By Doyle Rice
USA Today

Smoke from fires in Central America worsened a deadly tornado outbreak in the U.S. in 2011, according to a new study.

In late April of that year, violent tornadoes killed more than 300 people — mainly in Alabama and Mississippi — and caused $11 billion in damage, making it one of the costliest outbreaks on record, according to the National Climatic Data Center.

Though weather conditions in the U.S. were the primary cause of the severe weather outbreak, researchers say smoke particles from agricultural land-clearing fires in Mexico, Guatemala and Belize worsened the intensity of the storms.

The smoke lowered the base of the clouds and increased wind shear, said Gregory Carmichael, a professor of chemical and biochemical engineering at the University of Iowa and lead author of the study. Wind shear — a key ingredient in tornado formation — occurs when winds blow from different directions at different heights in the atmosphere.

"It is the first study to show smoke influence on tornado severity in a real-case scenario," Carmichael said. "Severe weather prediction centers do not include atmospheric particles and their effects in their models, and we show that they should at least consider it."

Weather experts not affiliated with the study gave it a mixed review.

"I'm somewhat skeptical of the impact," said Harold Brooks, a meteorologist with the National Severe Storms Laboratory. Smoke "could be important in marginal cases, providing enough help to get things into the right ballpark," he added.

Meteorologist Greg Carbin of the Storm Prediction Center said the link between smoke and violent tornadoes should be investigated further.

"Increased smoke/soot could enhance conditions for low-level storm rotation and intensification," but it could also act to suppress the development of more vigorous storms, Carbin said.