Tornado season becoming more intense

New Scientist : Although the number of tornadoes per year in the US has not increased over the past several decades, more tornadoes are occurring on a given day than ever before. James Elsner of Florida State University and colleagues report that since 2001 there has been at least one day per year when 32 or more tornadoes struck, and in 2011 there were six such days. Before 1980 it was rare to have as many as 16 in a day. The reason may be due to global warming, which is causing both Earth’s surface and upper atmosphere to heat up. Tornadoes form when cold air in the upper atmosphere allows warm air near the surface to quickly rise. With fewer periods of cold air high up, there are fewer opportunities for warm surface air to escape. But when it does, it “goes crazy,” says Elsner. Such shifts in atmospheric temperature may also be influencing other weather systems, such as hurricanes and rainstorms.