Vulnerability due to Nocturnal Tornadoes

Abstract

This study investigates the human vulnerability caused by tornadoes that occurred between sunset and sunrise from 1880 to 2007. Nocturnal tornadoes are theorized to enhance vulnerability because they are difficult to spot and occur when the public tends to be asleep and in weak building structures. Results illustrate that the nocturnal tornado death rate over the past century has not shared the same pace of decline as those events transpiring during the daytime. From 1950 to 2005, a mere 27.3% of tornadoes were nocturnal, yet 39.3% of tornado fatalities and 42.1% of killer tornado events occurred at night. Tornadoes during the overnight period (local midnight to sunrise) are 2.5 times as likely to kill as those occurring during the daytime hours. It is argued that a core reason why the national tornado fatality toll has not continued to decrease in the past few decades is due to the vulnerability to these nocturnal events. This vulnerability is magnified when other factors such as escalating mobile (or "manufactured") home stock and an increasing and spreading population are realized. Unlike other structure types that show no robust demarcation between nocturnal and daytime fatalities, nearly 61% of fatalities in mobile homes take place at night revealing this housing stock's distinct nocturnal tornado vulnerability. Further, spatial analysis illustrates that the American South's high nocturnal tornado risk is an important factor leading to the region's high fatality rate. The investigation emphasizes a potential break in the tornado warning dissemination system utilized currently in the United States.

Tornado fatalities and mobile homes in the United States

“…Tornadoes are significantly more lethal at night than during the day (Simmons and Sutter 2008a;Ashley et al 2008). We examine the timing of mobile home fatalities in the next section, and find that the mobile home problem particularly stems from tornadoes after dark.…”

“…This is consistent with a greater vulnerability of mobile homes, as tornadoes that might only damage permanent homes could destroy mobile homes and kill occupants. Mobile home fatalities are also more likely to occur at night, and so the mobile home problem is related to the vulnerability of nocturnal tornadoes (Simmons and Sutter 2008a;Ashley et al 2008). Fatalities do not occur at a higher rate at night in permanent homes, which suggests that the vulnerability of mobile homes at night results from residents being at home, not less effective warning transmission, consistent with Schmidlin et al (2009).…”

A Bayesian modelling framework for tornado occurrences in North America

“…Substituting b back into equation (2) to identify the population thresholds where nearly all tornadoes can be observed (p i (b)Z0.995), the threshold levels ranged from 6.2 to 7.2 persons km À 2 for F0-F5 tornadoes, which is consistent with the findings of other studies 2,21 , and 6.4 to 14.0 persons km À 2 for F2-F5 tornadoes ( Supplementary Figs 3 and 4). These predicted patterns may be related to the seasonal variability in the day length, which in turn affects the ability to observe tornadoes 22 . Moreover, because of the higher proportion of nocturnal tornadoes occurring in the winter months (10% in July to 40% in February for F2-F5 tornadoes), the daylight effect on the ability of observing tornadoes may be compounded 22 .…”

“…These predicted patterns may be related to the seasonal variability in the day length, which in turn affects the ability to observe tornadoes 22 . Moreover, because of the higher proportion of nocturnal tornadoes occurring in the winter months (10% in July to 40% in February for F2-F5 tornadoes), the daylight effect on the ability of observing tornadoes may be compounded 22 . Regarding the higher population threshold range derived for the F2-F5 ARTICLE tornadoes, we believe that it likely reflects the fact that significant tornadoes tend to be reported in more populated areas comparing with F0-F5 events.…”

A Model for U.S. Tornado Casualties Involving Interaction between Damage Path Estimates of Population Density and Energy Dissipation

“…We find 159 that the elasticity of the tornado casualty rate with respect to energy dissipation is positive and 160 statistically significant across most of the range of population density values, and that this elasticity 161 increases with population density. We also find that the elasticity of the tornado casualty rate with 2 The question of whether a tornado produces casualties is interesting but it has been addressed elsewhere (e.g., Ashley et al (2008) …”