The Connection Between Central American Farmers and Deadlier Tornadoes in the U.S.

Scientists link smoke from the burning of agricultural fields to more intense twisters.
(Photo: Roger Hill/Getty Images)
Feb 13, 2015· 1 MIN READ
Padma Nagappan is a multimedia journalist who writes about the environment, renewable energy, sustainability, agriculture, and biotechnology.

Tornado season in the United States has grown deadlier in recent years, and 2011 was one of the worst, spawning 1,691 twisters.

Now scientists at the University of Iowa have identified a possible culprit for more intense tornadoes: smoke. From Central America.

Smoke from fires set by farmers to burn agricultural waste that drifted north helped make the 2011 tornadoes in the U.S. more powerful, according to a new study.

As a growing global population leads to more land clearing to grow crops and thus more burning of forests and agricultural waste, the phenomenon could become more common.

The findings by University of Iowa atmospheric scientists are significant because they can help weather forecasters better predict the strength of storms and prepare residents of tornado-prone states to hunker down or evacuate.

Tornado season is just around the corner. Twisters begin ripping through Southern states in March and Plains states in late spring.

“Even without the smoke drifting up north, there would still have been an unusual number of tornadoes in 2011, but we found the smoke changed the parameters,” said Pablo Saide, a postdoctoral student at the University of Iowa and a coauthor of the study.

Tornadoes develop when warm, moist air meets cool, dry air, leading to instability that is compounded when the wind changes speed and height. This creates a horizontal spinning effect in the lower atmosphere, which gets pushed up by rising air into a vertical vortex and results in the raging tornadoes that occur during thunderstorms, tropical storms, and hurricanes.

Saide said smoke from the Yucatán peninsula, which includes parts of Mexico, Belize, and Guatemala, changed two key conditions for the 2011 tornadoes. First, it lowered the height of the cloud base, which dropped 650 feet. It also changed the wind shear, which is a variation in vertical wind speed. Both conditions intensify tornadoes.

“If winds at the lower level where we live are about 10 miles an hour, for example, then the wind one mile from ground level would be twice as fast,” Saide said, noting he was not referring specifically to the 2011 tornadoes. “Combined with lower clouds, this leads to higher chance of tornadoes and stronger tornadoes.”

How does smoke affect the clouds and wind shear?

Smoke increases the number of aerosol particles in clouds, which makes them brighter and more reflective. Soot from smoke also interacts with the sun and absorbs radiation, which heats nearby layers of air.

“Because of this, you get less solar radiation on the ground, which lowers temperature on land,” Saide said. “And the lower atmosphere is more stable, so together this brings the clouds down lower.”

He said the scientists plan to study other tornado outbreaks to see if smoke is a common trigger.