NOAA's National Centers for Environmental Information
Historical Records and Patterns
One of the main difficulties with tornado records is that a tornado, or evidence of a tornado must have been observed. Unlike rainfall or temperature, which may be measured by a fixed instrument, tornadoes are short-lived and very unpredictable. If a tornado occurs in a place with few or no people, it is not likely to be documented. Many significant tornadoes may not make it into the historical record since Tornado Alley was very sparsely populated during the 20th century.
Much early work on tornado climatology in the United States was done by John Park Finley in his book Tornadoes, published in 1887. While some of Finley's safety guidelines have since been refuted as dangerous practices, the book remains a seminal work in tornado research. The University of Oklahoma created a PDF copy of the book and made it accessible at John Finley's Tornadoes.
Today, nearly all of the United States is reasonably well populated, or at least covered by NOAA's Doppler weather radars. Even if a tornado is not actually observed, modern damage assessments by National Weather Service personnel can discern if a tornado caused the damage, and if so, how strong the tornado may have been. This disparity between tornado records of the past and current records contributes a great deal of uncertainty regarding questions about the long-term behavior or patterns of tornado occurrence. Improved tornado observation practices have led to an increase in the number of reported weaker tornadoes, and in recent years EF-0 tornadoes have become more prevalant in the total number of reported tornadoes. In addition, even today many smaller tornadoes still may go undocumented in places with low populations or inconsistent communication facilities.
With increased National Doppler radar coverage, increasing population, and greater attention to tornado reporting, there has been an increase in the number of tornado reports over the past several decades. This can create a misleading appearance of an increasing trend in tornado frequency. To better understand the variability and trend in tornado frequency in the United States, the total number of EF-1 and stronger, as well as strong to violent tornadoes (EF-3 to EF-5 category on the Enhanced Fujita scale) can be analyzed. These tornadoes would have likely been reported even during the decades before Doppler radar use became widespread and practices resulted in increasing tornado reports. The bar charts below indicate there has been little trend in the frequency of the stronger tornadoes over the past 55 years.
Timing
Because most tornadoes are related to the strength of a thunderstorm, and thunderstorms normally gain most of their energy from solar heating and latent heat released by the condensation of water vapor, it is not surprising that most tornadoes occur in the afternoon and evening hours, with a minimum frequency around dawn (when temperatures are lowest and radiation deficits are highest). However, tornadoes have occurred at all hours of the day, and nighttime occurrences may give sleeping residents of a community little or no warning.
Tornado Occurrence by Hour of Day for the United States
Tornado Occurrence by Hour by Region of the United States
In addition, tornadoes occur throughout the year. Because a tornado may occur at any time of the day or year somewhere in the United States, there really is no national tornado "season" (as there is with Atlantic hurricanes). Instead, each region may experience increased tornadic potential at different times of the year. Like with the diurnal pattern, for the United States (and hemisphere) as a whole, the months in which tornadoes are most likely correspond to the times of year with increased solar heating and strong frontal systems. Regionally, the frequency of tornadoes in the United States is closely tied with the progression of the warm season when warm and cold air masses often clash. Most of the early spring tornadoes in the United States tend to occur in the Southeast and South Central regions. Gulf States, such as Mississippi and Louisiana, are the frequent recipients of tornadoes from February to April. Late spring tornadoes generally spread a bit farther north, often into Kansas, Nebraska, and the Tennessee Valley region. By midsummer, most of Tornado Alley is active and tornadoes may occur throughout the United States. Late summer tends to bring some of the stronger tornadoes into the upper Midwest and Ohio valleys, and the pattern shifts back southward into the late autumn. The fewest tornadoes are documented during the winter months. Although rare, deadly winter outbreaks do occur.
Average Tornado Frequency by Month of Year 1991–2010
