Corresponding Author: Harold E. Brooks, NOAA/NSSL, Norman, OK 73069.

The 3 May 1999 Oklahoma City tornado was the deadliest in over 20 years, with 36 direct fatalities. In order to understand how this event fits into the historical context, we have examined the record of tornado deaths in the United States. Almost 20,000 deaths have been reported associated with more than 3600 tornadoes in the United States since 1680. A cursory examination of the record shows a break in 1875. Prior to then, it is likely that many killer tornadoes failed to be reported. When the death toll is normalized by population, a near constant rate of death is apparent until about 1925, when a sharp fall begins. The rate was about 1.8 people per million population in 1925, and was less than 0.12 people per million by 2000. The decrease in fatalities has resulted from two primary causes: a decrease in the number of killer tornadoes and a decrease in the number of fatalities in the most deadly tornadoes. Current death rates for mobile home residents, however, are still nearly what the overall national rate was prior to 1925 and are about 20 times the rate of site-built home residents. The increase in the fraction of the U.S. population living in mobile homes has important implications for future reductions in the death toll.

These considerations provide a background for understanding the fatalities in the 3 May 1999 outbreak. The tornado that hit the metropolitan Oklahoma City area killed 36 people, the most in the U.S. since the 10 April 1979 Wichita Falls, Texas tornado killed 42 (see Table 1 for a record of the most fatalities in a single U.S. tornado going backwards in time from the end of 2000). The total number of fatalities in the outbreak was 46, the most in the U.S. since the Ohio-Pennsylvania outbreak of 31 May 1985 killed 76. Of the 36 fatalities in the Oklahoma City tornado, 11 occurred in mobile homes, and 18 of the 46 deaths in the outbreak were mobile home residents. As we will discuss later, the problem of mobile home safety is the biggest obstacle to reduction of tornado deaths in the U.S.
 
 

2. The dataset and concerns
 
 

The primary dataset to be used is that of Grazulis (1993, hereafter G93; 1995), listing tornadoes with fatalities up through 1995. From 1996-2000, the records are those collected by NOAA's Storm Prediction Center (SPC, 2000) According to G93, the first death from a tornado in the United States was recorded in 1680 in Cambridge, Massachusetts. Tornado fatalities were reported infrequently over the following two centuries, and tornadoes with at least ten fatalities were reported very rarely. From 1680-1874, only four double-digit death tornadoes were recorded. In contrast, seven were recorded in the decade that followed. As a result, the primary emphasis of this paper is on the period 1875-2000. Along with the apparent break in the number of tornadoes with at least ten fatalities, another reason for concentrating on the period beginning in 1875 is clear when the number of killer tornadoes per year is considered (Fig. 1). In 1875, a large increase in the number of reports in the record occurred, with 30 killer tornadoes in that year alone.

It seems plausible, given the abrupt change in the number of killer tornadoes, that many killer tornadoes prior to 1875 went unreported. Given the rapid change in demographics in the U.S. in the 19th century, it is extremely difficult to estimate the amount of underreporting. Even since then, it is likely that some killer tornadoes have gone unreported, although underreporting is probably less of a problem since 1875.

The number of deaths in individual tornadoes is not always well-known, particularly for earlier tornadoes. G93 reports that possible deaths of slaves in the 1840 Natchez, Mississippi tornado are not recorded, as well as deaths on boats in the Mississippi River in the 1896 Saint Louis-East Saint Louis tornado. The latter event also provides evidence of conflicting numbers of deaths from different sources. G93 lists the number of deaths in Saint Louis as 137 and East Saint Louis as 118, consistent with Curzon (1896), who published a book on the tornado within two weeks of the event. The East Saint Louis Journal newspaper (Cook and Reed 2001) listed 128 deaths in East Saint Louis 4 days after the tornado, and Williams (1999) lists 140 dead in Saint Louis, giving a total of 268. We will use the G93 numbers throughout, with the caution that the exact number of deaths in many events cannot be known.

Another problem involves determining precisely which deaths are tornado-related. In the case of the 3 May 1999 Oklahoma City tornado, 36 people were killed directly by the tornado (e.g., hit by debris or thrown long distances), but several others could be viewed as indirect deaths. For instance, at least two people died of heart attacks in the vicinity of the damage path, a third died in a fall trying to get into a storm shelter outside of the tornado, and another died in a fire when candles, lit after his electrical power had gone off because of the tornado, caught his residence on fire (P. Archer, personal communication, 2000). Traditionally, National Weather Service procedure has been to consider only the direct fatalities, but determining that information, particularly early in the record, is impossible. There is no way to know how many of the deaths within the record are indirect, so there is a possible problem of inhomogeneity in the dataset.
 
 

3. General features of the record
 
 

The number of deaths per year in tornadoes has generally been much less in the last quarter of the 20th century than it was previously (Fig. 2). In particular, the number of deaths in the "big years" has dropped dramatically. From 1975-2000, only twice did 100 tornado deaths occur in a year (1984 and 1998). In contrast, it occurred 54 times from 1875-1974, including every year from 1916 through 1927. The 1910s through the middle 1930s, in fact, represent a period of very high fatalities, as shown by the smoothed curve on Fig. 2. From 1912-1936, the mean annual death toll was 260, almost five times as many as in 1976-2000, when the mean was 54.

The decrease in death toll is even more apparent when the rate of death is normalized by the population of the United States, available from the United States Census Bureau1 (Fig. 3). Prior to 1925, the smoothed rate was relatively constant, showing a slight increase from about 1.6 per million to 1.8 per million. Since that time, however, the rate has decreased substantially, to a little over 0.11 per million, a factor of roughly 15 less than the long-term average prior to 1925. Many factors, including the beginnings of tornado forecasting, improved communications, spotter networks, and changes in construction, likely have been involved in this decrease (see Doswell et al. 1999 for more discussion). There is no simple way to deconvolve the possible contributions from all those factors so that we do not assign a relative importance to any of them in particular. To put the change into perspective, however, a death rate of 1.8 per million people per year would have resulted in approximately 500 deaths in the year 2000, compared to the actual number of 40.

A long-term decrease in the rate of tornado deaths also means that the notion of an "average" tornado death toll is someone difficult to define. Taking a standard climatological period of 30 years makes little sense, given that the presence of such a strong trend. A more reasonable approach might be to look at the expected value of the trend at whatever point on the time series is of interest. In 2000, with the value of 0.12 per million and a population of almost 280 million, the expected death toll would be about 34, half of the 30-year mean value of 68. The decrease in the death toll also has changed the perception of what is a "big" year for tornado deaths. Lines estimating the 90th percentile death rate from 1925-2000 have been constructed to be parallel to the mean value (Fig. 3). Multiplying the values of the 90th percentile by the population in 1925 and 2000 gives estimated values of 475 and 80 deaths per year at those two times. From this perspective, the 1998 national death toll of 130 was just as far above the expected number of deaths as the 1925 national death toll of 794 (most of which came from the single "Tri-State" tornado). In fact, the 10th percentile value in 1925 is 61, so that a rare low death toll 75 years ago is almost as big as a rare high death toll now.

Identifying the underlying social and scientific reasons behind the decline in tornado deaths is difficult, but we can at least attempt to identify how the decrease has occurred from a statistical perspective. First, the number of killer tornadoes is, in general, less than it was 75 years ago (see Fig. 1). The median number of killer tornadoes per year from 1976-2000 was 17 and the maximum was 33. In contrast, the median from 1912-1936 was 39 and the minimum was 17.

Arguably more important than the number of killer tornadoes, the number of deaths in individual "high-death" tornadoes has decreased. This can be seen in at least two different ways. First, we can define a "high-death" tornado and examine how that definition changes with time. A simple, objective definition is to take all killer tornadoes within a five-year window and find the 95th percentile of the death tolls in those tornadoes (Fig. 4). Smoothing the 95th percentile value with a 3-point median filter, followed by an 11-point running mean, shows that it has dropped from 20-25 during the period 1900-1970 down to about 10-15 since 1975.

A second illustration of this is the return period of any particular value of the number of deaths in a single tornado. As an example, we have calculated how many days there were between two days with at least 10-death tornado on them since 1875, a total of 259 days as a function of time (Fig. 5). Prior to 1958, return periods of less than 10 days were common. Since then, the fewest days between 10-death tornado days has been 19 (between the Hall County, Georgia 12-fatality tornado on 20 March 1998 and the Birmingham, Alabama 32-fatality tornado on 8 April 1998).

We can compare the return period for 10-death tornadoes to the population of the U.S. By smoothing the return periods with a strong 5-point median and 19-point running mean, a shortening of the return period is apparent prior to 1925 with a lengthening of the period since then (Fig. 6). In the absence of mitigation efforts, it might be expected that the return period would have an inverse relationship to population. During the period before 1925, this is roughly true. Since then, however, as population has continued to increase, the return period has increased. Again, we cannot identify exactly why, but seems likely that improved forecasting, communications, spotter networks, and construction has provided significant mitigation of the life-threatening consequences of tornadoes.
 
 

4. The mobile home problem
 
 

Since 1975, the National Weather Service has separated out tornado fatalities associated with mobile homes. Data for other fatality locations (permanent2 homes, other buildings, vehicles, outdoors) are available online from the Storm Prediction Center (http://www.spc.noaa.gov/climo/) starting with 1985 (Table 2). The fraction of deaths occurring in mobile homes has been increasing since 1975 (Fig. 7). Using a Kendall's tau test (Press et al. 1992), the increase is statistically significant at the 95% confidence level. From 1976-1980, 24% of tornado deaths were in mobile homes. That fraction increased to 34% in 1986-1990 and 50% from 1996-2000.

In order to understand the increase, we need to look at the changing demographics of the U.S. The Census Bureau carries out a housing survey with the decennial census to determine the number of different kinds of housing units in use in the U.S.3 The mobile home fraction of housing units in the U.S. has increased significantly since 1950 (Table 3). Assuming the number of people in different kinds of housing is the same for each kind of unit, that means that 10 times as many people lived in mobile homes in 1990 compared to 1950. We can use these data to calculate the rate of fatalities in each kind of housing per million people, in a way similar to Fig. 3. For simplicity, we have assumed that the fraction of housing units changed linearly between the years the housing surveys were done and that the increase from 1980 to 1990 continued after 1990. The rate of death in mobile homes is dramatically higher than in permanent homes (Fig. 8). The mean (median) annual death rate from 1975-2000 in mobile homes was 1.23 (1.19) per million per year, while the mean (median) rate from 1985-2000 in permanent housing was 0.06 (0.04) per million per year. In short, the death rate is approximately 20 times as high in mobile homes compared to in permanent homes, and the mobile home rate is about 70% of the rate for the total U.S. population prior to 1925. There is no obvious trend in the rate of deaths over the time period of record.

The effect of the increasing population in mobile homes is important for the number of deaths in tornadoes nationally and may be especially serious in the future. We can make a simple model of the effect of mobile homes by using the population in mobile homes that underlies Fig. 8 and by assuming that the average death rate in mobile homes occurred each year since 1960. Further assuming that the mobile home population will continue to increase as it has, we can estimate the number of deaths in mobile homes in the future. Taking that number and dividing it by the total U.S. population gives a death rate for the total U.S. population if all deaths outside of mobile homes were eliminated. We can compare the model death rate to the long-term trend seen in Fig. 3 (Fig. 9). The mobile home rate is approaching the long-term overall trend and, in fact, will soon pass it. The raw data after 1990 in Fig. 3 could be interpreted as showing that the long-term decline has stopped, and that the death rate has leveled off, although it is too early to be sure of this change. Thus, the increase in mobile home residency may have reversed the approximately 70-year long trend of decreasing death rates in tornadoes. Only time will tell what the trend will be or if efforts to target this vulnerable population will help reduce deaths.

The increase in mobile homes as housing has been particularly large in the southeastern U.S. (Fig. 10). Given that this part of the U.S. gets a large number of tornadoes, and that many of them happen overnight when getting messages to people is particularly difficult, as shown by the tornadoes in Orlando, Florida (1998), Hall County, Georgia (1998), and Camilla, Georgia (2000), this is an area of great concern. Improvements in the accuracy and lead time of forecasts and warnings may have little effect on decreasing deaths if the problem of mobile home deaths cannot be addressed.
 
 

5. Summary
 
 

More than 18,000 people died in tornadoes in the U.S. from 1875 to 2000. The rate of death has been decreasing since 1925 from about 1.8 per million per year to 0.12 per million per year. This has resulted from both fewer killer tornadoes and from a decrease in the number of deaths in the largest fatality tornadoes. What used to be considered a relatively common number of fatalities, both from the perspective of a single tornado and annual death tolls, is now an extremely large number. We cannot determine the importance of the various factors that have helped the decrease, so that we cannot estimate the importance of forecasting or any other particular activity.

There remains one particularly vulnerable group of people in the U.S., residents of mobile homes. The rate of death is relatively close to the pre-1925 values in the U.S. The increase in use of mobile homes for housing has meant that they are an increasingly large component of the overall death toll. Half of all fatalities from 1996 to 2000 occurred in mobile homes, more than twice the fraction twenty years earlier. It seems likely that the fraction of deaths in mobile homes will continue to increase. Efforts to improve safety practices and communication of forecast and warning information for this group will be especially important in the future, if the long-term decrease in tornado fatalities in the U.S. is going to continue.
 
 

Brooks, H. E., and C. A. Doswell III, 2001: Normalized damage from major tornadoes in the United States: 1890-1999. Wea. Forecasting, 16, 168-176.

Cook, S., and S. Reed, 2001: List of tornado dead and injured.
 

Curzon, J., (Ed.) 1896: The Great Cyclone at St. Louis and East St. Louis, May 27, 1896. Cyclone Publishing Company, Saint Louis, MO, [Republished 1997 by Southern Illinois University Press, Carbondale, IL, ISBN 0-8093-2124-6.]
 

Doswell III, C. A., A. R. Moller, and H. E. Brooks, 1999: Storm spotting and public awareness since the first tornado forecasts of 1948. Wea. Forecasting, 14, 544-557.
 

Grazulis, T. P., 1993: Significant tornadoes, 1680-1991. St. Johnsbury, VT, Environmental Films, 1326 pp.
 

Press, W. H., W. A. Teukolsky, W. T. Vetterling, and B. P. Flannery, 1992: Numerical Recipes in FORTRAN. The Art of Scientific Computing. (Second Edition), Cambridge Univ. Press, 963 pp.
 

Williams, S. K., 1999: Listing of known dead of St. Louis from the cyclone.

Date	Location	Deaths
16 December 2000	Englewood, AL	11
3 May 1999	Oklahoma City, OK	36
10 April 1979	Wichita Falls, TX	42
21 February 1971	Cary-Pugh City, MS	58
25 May 1955	Blackwell, OK-Udall, KS	80
9 June 1953	Worcester, MA	94
8 June 1953	Flint, MI	115
9 April 1947	Woodward, OK	181
6 April 1936	Gainesville, GA	203
5 April 1936	Tupelo, MS	216
18 March 1925	Tri-State (MO-IL-IN)	695

Table 1: Most fatalities in a single tornado in the U.S. going back in time from the end of 2000. Location is most prominent site or sites hit by tornado.
 

Table 2: Annual tornado deaths by selected locations in U.S., 1975-2000. Total is the total number of deaths, Mobile is number of mobile home deaths, and Permanent is number of permanent ("site-built") housing deaths.

Table 3: Percentage of housing units in U.S. by mobile and permanent housing from U.S. Census Bureau decennial housing survey.

Fig. 2: Deaths per year in tornadoes in United States, 1875-2000. Light line with dots shows raw values. Heavy line is smoothed values. Smoother consists of finding median of 3 values and then applying a running mean over 5 medians.

Fig. 3: Death rate per million people per year in U.S., 1875-2000. Thin line with dots is raw rate, curved thick line is death rate, filtered by 3-point median and 5-point running mean filter, and straight solid lines are least squares fit to filtered death rate for 1875-1925 and 1925-2000. Dashed lines are estimates of 10th and 90th percentile death rates from 1925-2000.

Fig. 4: Deaths in 95th percentile tornado. Thin line with dots is raw value for 5-year period, centered on plotted year. Heavy line is filtered, using a 3-point median and 11-year running mean.

Fig. 5: Days between days with at least one 10-fatality tornado (return period). Dots are plotted at second date of pair.

Fig. 6: Smoothed 10-fatality tornado return period (5-point median, 19-point running mean) in days (dots with solid line) compared to 5 billion divided by US population (dashed line).

Fig. 7: Fraction of total deaths occurring in mobile homes, 1975-2000. Thin line with solid dots is for individual years and thick line with open dots is for 5-year average with points plotted at center of period.

Fig. 8: Death rate per million people per year in U.S. for mobile home residents (1975-2000, solid dots) and permanent home residents (1985-2000, open dots).

Fig. 9: Long-term trend in tornado death rate in U.S., based on 1925-2000, extrapolated out to 2020 (solid line) and death rate from model based on mean annual death rate in mobile homes, assuming no deaths occur except in mobile homes and that historical increase in fraction of U.S. population living in mobile homes will continue (dashed line).

Fig. 10: Fraction of total housing units that are mobile homes, based on U.S. Census decennial survey of housing. Southeastern states, including Arkansas, Louisiana, Mississippi, Alabama, Georgia, Florida, Tennessee, North Carolina, and South Carolina (solid line with open dots), central Plains states, including Oklahoma, Kansas, and Nebraska (dashed line with triangles), and rest of the U.S. (solid line with solid dots).