Washington, Nov 6 : A new study has determined that tornadoes that occur from midnight to dawn are 2.5 times more likely to prove fatal than twisters in the daytime.
The study, by scientists from the Northern Illinois University, also suggests that warning systems that have led to overall declines in tornado death rates might not be adequate for overnight events.
Nighttime tornadoes occur most frequently in the mid-South region of the United States.
Over the past century, the tornado death rate has declined, in large part because of sophisticated forecasting technology and warning systems.
But, the researchers found that the nighttime tornado death rate over the past century has not shared the same pace of decline as the rate for daytime tornadoes.
"The proportion of nocturnal fatalities and killer tornado events has increased during the last half century," said lead author Walker Ashley, an NIU meteorologist and professor of geography.
"Unfortunately, this nocturnal fatality rate appears to be a major factor for the stalled decline in national tornado-fatality tallies during the past few decades," he added.
The study found that from 1950 to 2005, 27 percent of tornadoes in the United States were nocturnal, yet 39 percent of tornado fatalities and 42 percent of killer tornado events occurred at night.
Ashley predicts that annual tornado fatalities might begin to rise.
In 2007 alone, 80 tornado fatalities were recorded, with 59 of those fatalities occurring between sunset and sunrise. Nineteen of 26 killer tornadoes that year occurred at night.
So far this year, 123 tornado fatalities already have been recorded-nearly double the annual average.
"The tornado death rate has bottomed out and is probably going to increase due to several factors," Ashley said.
"Because of population growth and development patterns, including urban sprawl, tornado risk to the populace has increased in recent decades. Tornadoes are impacting larger populations that are more spread out, resulting in higher tornado death tallies," he added.
The most dangerous window of time for a tornado, according to the study findings, is the period from midnight to sunrise.
Tornadoes during this time period are 2.5 times as likely to kill as those occurring during the daytime hours.
"Because most people go to bed after the late evening news, they are sleeping and unaware of televised weather alerts," Ashley said.
"And warning sirens give us a false sense of security. They're not designed for warning people who are already indoors. We're not seeing a forecasting problem but rather a communication breakdown," he added.