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Tornadoes Scott R. Lillibridge, M.D. Centers for Disease Control & Prevention

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TornadoesScott R. Lillibridge, M.D.Centers for Disease Control & Prevention

INTRODUCTIONBackground and Nature of the ProblemTornadoes are funnel-shaped wind storms that occur when masses of air with differing physical qualities (e.g., density, temperature, humidity and velocity) collide (1). These violent rotating winds converge to form a vortex, which is usually narrow at the base and which gives a tornado its typical funnel-shaped appearance. Air and debris are actively drawn into the base of the vortex as the tornado moves across the ground, resulting in a path of destruction. Tornadoes in the northern hemisphere rotate counter clockwise, while those in the southern hemisphere rotate clockwise (2).

Winds associated with tornadoes can reach speeds in excess of 250 miles per hour (mph) (1,3). Because weather conditions that create tornadoes may be present over a large geographic region, tornado outbreaks, defined as six or more tornadoes, may occur within a relatively short period of time (2). For example, in 1974, an outbreak of 148 tornadoes throughout the eastern United States affected 13 states and resulted in approximately 300 deaths and 6,000 injuries (4). The cost of property damage caused by a single outbreak can be in excess of 200 million dollars (5).

As shown in Table 11-1, tornadoes are rated by the Fujita-Pearson Tornado Scale (F0 through F5) on the basis of the estimated wind speed of their vortices and the width and length of their paths (3,6). Most tornadoes (60%) are considered weak (F0,F1), with wind speeds less than 113 mph; these have limited potential to cause injury or destroy property. However, 1-2% of all tornadoes are considered violent (F4-F5), with wind speeds greater than 206 mph; these tornadoes are highly destructive and account for more than 50% of all tornado-related deaths in the United States (Table 11-2).

Because the force of a tornado is strongly associated with its potential to cause injury and death, the number of violent tornadoes (F4,F5) per area of land mass may provide a more accurate representation of a state's public health risk from tornadoes. States with high concentrations of F4 and F5 tornadoes include Oklahoma, Indiana, Iowa and Kansas (2). Among all states, Florida has the highest concentration of tornadoes (tornadoes per 10,000 square miles) (Table 11-3), although tornadoes in Florida tend to be weak (F0,F1) and thus to have limited public health impact.

In the United States, most tornadoes (59%) travel toward the northeast at an average ground speed of 40 mph (2). However, tornadoes have been noted to move along the ground at velocities up to 75 mph, remain stationary, or even reverse course. The average length of a tornado is 4.4 miles and the average width of a tornado is 128 yards. Most tornadoes last for only a few short minutes; however, maximum strength tornadoes have been known to travel more than 200 miles (Table 11-4), persist for hours, and span 3 miles in width (2,3). In the United States, the peak time of day for the occurrence of tornadoes is between 5 and 6 p.m. (Figure 11-1).

The time of year is also an important factor related to the development of tornadoes (Figure 11--2). April, May, and June are the peak months for the occurrence of tornadoes in the United States; however, the months of peak occurrence vary considerably from state to state. For example, the peak season for tornadoes in North Dakota is actually June, July, and August. Although tornadoes occur most frequently in certain geographical regions, at certain times of days, and during certain months, tornadoes have actually been recorded at all hours of the day and in all months of the year. Tornadoes have also been recorded in Alaska and Hawaii.

The property destruction that accompanies a tornado is directly related to the strength of its destructive winds. These forces can be sufficient to sweep houses off their foundations and carry debris hundreds of miles. Tornado outbreaks with multiple simultaneous tornadic storms may create substantial and wide-ranging demands on state and local emergency services and health departments (5). The disruption of electrical power and telephone services and the financial losses due to tornadoes can be catastrophic to a community (3). Because of the highly localized path of destruction that accompanies a tornado, the infrastructure of an affected community usually remains intact.

Scope and Relative Importance of Tornadoes North America is the most tornado-active continent and is known for particularly forceful and destructive tornadoes (2). In North America most of these destructive storms occur within the continental United States (10). Among all natural hazards in the United States, only floods and lightning result in more weather-related deaths (11). Other countries, such as Canada, Russia, Australia, China, India, and Bangladesh, are also tornado-prone and have recorded significant tornado-related disasters (2).

Factors that Contribute to Tornado DisastersTornadoes pose their greatest danger to public health as they move through heavily populated areas (5,8). Unlike other disasters, such as hurricanes, which are preceded by a lengthy warning period that allows response officials to evacuate the vulnerable population, tornadoes occur suddenly. Therefore, unless disaster mitigation steps are undertaken prior to the tornado, preventing death and injury from these storms may be impossible. Inadequate warning and a lack of suitable sheltering for the population at risk are the main contributing factors to the adverse public health effects of tornadoes.

Unfortunately, many community residents do not have access to appropriate storm shelters (5). A suitable storm shelter for people threatened by tornadoes is usually defined as a subgrade (underground) structure such as a basement or a structure specifically designed to provide temporary protection from adverse weather conditions. People in multiple-family dwellings and other tornado-vulnerable communities such as mobile home parks pose a special storm sheltering challenge for disaster management officials, particularly when no community storm shelters are located nearby.

FACTORS AFFECTED PROBLEM OCCURRENCE AND SEVERITYBecause of recurrent weather patterns favorable for the formation of tornadoes, certain geographic regions are at higher risk for the development of tornadoes than other areas. For example, from 1953 through 1991, five states (Texas, Oklahoma, Florida, Kansas, and Nebraska) accounted for 11,935 (40%) of the 29,953 tornadoes that occurred in the United States (2) (Figure 11--3).

Public Health Impacts: Historical PerspectiveFrom 1953 through 1991, the United States averaged 768 tornadoes per year (14). During that period, an average of 93 people were killed each year by tornadoes in the United States. In 1882, in one of the earliest and most comprehensive accounts of U.S. tornadoes, U.S. Army Sergeant J.P. Finley reported on 600 tornadoes that occurred from 1860 through 1880 (15). While stationed in the midwestern region of the United States, he investigated many reports of tornadoes. He traced their paths and recorded their patterns of damage.

Sergeant Finley also established in the midwestern United States a tornado surveillance network composed of more than 1,000 people who routinely collected and reported details concerning the occurrence and effects of local tornadoes. Such a program was the first comprehensive and systematic weather reporting system, and it set the stage for the development of a modern national meteorological observational system. Many of Sergeant Finley's original observations and safety recommendations have stood the test of time.

Interestingly, although the annual number of tornadoes reported in the United States generally increased from 1921 through 1990, the annual number of tornado-related deaths actually decreased (Figure 11- 4) (2,14). One reason for the decrease in mortality associated with tornadoes is the increasing effectiveness of the National Weather Service's (NWS's) severe storm warning systems. Since 1952, when the NWS first began broadcasting tornado warnings, the number of tornado-related deaths has been declining steadily (2).

FACTORS INFLUENCING MORBIDITY AND MORTALITY ASSOCIATED WITH TORNADOESNatural factorsAs shown in Table 11-2, a population's risk for injury and death from a tornado increases with the tornado's strength. Fortunately, violent (F4 and F5) tornadoes are rare (2). Another factor related to the risk from a particular tornado is the length of its path. However, the association between the length of a tornado's path and its destructive power is somewhat confounded by the fact that tornadoes with long paths also tend to be the most powerful and destructive (10).

Human FactorsHuman factors also contribute to the severity of the adverse public health consequences associated with tornadoes. Epidemiologic studies have identified several risk factors for injury and death during tornado disasters, including the following: 1) residing in mobile homes; 2) being more than 60 years old; 3) remaining in a vehicle; 4) failing to seek shelter during a tornado warning; and 5) being unfamiliar with tornado-warning terminology (2,5,8-9,16,18-19). Although the number of tornado-related deaths in the United States is decreasing, the number of tornado-related deaths among people residing in "mobile homes" may be growing at an alarming rate (5).

As consistently demonstrated in several studies, tornado-related morbidity and mortality rates are higher when no effective storm warnings are issued and when no suitable storm shelters are available. Furthermore, the effectiveness of official warning-dissemination systems is limited if populations are unable to understand the warnings because they are delivered in a culturally inappropriate manner (20). For example, during a tornado disaster in 1987, the unincorporated rural community of Saragosa, Texas (population 200-415, largely Hispanic), was struck by a single, violent, maximum-strength tornado (F-5) that killed 30 people and injured 131 more (20).

PREVENTIONAND CONTROL MEASURESWarningOne of the most important prevention tools that NWS warning meteorologists use to reduce a population's risk from tornadoes is the NEXRAD (Next Generation Weather Radar) doppler radar weather system (22). This new technology provides tornado detection that is from 30% to 60% more sensitive than conventional radar at distances of more than 200 miles (23). Doppler weather radars are able to detect the movement of tornadoes an average of 20 minutes before touchdown.

The resulting tragedy illustrates the importance of expanding the availability of new technology such as doppler radar to all tornado-prone regions. In addition to improvements related to tornado-detection, the NWS is undergoing a major nationwide modernization program to improve the dissemination of weather hazard information through radio, television, and telephone. Storm warnings are increasingly being incorporated into the commercial news media (e.g., television's 24-hour "Weather Channel") (24). Continued improvements in tornado detection and warning dissemination are credited with the decrease in the annual number of observed tornado-related deaths in the United States (2).

Epidemiology: Assessment and SurveillanceEpidemiologists may be required to provide technical assistance in the conduct of emergency health assessments of populations affected by tornado disasters (25-26). Such assessments following tornado disasters are usually conducted by following the path of the storm in a simple door-to-door fashion or by assessing the needs of people who have been displaced from their homes into temporary shelters. Because tornadoes have their greatest adverse effect on public health when they move into populated areas, assessment teams should initially focus on highly urbanized residential areas when resources are limited.

Postdisaster public health surveillance of a population affected by a tornado may be an important component of the emergency public health response (27). Information obtained from such surveillance can be used to detect epidemics of infectious disease, injuries, and environmentally related illness that occur within a population as a result of a tornado disaster (28-29). To be effective, postdisaster surveillance activities for health and medical conditions should be based at clinical sites and at both official and unofficial shelters where displaced persons have collected.

Engineering and Legal ControlsResidents of "mobile homes" have been identified as being at especially high risk for death and injury from tornadoes (8,18). The most graphic example occurred in Andover, Kansas, in 1991, when an F-5 tornado struck a mobile-home community and destroyed 205 residential units. Of the 38 unsheltered residents who remained within the mobile-home community, 11 (29%) were killed, 17 (45%) were hospitalized, and 9 (24%) sustained injuries that were treated on an outpatient basis. Incredibly, only one unsheltered person escaped injury.

Medical Treatment and RehabilitationBecause tornadoes create destruction along paths that may span many miles, quickly determining where the majority of casualties occurred can be difficult. Following a major tornado, communication systems are frequently disrupted and highways are often blocked with debris. Many patients with minor injuries may suddenly converge on local hospital emergency departments and overwhelm available staff and material resources. This may hinder efforts by the hospital staff to prepare for more severely injured patients who may arrive later because of their need for extrication, field stabilization, and ambulance transport.

Public Awareness and EducationTornado watches and warnings issued by the NWS to the media and to community organizations are the primary means of alerting the public about an approaching tornado (4). An NWS Tornado Watch means that weather conditions are conducive to the development of tornadoes. An NWS Tornado Warning means that a tornado has been sighted by ground observers or has been detected by advanced technology. One limitation to this nomenclature is that the local population may not understand the difference in meaning between the two terms.

CRITICAL KNOWLEDGE GAPSGiven the frequency of tornadoes within the United States, far too little investigation has been done to determine risk factors for injury and death in order to develop more effective strategies for protecting the public from the hazardous effects of tornadoes. Further epidemiologic studies are clearly needed. Numerator data describing the age, sex, and race of people injured or killed and denominator data describing the population at risk are necessary to develop rates of tornado-related morbidity and mortality (37).

Current severe-weather morbidity and mortality surveillance systems (e.g., the National Climatic Data Center) collect only limited information on the circumstances associated with tornado-related injuries or deaths. Medical records and coroner reports are usually not examined by those collecting information on the effects of adverse weather events in order to obtain further data on the mechanism of injury or the cause of death. Little information is collected on the shelter-seeking behavior of victims at the time of injury or death or on whether victims had been adequately warned that a tornado was approaching.

METHODOLOGIC PROBLEMS OF EPIDEMIOLOGIC STUDIESLike most natural disasters, tornadic storms create situations in which the size of the affected population is difficult to determine. Estimating the size of populations affected by tornadoes is especially difficult because tornadoes often take meandering paths of destruction across a community and, thus, the entire community may not be uniformly affected. Another key element of any epidemiologic investigation of a community affected by a tornado is determining the amount of warning time that population had prior to the tornado's impact.

Another difficult task in comparing injuries and deaths associated with any specific tornado is determining the extent to which various building types (e.g., woodframe, brick) tend to resist wind damage. Highly resistant structures would be expected to protect their occupants better. In addition, the windspeed gradient associated with a tornado may differ significantly at distances of only a few meters from the funnel cloud. Therefore, the extent to which a person's injuries reflect the full effect of the winds associated with a particular tornado is often unclear.

RESEARCH RECOMMENDATIONSConduct research to define the optimum use of new warning technology. Although the NWS has developed improved tornado prediction and detection methods (2,22), the communication of this information to the public can still be improved. Automatic telephone warning systems and NWS storm warning radios show great promise. In addition, research to develop storm-warning technology that is specific for both indoor and outdoor populations is required if communities are to be fully protected.

SUMMARYTornadoes remain the most frequent adverse weather event likely to result in a disaster in the United States. Unfortunately, preventing tornadoes is currently beyond our technical capabilities. Research results have illustrated the life-saving importance of providing adequate warning and shelter to populations at risk. These prevention measures are particularly important in states with large numbers of violent tornadoes (e.g., Kansas, Iowa, Illinois, Indiana, and Oklahoma) and in locations where many people live in mobile homes (5,39). The National Weather Service continues to modernize its nationwide adverse weather warning system.


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