Tornadoes – Introduction


According to the Glossary of Meteorology (AMS 2000), a tornado is:

“a violently rotating column of air, pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud.”

Literally, in order for a vortex to be classified as a tornado, it must be in contact with the ground and the cloud base.

Weather scientists haven’t found it so simple in practice, however, to classify and define tornadoes. For example, the difference is unclear between an strong mesocyclone (parent thunderstorm circulation) on the ground, and a large, weak tornado.

There is also disagreement as to whether separate touchdowns of the same funnel constitute separate tornadoes. Meteorologists also can disagree on precisely defining large, intense, messy multivortex circulations, such as the El Reno tornado of 2013, with respect to the parent mesocyclone and surrounding winds of damaging intensity.

It is well-known that a tornado may not have a visible funnel. Mobile radars also have showed that tornadoes often extend outside an existing, visible funnel. At what wind speed of the cloud-to-ground vortex does a tornado begin? How close must two or more different tornadic circulations become to qualify as a one multiple-vortex tornado, instead of separate tornadoes? There are no firm answers.[2]

Where and when do tornadoes occur?

Tornadoes occur in many parts of the world, including Australia, Europe, Africa, Asia, and South America. Even New Zealand reports about 20 tornadoes each year. Two of the highest concentrations of tornadoes outside the U.S. are Argentina and Bangladesh.

Tornadoes that hit Washington, D.C. suburbs in September 2001 illustrate a fact often stated by weather forecasters: “Tornadoes can and do happen any time of the year in just about any location.”

Tornado Alley in the U.S. includes parts of Texas, Oklahoma, Nebraska, eastern Colorado and western Iowa, and is characterized by a high frequency of strong and violent tornadoes and a relatively consistent season from year to year.

Tornado season usually means the peak period for historical tornado reports in an area, when averaged over the history of reports. There is a general northward shift in “tornado season” in the U. S. from late winter through mid summer. The peak period for tornadoes in the southern plains, for example, is during May into early June. On the Gulf coast, it is earlier during the spring; in the northern plains and upper Midwest, it is June or July.

Tornadoes can appear from any direction. Most move from southwest to northeast, or west to east. Some tornadoes have changed direction amid path, or even backtracked.

Tornado Watch or Warning?

What is the difference between a Tornado Watch and a Tornado Warning issued by the National Weather Service?

  • Tornado Watch: Be Prepared! Tornadoes are possible in and near the watch area. Review and discuss your emergency plans and check supplies and your safe room. Be ready to act quickly if a warning is issued or you suspect a tornado is approaching. Acting early helps to save lives! Watches are issued by the Storm Prediction Center for counties where tornadoes may occur. The watch area is typically large, covering numerous counties or even states.
  • Tornado Warning: Take Action! A tornado has been sighted or indicated by weather radar. There is imminent danger to life and property. Move to an interior room on the lowest floor of a sturdy building. Avoid windows. If in a mobile home, a vehicle, or outdoors, move to the closest substantial shelter and protect yourself from flying debris. Warnings are issued by your local forecast office. Warnings typically encompass a much smaller area (around the size of a city or small county) that may be impacted by a tornado identified by a forecaster on Radar or by a trained spotter/law enforcement who is watching the storm.

How do tornadoes form?

Only about one thunderstorm in a thousand produces tornadoes. Most tornadoes form during rotating supercell thunderstorms.

The most destructive and deadly tornadoes occur from supercells–which are rotating thunderstorms with a well-defined radar circulation called a mesocyclone. Supercells can also produce damaging hail, severe non-tornadic winds, unusually frequent lightning, and flash floods. Tornado formation is believed to be dictated mainly by things which happen on the storm scale, in and around the mesocyclone.

How the column of air begins to rotate is not yet completely understood, but one way the rotation appears to happen is when winds at different altitudes blow at different speeds, creating wind shear. For example, a wind at 1000 feet above the surface might blow at a speed of five miles per hour and a wind above that, at 5000 feet, might blow at 25 miles per hour. A horizontal rotating column of air can form at the boundary between these two winds.

If the rotating column of air gets caught in the flow of air moving up into the storm (an updraft), the spin tightens and speeds up, much like a skater’s spins faster when arms are pulled close to the body, creating a funnel cloud. The rain and hail in the thunderstorm cause the funnel to bend downward. If it touches the ground, it’s a tornado.

Most tornadoes spin in the same direction – counterclockwise when viewed from above. A few spin clockwise. The direction that a tornado spins is probably related to the direction of rotation in the thunderstorm.


  1. American Meteorological Society Glossary:
  2. NOAA – The Online Tornado FAQs:
  3. National Atlas – When and Where Do Tornadoes Occur?:
  4. NOAA – The Online Tornado FAQs:
  5. NOAA – The Online Tornado FAQs:
  6. Image Source: [Accessed March 15, 2014]
  7. NOAA – The Online Tornado FAQs:
  8. NOAA – National Severe Storms Laboratory (NSSL) – Vortex 2: