The national news is dominated by yet another set of extraordinary tornadoes in the southeastern U.S. Last week, Tuscaloosa, Ala., was devastated, and the death toll is still unknown as searchers comb through the debris. But on April 27 alone, more than 160 tornadoes were reported, causing at least 300 deaths (210 of those in Alabama), the second deadliest tornado day in U.S. history.
The death toll from this storm is second only to the more than 300 killed in the legendary April 3-4, 1974, “Super Outbreak,” which caused death and destruction from Michigan, Indiana and Ohio to Alabama and Georgia.
These storms follow closely on the heels of major tornado outbreaks across the Midwest and Southeast in February, March and early April.
What is going on here? The basic causes of tornadoes are well understood. Typically, a warm moist air mass from the Gulf of Mexico moves north and meets cooler, drier air from the northern Plains and the Rockies. When these collide, a strong front develops, which causes a big horizontal cylindrical vortex to form as the warm air slides beneath the cold air and thunderheads grow. If there is also strong shear from the jet stream, the horizontal cylindrical spiral of air will tilt into a vertical funnel. If it is big enough, it will touch the ground and become a tornado.
Because tornadoes are generated when these different air masses come into contact, they are most common in the spring, when the weather is transitioning from cold on the northern Plains to hot on the Gulf Coast. The U.S. has by far the lion’s share of the world’s tornadoes due to its unique geography: the Rocky Mountains on the west funneling and blocking air masses, the warm Gulf air moving up the south and cold air from Canada to the north. The only other country in the world with significant tornadoes? Bangladesh, which also has high mountains. The world’s deadliest tornado occurred not in the U.S., but in Bangladesh on April 26, 1989, killing more than 1,300 people.
So far, the U.S. tornado season is getting off to a rip-roaring start, with more than 1,051 tornadoes reported, at least 444 of those confirmed. It could break the previous records for tornado seasons if this trend continues, although there have been some spectacular early tornado seasons in the past that later fizzled and ended up being average or below average years.
But the good news is that these numbers don’t yet approach the deadliest tornadoes in U.S. history, including more than 700 killed and 2,027 injured by the Tri-State Tornado of March 18, 1925, which carved a path of destruction for hundreds of miles from Missouri to Indiana.
Today, there may be numerous dangerous tornadoes in a given year, but a higher percentage of people survive because of warnings and shelters and better building construction — even though our population growth is putting more people in harm’s way.
Are tornadoes becoming more frequent than ever before? Here, the evidence is inconclusive. Data from the National Oceanographic and Atmospheric Administration seem to show a fairly stable but fluctuating trend of about 800 to 1,000 tornadoes per year between 1970 and 1988. Then, starting in 1989, the trend climbs to about 1,200 tornadoes per year, with a big spike of 1,300 to 1,400 tornadoes in 1998 and 2003, 1,692 in 2008, 1,156 in 2009, and 1,282 in 2010.
As far as I know, no one has done a careful statistical analysis to see if this trend is meaningful and whether it holds up against the records over the past century.
But it would not surprise me to find that the warming of the tropics that drives hurricanes also means more energy in those Gulf Coast warm moist air masses that cause tornadoes.
Donald R. Prothero, the author of “Catastrophes: Earthquakes, Tsunamis, Tornadoes, and Other Earth-Shattering Disasters,” is a professor of geology at Occidental College in Los Angeles.
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