Video courtesy of TED-Ed.
“Tornadoes are rapidly rotating columns of air that make contact with the ground. In general, we consider them to be intense, columnar vortices in contact with the ground that is capable of inflicting damage. When the air is too dry or the pressure not low enough for a condensation funnel to form or to extend down to the ground, the tornado may be visualized as a column of rotating dust or debris or not visualized at all if there are no visible particles available to be lofted” (Bluestein, 307).
According to National Geographic:
- The winds of a tornado can reach speeds of up to 480km per hour or just under 300 mph!
- The duration of tornadoes and the distance they travel can vary. Most last less than ten minutes and travel 3 to 6 miles before disappearing.
- The formation of a tornado is so complex that scientists still don’t completely understand it. And what’s more, the unpredictability of tornadoes makes them difficult – and dangerous – to study. A tornado will demolish everything in its path, including measuring equipment.
As of the year 2012 the technology used for tornado research according to Howard B. Bluestein (2013), “involves observing tornadoes with ground-based, rapid-scan (both electronic and mechanical) mobile, Doppler radars; mobile, rapid-scan (mechanically scanning—not electronically scanning), polarimetric Doppler radar; mobile, pulsed Doppler lidar, instrumented surface probes; UAVs; and three-dimensional numerical simulation experiments with grid spacing down to tens of meters in non-hydrostatic cloud models and 1m LES models” (p. 234).
Before we get into why a tornado happens, the conditions that need to be met for a tornado or what scientists do to calculate a tornado, we must first define what a tornado is. I am sure everyone has heard of a tornado, as it is one of the most common forms of natural disasters. Tornadoes as defined by Howard B. Bluestein (2013), “are rapidly rotating columns of air that make contact with the ground” (p. 307). Tornadoes typically last from a few seconds to an hour, most last around 10 min. They usually have a diameter of 200m but can be as narrow as 10m or as wide as 2km, with wind speeds of 20 to 140ms-1.
(Old drawing (or photograph) of a tornado in the U. S. in 1884 (courtesy of History of Science Collections, University of Oklahoma Libraries).
“Tornadoes occur all over the world, but are most common in the Great Plains of the U. S. and just to the east during the spring, when there is a ready supply of moisture from the relatively warm Gulf of Mexico and climatologically there is a jet of air aloft that enables there to be vertical wind shear strong enough to support supercells, the most prolific parent storms of the strongest tornadoes (p. 326).”
“As air subsides in the lee of the Rocky Mountains, it warms and produces a trough of low pressure. In response to this surface trough, surface geostrophic winds become southerly and, under the influence of surface friction, the winds become southeasterly over the states of the Great Plains. (p. 326).
“Periodic incursions of cold air behind frontal zones, however, act to remove the potential instability needed for convective storm development. The frequency of occurrence of tornadoes varies from decade to decade, but the location of Tornado Alley is consistently from Texas, northward through Oklahoma, Kansas, Nebraska, and into Iowa. (p. 328).
The figure shows the peak month that tornadoes occur in the United States.
“The dryline, a boundary separating moist, relatively cool Gulf of Mexico air from dry, relatively warm continental air that has had contact with higher terrain to the west, is a frequent locus of storm initiation, since just east of it the equivalent potential temperature is greatest; thus, the potential for reaching convective temperature is maximized at and just east of the dryline and CAPE is greatest there also, if the conditions aloft are uniform. (p. 330)
Noteworthy discussion from ENS 151:
We discussed in our earth science class that a thunderstorm requires warm air, moist air, instability (lifting ), and that includes high temperatures. Thunderstorms are most common in the afternoon and early evening. Thunderstorms are first developed during the cumulus stage, which is a thunderstorm starting to form, and it provides moisture that condenses and builds the cloud. Next, the mature stage is marked by the heavy precipitation and the cloud continues to enlarge. Lastly, when the updrafts disappear, the precipitation becomes light, and then stops, and then the cloud finally starts to evaporate.
Keeping the ideas behind daytime heating in mind, you can see, in the state of Illinois, tornadoes generally occur in the mid-afternoon to the late-evening time frame in the chart below.
(Data chart provided by Dr. Jim Angel, State Climatologist in Tornado Plots For Illinois)
Springfield, IL - Tornadoes of 2006 