Photograph by Julie Denesha, Getty Images
Published May 23, 2011
The devastating tornado that killed at least 116 people in Joplin, Missouri (map), Sunday may have been spawned in part by warmer-than-normal water temperatures in the Gulf of Mexico, according to meteorologist Jeff Masters.
Tornadoes can spin into being when warm, moist air masses and cold, dry air masses collide, forming storms. The warm air ascends through the colder layers, and given the right wind conditions, the updraft may begin spinning, creating a so-called mesocyclone.
The mesocyclone can eventually form a stormy funnel cloud, which becomes a tornado when it touches down. But a continuous supply of warm air is needed to keep the tornado going—which is where the Gulf of Mexico comes in.
Flowing north into the United States' Tornado Alley region—between the Rocky and Appalachian Mountains—the Gulf's warm, moist air is generally a major factor in tornado formation each spring, said Masters, meteorological director for the Weather Underground website.
Making matters worse this spring, surface water temperatures in the Gulf of Mexico are currently about two degrees Fahrenheit (just under one degree Celsius) above normal in some areas—which might help explain why dozens of tornadoes struck Missouri and other midwestern U.S. states last weekend, he said. (See a U.S. map.)
Joplin, Missouri, Tornado Among Strongest
Flattening buildings, tossing cars like toys, and even ripping up pavement, the tornado that tore through Joplin Sunday may have been an F5—the highest level on the Fujita scale, which ranks tornadoes based on wind speed and damage potential—Masters said.
"A tornado can't rip up pavement unless it's at least an F4, and that kind of damage is most commonly associated with an F5 storm," he said.
An F4 tornado has winds from 207 to 260 miles (333 to 418 kilometers) an hour. An F5 twister has winds from 261 to 318 miles (420 to 511 kilometers) an hour.
Tornadoes Right on Schedule
Despite the Joplin disaster and a record-setting outburst of tornadoes in April, which devastated parts of the U.S. South, intense tornadoes are not becoming more common, Masters said.
"There's no evidence of a change in violent tornadoes in the last 60 years," he said. In terms of tornado fatalities, 2011, with more than 450, ranks as the ninth deadliest in U.S. history, according to the National Severe Storms Laboratory, the Associated Press reported.
In fact, the weekend's midwestern tornadoes fit into the typical tornado-season pattern.
In Tornado Alley—where warm moist air from the Gulf meets cooler, dry air sweeping down from Canada—twisters often form first in the Deep South in April. (See pictures of a monster tornado in Alabama this past April.)
As spring progresses, the hot-cold air clashes move north, making the Midwest a May tornado hot spot. Come June—the last month of the U.S. tornado season—Minnesota and the Dakotas usually see their own tornado surge.
Photojournalist Allison Shelley documented Haiti for a year after the 2010 quake. She went back this month to check on rebuilding progress.
An innovative mapping project could help indigenous people claim ancestral lands—and protect ancient forest.
With little known about sea snakes, scientists worry that massive harvests could be damaging wild populations.
The Future of Food
How do we feed nine billion people by 2050, and how do we do so sustainably?
We've made our magazine's best stories about the future of food available in a free iPad app.