Tornado Alley is a colloquial term for the area of the United States where tornadoes are most frequent. Although an official location is not defined, the areas in between the Rocky Mountains and Appalachian Mountains are the areas usually associated with it. Tornadoes are most common here because it is the region where warm, moist air from the Gulf of Mexico meets cold, dry air from the Rocky Mountains and Canada, creating intense, tornado-producing thunderstorms known as supercells.
Tornado geography 
Though no state is entirely free of tornadoes, they occur more frequently in the plains between the Rocky Mountains and Appalachian Mountains. According to the storm events database of the National Climatic Data Center, Texas reports more tornadoes than any other state, though the very large land area should be taken into account. Kansas and Oklahoma rank first and second respectively in the number of tornadoes per square mile. Florida also reports a high number and density of tornado occurrences, though tornadoes there rarely approach the strength of those that sometimes occur in the southern plains.
Though Tornado Alley is considered to be in areas of the Central United States, no official definition of the term has ever been designated by the National Weather Service. According to the National Severe Storms Laboratory FAQ, "Tornado Alley" is a term created by the media as a reference to areas that have higher numbers of tornadoes. Over the years, the boundaries of Tornado Alley have not been clearly defined, but the differences are the result of the different criteria used to define the region. Ninety percent of tornadoes hit this region of the U.S because cold, dry air from Canada and the Rocky Mountains meets warm, moist air from the Gulf of Mexico and hot, dry air from the Sonoran Desert, which causes atmospheric instability, heavy precipitation, and many intense thunderstorms.
The most common definition of Tornado Alley is the location where the strongest tornadoes occur more frequently. The core of Tornado Alley consists of northern Texas (including the Panhandle), Oklahoma and Kansas. However, Tornado Alley can also be defined as an area reaching from central Texas to the Canadian prairies and from eastern Colorado to western Pennsylvania. It can also be disputed that there are numerous Tornado Alleys. In addition to the Texas/Oklahoma/Kansas core, such areas also include the Upper Midwest, the Ohio Valley, the Tennessee Valley and the lower Mississippi valley.
The term "tornado alley" was first used in 1952 by U.S. Air Force meteorologists Major Ernest J. Fawbush (1915–1982) and Captain Robert C. Miller (1920–1998) as the title of a research project to study severe weather in parts of Texas and Oklahoma.
The nickname "Dixie Alley" is sometimes used for the areas in the southeastern U.S. – notably the lower Mississippi Valley and the upper Tennessee Valley. This region is particularly vulnerable to violent, long tracked tornadoes. Much of the housing in this region is less robust than in other parts of the USA and many people live in mobile homes. As a result, tornado related casualties in the southern USA are particularly high. One prime example was the April 25–28, 2011 tornado outbreak. The term was coined in 1971 by Allen Pearson, former director of the National Severe Storms Forecasting Center.
In the heart of tornado alley, building codes are often stricter than those for other parts of the U.S., requiring strengthened roofs and more secure connections between the building and its foundation. Other common precautionary measures include the construction of storm cellars, and the installation of tornado sirens. Tornado awareness and media weather coverage are also high.
Some studies suggest that there are also smaller tornado alleys located across the United States.
Number of U.S. tornadoes per state 
These figures, reported by the National Climatic Data Center for the period between January 1, 1950 and July 31, 2009, show the ten most affected states. As reports are taken from individual counties within States, sometimes the same tornado can be reported more than once as it crosses county lines.
- Texas: 8,049
- Kansas: 3,809
- Oklahoma: 3,443
- Florida: 3,032
- Nebraska: 2,595
- Iowa: 2,368
- Illinois: 2,207
- Missouri: 2,119
- Mississippi: 1,972
- Alabama: 1,844
See also 
- Tornado FAQ
- "Tornado Climatology". National Climatic Data Center. January 29, 2007. Retrieved 2007-04-26.
- Jeremy Singer-Vine (May 23, 2011) "How did "Tornado Alley" get its name?," Slate (on-line magazine).
- John P. Gagan, Alan Gerard, and John Gordon (December 2010) "A historical and statistical comparison of "Tornado Alley" to "Dixie Alley", " National Weather Digest, vol. 34, no. 2, pages 146-155; see especially page 146.
- "Weather officers commended," Take-Off (newspaper of Tinker Air Force Base; Midwest City, Oklahoma), January 16, 1953.
- Results of search of Google Books for "tornado alley".
- Gagan et al. (2010), page 147.
- Broyles, Chris; C. Crosbie (October 2004). "Evidence of Smaller Tornado Alleys Across the United States Based on a Long Track F3-F5 Tornado Climatology Study from 1880-2003". 22nd Conference on Severe Local Storms. Hyannis, MA: American Meteorological Society.