S., although not by a sufficient amount to make it distinguishable from natural variability, at least not yet.
Computer-model projections of how instability may change in the future, though, show that this tornado ingredient is likely to increase because of warming surface temperatures and the addition of moisture in the air through evaporation.
Instead, an increase in tornado counts of EF-0 or stronger tornadoes has been attributed to an uptick in observations of very weak tornadoes.
The Enhanced Fujita Scale measures tornado strength based on the extent and type of damage that they cause (no surface weather station has ever survived a direct tornado strike to take wind measurements from inside a twister).
That increase in atmospheric instability could boost the number of days with severe thunderstorms in parts of the U. One study, published in the Proceedings of the National Academy of Sciences in 2007, found that a doubling of greenhouse gases in the atmosphere would boost the number of severe thunderstorm days in parts of the U.
S., particularly in the Southeast and along the East Coast.In 2011, there was a La Niña event in the tropical Pacific Ocean, which has been linked to active tornado seasons in the U. The drought continued into the first half of this year, before shrinking back to the west, and this tornado season got off to a very slow start. The results of a computer-modeling study comparing the projected summer climate in 2072–2099 from 1962–1989.CAPE is predicted to rise enough to overwhelm a slight decrease in vertical wind shear, leading to an increase in severe thunderstorm days, particularly in the eastern states. A key ingredient for producing tornadoes is a warm, moist, and unstable atmosphere.It's difficult to tease out trends from historical tornado data, due to changing reporting practices, population growth, and the advent of advanced radar technology that has allowed meteorologists to spot more tornadoes now than ever before.It's thought that many tornadoes — particularly weaker ones — were missed in the early decades of recordkeeping, and construction methods have also changed with time.A CAPE value above 2,500 is considered to be an indication of extreme instability in the atmosphere, which means that if a trigger comes along for a storm, such as a cold front, storms could develop rapidly and quickly turn severe.The high amount of instability on Monday helped thunderstorms explode along a slowly moving frontal boundary separating hot, humid air to the east from comparatively cooler and drier air to the west.Similarly, there is no evidence to indicate that EF-4 and EF-5 tornadoes — like the one that decimated a large swath of Moore — are becoming more frequent or severe.Such tornadoes are rare — they comprise less than 1 percent of the total number of tornadoes — yet they are the most reliable killers, accounting for 70 percent of tornado fatalities.According to the Storm Prediction Center, in 2012 there were just 939 tornadoes and 70 fatalities.Large-scale climate patterns can influence tornado seasons. In 2012, much of the country was hit with widespread drought, which dried up the severe thunderstorm season just as it dried up wheat fields across Tornado Alley. had the longest-ever streak of days without a tornado fatality Because historical tornado data is not considered very reliable or consistent, scientists have focused especially closely on how a warming climate is altering the balance of ingredients that go into producing a tornado.