The only continent that has not recorded a tornado is Antarctica. Otherwise, tornadoes can happen in just about any location.
(Credit: R. Erdrich, Abilene Reporter-News)
North America, and specifically the United States, accounts for more than 75% of the world’s reported tornadoes. Other tornado-prone areas around the globe include western Canada, Argentina, the western Mediterranean, Japan, Bangladesh, Australia, New Zealand and South Africa.
Tornadoes form in regions of the atmosphere in which warm moist air is surmounted by warm dry air, large vertical wind shears exist, and weather systems such as fronts or thunderstorm complexes can force air upward. The U.S. provides these three ingredients in abundance. The distribution of tornado reports reveals a “tornado alley” of highest frequency in the central Great Plains. In this region, warm moist low-level air from the Gulf of Mexico is often capped by very dry air blown out over the Plains at mid-levels from the Mexican Plateau. This type of stratification sets the stage for the explosive thunderstorms that can spawn tornadoes. Another area of frequent tornado occurrence is over the Southeast.
Although tornadoes can occur at any time of day or night, favorable tornadic conditions benefit from solar heating. Therefore, the most likely times for tornadoes are late afternoon or early evening. In fact, more than half of all U.S. tornadoes occur between 3 and 7 p.m.
The total number of tornadoes in the U.S. each year has stayed relatively consistent over the past several decades. Recent research has indicated that there are now fewer days with tornadoes each year, but there are more tornadoes on the days when they do occur, indicating that they tend to now happen in more concentrated bursts over fewer days of the year.
Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.
