tornadoes FAQ

Tornadoes: Frequently Asked Questions About the Power of Nature

In an average year, about 1,000 tornadoes are reported across the United States, resulting in 80 deaths and over 1,500 injuries. A tornado is a violently rotating column of air extending from a cumuliform cloud and in contact with the ground. A tornado usually (but not always) is visible due to condensation and dust and debris within the rotating column. Strong and violent tornadoes are most likely to occur with supercell thunderstorms, but tornadoes also can occur within bowing lines of storms (i.e., bow echoes). Tornadoes originate from the energy released in a thunderstorm, however they account for only a small fraction of the total energy within a storm. What makes tornadoes dangerous is that their energy is concentrated into a very small area. Ongoing research continues to determine exactly how part of a thunderstorm's energy becomes concentrated into a tornado.

Where and when do tornadoes occur?

Tornadoes are possible anywhere in the United States, but are most common in the central plains east of the Rocky Mountains and west of the Appalachians. They usually occur in the late afternoon and evening during the spring and summer, being most common in the south in early spring and in the northern tier of states in the summer. However, tornadoes can occur on any day of the year and at any hour. They also can form in many other parts of the world, including Australia, Europe, Africa, Asia, and South America.

How do they form and can they be predicted?

The idea that tornadoes may form as a result of warm moist air colliding with cold Canadian air and dry air from the Rockies is a gross oversimplification. Many thunderstorms form under those conditions, which never come close to producing tornadoes. Even when the large-scale environment is quite favorable for tornadoes, not every storm spawns one. Instead, tornadoes form via complicated storm-scale processes which depend on the interaction of the storm's internal dynamical structure including its forward and rear flank gust fronts (see below) with the influx of buoyant, sheared air immediately around and under the rotating updraft (mesocyclone) within the storm. In fact, recent theories suggest that the temperature and moisture characteristics of the rear flank downdraft of a supercell are very important in spawning a tornado. Although prediction of the exact location and intensity of tornadoes is difficult, atmospheric conditions associated with the formation of the parent severe storms often are well-forecast.

A tornadic supercell thunderstorm in north-central Kentucky In May 1996 as shown by National Weather Service Doppler radar. Important radar signatures are annotated, including the location of large hail, the forward and rear flank downdrafts, hook echo, and tornado location.

Once thunderstorms develop, the National Weather Service's WSR-88D Doppler radar is used to evaluate severe weather and tornadic radar signatures in order to make short-term predictions of tornado occurrence. If a tornado is suspected, a tornado warning is issued for the affected area. However, while Doppler radar can interrogate severe storms very well, it cannot always detect the small-scale processes that actually lead to tornadogenesis. Therefore, trained storm spotters, public officials, and the media also are vital in the warning process by gathering, relaying, and disseminating pertinent observed weather information. This partnership facilitates the proper detection and warning of tornadoes for the general public.

What type of damage can they do?

Damage results from the strong winds within a tornado. The degree of damage and speed of these winds vary widely, depending on the intensity of the tornado. The Fujita Scale is designed to assess damage produced by tornadoes, then estimate associated wind speeds by assigning an "F" rating. The scale ranges from F0 (weak tornado with winds up to 72 mph) to F5 (very violent tornado with winds around 300 mph). The vast majority of tornadoes range from F0-F3. All tornadoes produce damage, but the most violent ones can cause automobiles to become airborne, rip homes to shreds, and turn broken glass and other debris into lethal missiles. The biggest tornado threat to human beings is from flying debris in the wind. It is important to note that straight-line winds from a thunderstorm can do damage similar to an F0 or F1 tornado.

What is the smallest, largest, and average size?

Tornadoes can vary significantly in size and intensity. Thus, the easiest way to answer this question is to assess the size of the damage path. However, the term "average" can be misleading, since the majority of tornadoes are small compared to the infrequent large events. With this said, the typical tornado damage path is about one or two miles, with a width of around 50 yards. The largest tornado path widths can exceed one mile, while the smallest widths can be less than 10 yards. Widths can even vary considerably during a single tornado, since its size can change during its lifetime. Path lengths can range from a few yards to more than 100 miles. A key point to remember is that the size of a tornado is not necessarily an indication of its intensity. Large tornadoes can be relatively weak, while small tornadoes occasionally can be violent.

How long and fast is a tornado on the ground?

Detailed statistics are not available to answer this question. Nevertheless, ground time can range from an instant to several hours, although the typical time is around 5 to perhaps 10 minutes. Supercell tornadoes tend to be longer-lived, while those pawned by squall lines and bow echoes may only last for a few minutes. Tornado movement can range from virtually stationary to more than 60 miles per hour.

What is a multiple-vortex tornado?

A multiple-vortex tornado contains two or more small, intense sub-vortices rotating around the center of the larger tornadic circulation. Occasionally visible, these vortices may form and die within a few seconds, and can occur in all tornado sizes, from huge "wedge" tornadoes to narrow "rope" tornadoes. Sub-vortices can cause narrow, short, extreme swaths of damage that sometimes arc through tornado tracks.

More information about tornadoes can be found on-line at www.spc.noaa.gov/faq/tornado.
 

Back to Training Documents