social, economic, and tornado data. Social and economic data were obtained from the U.S. Census Bureau and the Texas State Property Tax Board. The social and economic variables selected for this analysis (Table 1) were chosen because, as reported in the previous section, they are potential indicators of tornado vulnerability. Tornado data were obtained from the Storm Prediction Center (SPC) severe weather database files (SPC 2010) for significant (F2 and F3) and violent (F4 and F5) tornadoes, as rated by the Fujita scale (F scale), occurring in Texas from 1950 to 2008. This study focuses on significant and violent tornadoes because they are responsible for the majority of tornado-related casualties (Merrell et al. 2005; Simmons and Sutter 2011). Furthermore, significant and violent tornadoes have been reported relatively consistently over time (Brooks and Doswell 2001), which will provide a more consistent measure of tornado incidence. This study therefore focuses on significant and violent tornadoes, excluding those rated weak (F0 and F1). Also, this study is focused on the impact of tornadoes on Texas counties and it therefore assigns a tornado count to every county of the track. For instance, a tornado that tracks through three counties would be assigned as a count to each of the three counties.
As with previous vulnerability research (e.g., Cutter et al. 2003; Hout et al. 2010; Simmons and Sutter 2011), all variables in this study are aggregated at the county level. County-level tornado vulnerability assessments are practical given that National Weather Service Weather Forecast Offices (WFOs) issue severe weather warnings at the county and parish levels and because political organization for emergency resources is organized by county.
b. Conceptual model
Vulnerability can be thought of as the potential for loss (Mitchell 1989) or susceptibility to harm (Boruff et al. 2003). The concept of vulnerability has been associated with various definitions, often depending on discipline (Alwang et al. 2001; Cutter 1996). This diversity of definitions has led to various methods of quantifying vulnerability (Alwang et al. 2001).
The methodology used in this study to quantify the tornado vulnerability of Texas counties is adapted from Pielke and Pielke’s (1997) framework developed to assess the vulnerability of certain locations to tropical cyclones. According to this framework, the vulnerability of a location to a particular hazard is a function of the societal exposure to and incidence of that particular hazard. Societal exposure is determined through an assessment of the people and property exposed to a particular hazard and incidence is determined by some measure of the frequency, location, and intensity of a particular hazard.
Following the above framework, this study defines the vulnerability of Texas counties to tornadoes as the potential for a county to suffer tornado-related losses and is thus a function of tornado incidence and societal exposure (Fig. 1). Because tornado incidence is determined by some measure of frequency, location, and intensity, this study develops tornado incidence scores for each county in Texas based on tornado frequency and intensity. Models produced by Merrell et al. (2005) report that a one-category increase in the F-scale rating of a tornado increases its expected casualties by nearly an order of magnitude. However, rather than increasing the weight of tornadoes by an order of magnitude in association with their F-scale rating, this study uses a more conservative approach by weighting tornadoes by their respective F-scale rating. Thus, tornado incidence scores are derived by summing the tornado frequency within each F-scale category multiplied by their respective F-scale rating. As an example of the tornado incidence score, a county with three F2s, eight F3s, four F4s, and one F5 would receive a tornado incidence score of 51 [3(2) + 8(3) + 4(4) + 1(5)]. The final scores are area normalized (tornado incidence score per square kilometer) to account for variable county size.
Societal exposure is determined by assessing the people and property at risk. Because of the multidimensional nature of societal exposure, this study uses the principle components analysis (PCA) method presented by Cutter et al. (2003) and later discussed in Cutter and Finch (2008), to derive societal exposure scores. Essentially, this method provides a social vulnerability index for a location based on multiple socioeconomic variables. Although originally developed to assess social vulnerability, the general concept—that of using PCA to create an index—is applicable to the development of societal exposure scores for Texas counties that can be combined with their corresponding tornado incidence scores to assess county-level tornado vulnerability. Cutter et al. (2003) similarly suggested the coupling of their social vulnerability index with some measurement of specific hazard event frequencies, which would provide a measure of the vulnerability of a location to a particular hazard.
The societal exposure scores developed in this study consist of variables that have been shown to increase a location’s susceptibility to tornadoes (Table 1). The PCA with varimax rotation extracted three principle components that explained 78% of the societal exposure variance among Texas counties (Table 2). During the PCA, factor scores for each of the three principle components were saved as new variables. These new variables were summed to develop the final societal exposure score for each county.
c. Vulnerability assessment methods
Tornado incidence and societal exposure scores can be combined in multiple ways to assess overall tornado vulnerability. This study presents three methods of assessing and visually representing tornado vulnerability. Prior to mapping and combination, tornado incidence and societal exposure scores were normalized (Z scores) to facilitate comparison. Once normalized, their distributions were transformed so that all values are nonnegative.
The first method (method 1) does not combine tornado incidence and societal exposure scores into a composite index but rather displays them individually. This facilitates the representation of the various combinations of tornado incidence and societal exposure that a county may obtain (Fig. 2). Counties with high tornado incidence and high societal exposure are considered the most vulnerable to tornadoes. This study uses the median societal exposure and tornado incidence scores as the break between high and low exposure and incidence; however, societal exposure and tornado incidence distributions can be broken into more than two groups to illustrate more combinations of exposure and incidence.
The second method (method 2) derives tornado vulnerability scores for each county by summing their respective normalized, nonnegative tornado incidence and societal exposure scores. This method is similar to the additive approach taken by Dixon and Fitzsimons (2001) and Herbert et al. (2005). With an additive approach, the most vulnerable counties will be those that have the greatest combined tornado incidence and societal exposure scores. Method 2, therefore, may highlight counties with high exposure or incidence but low incidence or exposure, respectively. In other words, counties with a high level of tornado incidence but low societal exposure may receive a high vulnerability score. The same may occur for counties with high societal exposure but low tornado incidence. This tendency was noted by Herbert et al. (2005) when using a similar additive method to determine the vulnerability of Texas coastal counties to tropical storms.
The third method (method 3) is intended to highlight those counties with relatively high levels of both tornado incidence and societal exposure, thus dampening method 2’s tendency to highlight counties with high exposure or incidence but low incidence or exposure, respectively. In method 3, each county exposure score is divided by the maximum exposure score, thus transforming all scores to a scale ranging from zero to one. The same process was done for each county’s tornado incidence score. The transformed societal exposure and tornado incidence scores for each county were then multiplied to provide an overall tornado vulnerability score ranging from zero to one, with zero being the lowest vulnerability and one being the highest vulnerability. This method will therefore assign higher tornado vulnerability scores to those counties with relatively high levels of both tornado incidence and societal exposure. For example, a county with a high societal exposure score (e.g., 0.95) and low tornado incidence score (e.g., 0.15) would receive a relatively low overall vulnerability score (0.14), whereas a county with relatively high exposure and incidence scores (e.g., 0.50 each) would receive a higher overall vulnerability score (0.25).
Tornado vulnerability scores for each of the three methods are displayed on Texas county base maps along with National Weather Service county warning areas (CWAs; Fig. 3). It seems practical to jointly display CWAs along with their tornado vulnerability levels because WFOs are responsible for issuing severe weather warnings for the counties and parishes located within their CWA.
a. Tornado vulnerability: Method 1
According to method 1, the majority (62%) of Texas counties have either low incidence and high exposure or high incidence and low exposure (Table 3). Only 19% of Texas counties have high incidence and high exposure. These counties are considered to be the most vulnerable to tornadoes and are primarily located in the northern and northeastern portions of the state (Fig. 4). By illustrating the two components of tornado vulnerability separately, method 1 allows visualization of the various combinations of tornado incidence and societal exposure that a county may obtain. For instance, the southwest and west sections of Texas are dominated by low incidence, high exposure counties. These counties are susceptible to the damaging impacts of tornadoes due to their high exposure levels, but they have low tornado incidence scores. However, it must be noted that such counties, because of their societal structure, could be significantly impacted by a future tornado. High incidence, low exposure counties are spread throughout the eastern half of Texas and the panhandle region. These counties are frequented by more tornadoes, but they are not as susceptible because their level of exposure is relatively low. Once again, it is important to note that these counties may still be significantly impacted.
b. Tornado vulnerability: Method 2
According to method 2, 75% of Texas counties have low or very low tornado vulnerability and 25% have high or very high vulnerability levels (Table 4). Loving County, which received the highest societal exposure score, also received the highest tornado vulnerability score. The counties with the lowest tornado vulnerability scores are Fort Bend (Houston/Galveston, Texas, CWA) and Hartley (Amarillo, Texas, CWA). Counties identified as having high and very high tornado vulnerability are scattered throughout the state (Fig. 5). Note the counties in the western and southern portions of Texas that are identified as highly or very highly vulnerable; these counties have very low levels of tornado incidence but have high societal exposure. Cutter and Finch (2008) also found the counties located along the Texas–Mexico border are highly vulnerable to natural hazards and attributed their vulnerability to their poverty levels.
The majority of the counties identified as highly or very highly vulnerable are located in the northern section of the state, within the Oklahoma City, Oklahoma; Dallas/Fort Worth, Texas; and Shreveport, Louisiana, CWAs. The largest cluster of highly vulnerable counties is located in the Shreveport CWA, where all but three counties are classified as high or very high. Spatial cluster analysis (Getis–Ord general G; Chang 2010) indicates that the clustering of counties with high vulnerable scores is statistically significant (Table 5).
c. Tornado vulnerability: Method 3
According to method 3, 85% of Texas counties are classified as low or very low tornado vulnerability and 15% are high or very high (Table 4). Marion County, which received the highest tornado incidence score, also received the highest tornado vulnerability score. A total of 19 counties had equally low tornado vulnerability scores because of a tornado incidence score of zero (i.e., no tornadoes) or negligible societal exposure scores. The high and very high vulnerable counties are less dispersed in method 3 than was seen with method 2 (Fig. 6). Most noticeably, there were no counties identified as having high or very high tornado vulnerability in south or southwest Texas. Similar to that seen with method 2, the largest cluster of highly vulnerable counties is located in the Shreveport CWA, where all but four counties are classified as high or very high. Again, spatial cluster analysis indicated that the clustering of highly vulnerable counties is statistically significant (Table 5).
This study has illustrated three vulnerability assessment methods. Counties repeatedly identified as the most vulnerable by the individual assessment methods can be considered the most highly vulnerable counties to tornadoes. A total of 27 counties were identified as highly vulnerable counties by two out of the three methods (Table 6), and 21 were identified to be highly vulnerable by all three (Table 7). Even though the three methods used in this study did not provide identical outputs, general trends can be observed. For instance, it is obvious from Fig. 7 that the most vulnerable counties are located in the northern half of the state, particularly in the northeast within the Shreveport CWA. All but three counties within the Shreveport CWA were identified as highly vulnerable.
Of the three methods provided in this study, method 1 is the most flexible in that a variety of break points may be utilized in parsing the data, which will allow the visualization of multiple combinations of tornado incidence and societal exposure. Method 3 is useful for identifying counties that have high levels of both tornado incidence and societal exposure. Method 2 is most susceptible to statistical outliers, whereas method 3 minimizes the impacts of those outliers. Method 2 also has a tendency to classify counties as highly vulnerable because of their societal exposure, even though they may experience very few tornadoes, such as Loving County. However, this information still has the utility of identifying counties that have the potential to be substantially impacted by tornadoes because of high societal exposure, even though tornadoes are infrequent.
The information on tornado vulnerability provided in this study is of primary interest to county-level emergency managers interested in their particular site and situation. This information, however, may also be used at the regional scale (e.g., CWAs and councils of governments) for general identification of problematic areas. Likewise, state-level officials and planners may use this information to facilitate design and implement hazard awareness programs in counties and regions with high vulnerability.
The methods presented in this study can be refined to include additional variables. For example, people are more susceptible to nocturnal tornadoes and those occurring in the winter season (Ashley et al. 2008; Simmons and Sutter 2011). Accounting for these tornadoes in the tornado incidence score would be an insightful addition to the current results. Also, other socioeconomic variables such as educational attainment and median home age, which have been shown to be statistically significant determinants of tornado-related fatalities (Simmons and Sutter 2011), can be easily incorporated into the PCA to provide a more comprehensive societal exposure score.
The PCA approach taken in this study produces composite scores, which makes it difficult to determine the contribution by individual variables to tornado vulnerability. Because all variables do not equally contribute to tornado vulnerability, there is a need to determine a defensible weighting scheme for the variables (Cutter et al. 2003). The regression-based approach taken by Simmons and Sutter (2011) provides regression coefficients that indicate the contribution of individual variables and thus could be used in conjunction with the present methodology to determine a weighting scheme for the variables used in the PCA.
Some regions in the tornado vulnerability maps (Figs. 4–6) exhibit marked differences in tornado vulnerability among adjacent counties. For example, Childress County, located in the Lubbock, Texas, CWA, is classified as low incidence and low exposure by method 1 but is surrounded by counties classified as high incidence and high exposure (Fig. 4). Edge effects such as this are located where tornado reports may be underreported or large differences in socioeconomic variables among adjacent counties exist. Other considerations with the methods presented in this study are that they represent a snapshot of socioeconomic conditions, which are in constant flux, and the tornado incidence scores are subject to change with each new season. Therefore, tornado vulnerability analyses such as these are best suited to automated systems that can provide frequently updated output.
This study assessed the vulnerability of Texas counties to tornadoes using both the incidence of and societal exposure to tornadoes. Three simple quantitative vulnerability assessment methods were performed. Method 1 presented societal exposure and tornado incidence separately, rather than combining them into an overall score, and therefore facilitated the visualization of both components of tornado vulnerability for each county. Method 2 highlighted counties with the highest combined societal exposure and tornado incidence scores. Counties that had high tornado vulnerability scores with method 2 therefore had relatively high levels of societal exposure, tornado incidence, or both. Method 3 highlighted counties with relatively high levels of both societal exposure and tornado incidence.
The use of three vulnerability assessment methods facilitated the identification of counties that were repeatedly categorized as vulnerable. All three methods indicated that the most vulnerable counties are primarily located within the northern and northeastern portion of Texas, with statistically significant clustering of highly vulnerable counties in the Shreveport, Louisiana, CWA; however, there was variability among the methods. The use of three methods also illustrated that the spatial distribution of vulnerability is sensitive to the choice of quantitative method employed.
R.W.D. received partial support for this work through the Texas State University faculty development leave program. We wish to acknowledge insightful comments from the reviewers.
This article is included in the Tornado Warning, Preparedness, and Impacts Special Collection.