For the book by William S. Burroughs, see Tornado Alley (book).

A diagram of tornado alley's rough location (red), and its contributing weather systems

Tornado Alley is a colloquial term that most often refers to 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.^[1]

Tornado geography[link]

Tornado activity in the United States.

Although no U.S. state is entirely free of tornadoes, they are most frequent in the plains between the Rocky and Appalachian Mountains. According to the storm events database of the National Climatic Data Center, Texas reports more tornadoes than any other state, though this state's very large land area should be taken into account. Kansas and Oklahoma are second and third respectively for sheer number of tornadoes reported but report more per land area than Texas. However, the density of tornado occurrences in northern Texas is comparable to Oklahoma and Kansas. Florida also reports a high number and density of tornado occurrences, though only rarely do tornadoes there approach the strength of those that sometimes strike the southern plains.^[2].

Definition[link]

Although Tornado Alley is considered to be in the areas of the Central United States, no official definition of the term has actually been produced by the National Weather Service. According to the National Severe Storms Laboratory FAQ,^[3] "Tornado Alley" is a term created by the media to refer to areas that have greater 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. 90% 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 most frequently. The core of Tornado Alley consists of northern Texas (including the Panhandle), Oklahoma and Kansas. However, Tornado Alley can be also be defined as an area stretching from central Texas to the Canadian prairies and from eastern Colorado to western Pennsylvania. It can also be argued that there are numerous Tornado Alley's. In addition to the Texas/Oklahoma/Kansas core, such area's include 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.^[4]

Variations[link]

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.

The term "Dixie Alley" was coined in 1971 by Allen Pearson, former director of the National Severe Storms Forecasting Center.^[5]

Impact[link]

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.^{[citation needed]} 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.^[6]

Number of U.S. tornadoes per state[link]

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.

1. Texas: 8049
2. Kansas: 3809
3. Oklahoma:3442
4. Florida: 3032
5. Nebraska: 2595
6. Iowa: 2368
7. Illinois: 2207
8. Missouri: 2119
9. Mississippi: 1972
10. Alabama: 1844

References[link]

See also[link]

• List of tornadoes and tornado outbreaks
• Tornado climatology
• Hurricane Alley

External links[link]

• NSSL Tornado Climatology
• Climatological Estimates of Local Daily Tornado Probability for the United States
• Tornado hazards in the United States
• Statistical modeling of tornado intensity distributions