10.5: Tornadoes

What is a Tornado?

A tornado, also known as a twister, is a rapidly rotating column of air that extends vertically from the surface of the Earth to the base of a thunderstorm. Recall from the prior chapter that supercell storms can produce tornadoes. A supercell is a severe thunderstorm with rotation. Typically, the rotation of a supercell thunderstorm is driven by directional wind shear in the environment. Occasionally, a supercell storm produces a tornado due to rapidly rotating air in cyclostrophic balance. We learned in Chapter 8 that cyclostrophic balance occurs when the pressure-gradient force equals the centrifugal force. As the rotating air motion in a supercell is vertically stretched, it narrows and speeds up. To picture this, imagine a figure skater pulling their arms inward as they spin. Their diameter narrows and their rotation speed increases. The same physics—conservation of angular momentum—occurs as a tornado forms. Figure \(\PageIndex{1}\) depicts a tornado in Elie, Manitoba, in June 2007.

Figure \(\PageIndex{1}\): F5 tornado viewed from the southeast as it approached Elie, Manitoba, on Friday, June 22, 2007. (CC BY-SA 3.0; Justin1569 via Wikimedia Commons.)

Predicting Tornadoes

Tornadoes are difficult to forecast with sufficient lead time. Typically, a Tornado Watch will be issued by the Storm Prediction Center when all of the ingredients for a tornado are present in the atmosphere. These ingredients are the same as for a severe thunderstorm: warm, moist air; instability (quantified by large values of CAPE); potential for a trigger; and, of course, directional wind shear to provide rotation. A Tornado Warning is issued when a tornado is imminent or occurring. For example, one may be issued when a funnel cloud is spotted or when a “hook echo” is observed on a weather radar, as illustrated in Figure \(\PageIndex{2}\). The key feature is the bottom-left region of the storm, where a rain band wraps around a region of dry inflow air. At the cusp of the hook, a tornado would form if it were occurring. The strong rotation in a tornado creates this radar signature. Forecasting tornadoes is called “nowcasting.” Although very little advance notice can be given to the public, forecasters do their best to provide as much notice as possible.

Figure \(\PageIndex{2}\): Hook echo appears on a weather radar when strong rotation in a tornado flings rain and ice in a spiral pattern around a dry air region. (Public Domain; White Fox via Wikimedia Commons).

The Fujita Scale

The Fujita Scale is a scale for rating tornado intensity, based primarily on the damage tornadoes inflict on human-built structures and vegetation. Thus, the strength or scale of a Tornado is categorized only after it occurs. The scale was introduced in 1971 by Dr. Tetsuya Fujita of the University of Chicago, in collaboration with Allen Pearson, then head of the National Severe Storms Forecast Center (now the Storm Prediction Center). In 2007, the Fujita Scale was replaced by the Enhanced Fujita Scale (EF), a modified version of the original scale that is currently used to classify tornado strength and wind speed based on observed damage. Table \(\PageIndex{1}\) lists the six categories of the EF scale along with the associated wind speed and damage.

Table \(\PageIndex{1}\): The Enhanced Fujita Scale

Scale	Wind Speed (3 sec. gust, mph)	Damage
EF0	65-85	Minor
EF1	86-110	Moderate
EF2	111-135	Considerable
EF3	136-165	Severe
EF4	166-200	Extreme
EF5	>200	Massive/Incredible

Tornado Occurrence

Tornadoes occur in many regions worldwide, 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 in Argentina and Bangladesh. But the central US is the world’s tornado hot spot. Approximately 1,200 tornadoes occur in the U.S. each year. Because official tornado records date back only to 1950, we do not know the actual annual average number of tornadoes. Additionally, tornado spotting and reporting methods have undergone significant changes over the last several decades, resulting in a greater number of tornadoes being observed.

Tornado Alley refers to a broad area of relatively high tornado occurrence in the central United States. As a colloquial term, there are no definitively set boundaries, as illustrated in Figure \(\PageIndex{3}\). Still, the area common to most definitions extends from Texas through Oklahoma, Kansas, Nebraska, South Dakota, Iowa, Minnesota, Wisconsin, Illinois, Indiana, Missouri, Arkansas, North Dakota, Montana, Ohio, and eastern portions of Colorado, New Mexico, and Wyoming.

Figure \(\PageIndex{3}\): A unique interaction occurs when dry cold air moving south from Canada and northwestern U.S. meets warm moist air traveling north from the Gulf of Mexico. When the jet stream is positioned over this area (Specifically when the eastern half of an upper-level trough in the jet steam present), a mid-latitude cyclone develops. Severe thunderstorms form along a long trough extending from the center of the mid-latitude cylone over central Kansas down to central Texas. Thus, states like Texas, Oklahoma, Kansas, and Nebraska experience numerous tornadoes annually. This region is commonly known as Tornado Alley, which is sandwiched between the warm moist air from the Gulf of Mexico, and the cold dry air from Canada and the Northwestern U.S.. (CC BY-SA 3.0; Dan Craggs via Wikimedia Commons)

This region is particularly conducive, since it has all the necessary ingredients for tornado formation:

1. Flat, dry terrain: Tornadoes are more easily formed in regions with flat, dry terrain.
2. Atmospheric instability: Warm, moist air from the Gulf of Mexico combines with cool, dry Rocky Mountain winds.
3. Competing air masses: When competing air masses converge, warm air rises and cool air sinks, producing severe thunderstorms and sometimes supercells. If the two air masses clash at different speeds or in different directions, a funnel cloud can form, leading to a tornado.

While Tornado Alley is widely recognized as a tornado hotspot, tornadoes and tornado-like features can occur elsewhere. A dust devil and a waterspout are tornado-like features that are associated with cumuliform and sometimes cumulonimbus clouds. However, dust devils and waterspouts are typically not associated with supercells. Waterspouts look like tornadoes but form over water surfaces and do not reach the destructive wind speeds of tornadoes.