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Submitted by George Elliott on Sun, 06/23/2013 - 7:27am.
We all know tornadoes can occur at any time of the day, but not long ago it was a common misperception to believe that tornadoes at night were either weaker, and/or less desctructive. Not only that, a lot of people were under the impression that there are very few nighttime tornadoes. But, we now know these assumptions are wrong, and with increasing evidence, it appears that there's a lot more going on in the severe weather environment at night that produce tornadoes.
Evidence shows that in areas where significant late afternoon and evening tornadoes (only the stronger F2-F5 categories) was less likely, there was a higher likelyhood of nocturnal tornado activity. These nocturnal environments may be characterized by only marginal thermodymic energy, such as you see during the peak heating during the late afternoon, but also have great amounts of directional wind shear (winds changing direction with height), as well as strong low and mid-level (especially low-level) jet stream flow (narrowly focused high-speed winds).
All this can lead to the grim truth that injuries and deaths during nocturnal tornadoes is quite high. A new study by Northern Illinois University scientists underscores the danger of nighttime tornadoes and suggests that warning systems that have led to overall declines in tornado death rates might not be adequate for overnight events, which occur most frequently in the nation's mid-South region, which includes Arkansas, Louisiana, Tennessee, Mississippi, and Alabama.
The number of tornado deaths have decreased a lot over the past 50 years, however, the rate of death from the nocturnal tornado activity has not decreased. This is all the more important as the population grows, especially in areas most vulnerable to nocturnal tornadoes.
Additionally, a 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.
Tornadoes that occur between midnight local time and sunrise seem to have a 2.5 times the death rate as compared to afternoon storms. This is not due to storm strength, lengths, etc., but due to the fact that getting the warnings out to the sleeping population is extremely difficult, especially since people pay very little attention to the need to monitor weather conditions 24 hours a day (such as with weather radios).
Other reasons for higher nighttime tornado injuries and deaths are:
-- Tornadoes are difficult for the public and trained spotters to see; people are more likely to be in structures that are more susceptible to damage, such as single-family homes and mobile or manufactured homes as opposed to schools and large office or workplace buildings, and
By: George Elliott