Breadcrumb Home Deadliest tornado outbreak in decades was fueled by smoke from land clearing Wednesday, February 11, 2015 http://news.nationalgeographic.com/news/2015/02/150209-tornado-smoke-aerosol-st… A University of Iowa study has found that land-clearing fires in Central America intensified the tornadoes of April 27, 2011, the worst day of U.S. tornadoes since 1974.