Smoke from fires in Mexico and Central America may have worsened one of the largest tornado outbreaks in recent decades, a new study suggests. In the afternoon and evening of 27 April 2011, more than 120 tornadoes ravaged a swath of the southeastern United States, killing 313 people. (Tornado shown, which occurred later that year, was not part of the historic outbreak.) At the same time, meteorologists noted several layers of smoke-filled air over the region that could be tracked back to fires set to clear agricultural lands for spring planting in eastern Mexico and Central America. Researchers have now used a computer model to assess the possible effects of smoke on the region’s weather that day. In the simulations that included smoke-filled air, storm clouds were lower and thicker (and average wind speeds at 1 kilometer above ground were higher) than they were in the no-smoke models, the team reported online ahead of print in Geophysical Research Letters. The smoke-induced increase in wind speed at an altitude of 1 km enhanced a phenomenon called wind shear, the difference in wind velocity between one layer of the atmosphere and another. Although lower, thicker storm clouds and increased wind shear don’t directly cause tornadoes, they have been shown to intensify a tornado’s strength if one does form. As a result, the presence of smoke wafting into a region from distant fires should be included in weather models, the researchers say.