Tornadoes

What exactly is a tornado? How does it form? (Tornadogenesis.) What are the main types of tornado? How is the strength of a twister rated by the Enhanced Fujita Scale? Does global warming affect tornadoes? Where and when do tornadoes occur? Which was the most powerful tornado ever? We answer all these questions and more.
Tornado in Manitoba
F5-strength tornado, Manitoba, Canada. Photo: Justin Hobson (CC BY-SA 3.0) GNU Free Documentation License, version 1.2

What is a Tornado?

Tornadoes are extreme weather events that are spawned by giant thunderstorms, known as a “supercells”.

Most are characterized by a vertical column of rapidly spinning air that emerges from the cumulonimbus cloud base of a storm system, until it makes contact with the ground. Once ‘touchdown’ is achieved, the tornado or ‘twister’ is capable of chewing up anything in its path, from farm animals to large buildings.

In fact, along with hurricanes and cyclones, tornadoes are among the most violent weather phenomena on the planet. Their winds can destroy bridges, flip trucks and trains, deform skyscrapers – they can even tear the bark off trees.

Tornadoes – especially violent ones – occur far more often in the United States than in any other country. Although they can occur in all states and at any time of the year, they appear most often in a swathe of central and southeastern states nicknamed “tornado alley”, during a ‘tornado season’ which lasts from March to June. In 2020, the US experienced more than 1200 tornadoes, which caused 78 fatalities and $4.4 billion worth of damage.

The average tornado measures about 150 meters (500 feet) in diameter, has a wind speed of less than 112 km/h (70 mph), travels about 30 miles an hour, and lasts about ten minutes before dissipating. 1

However, the most powerful tornadoes have wind speeds of over 480 km/h (300 mph), and can expand to more than 4 km (2.5 miles) across. These monsters can blast a trail for hundreds of kilometers.

On March 18, 1925, for instance, the “Tri-State Tornado” created a record-breaking 350 km (219 mile) long path from southeast Missouri through southern Illinois and into southwest Indiana. The storm devastated a number of towns, injured over 2,000 people and caused 695 deaths. During its 3-5 hour journey, its over-the-ground speed was recorded as 116 km/h (73 mph).

Tornadoes are strongly influenced by temperature, thus giving rise to the idea that climate change is responsible. However, things are not so simple. Scientists still don’t fully understand the other factors that affect tornado formation, such as changes in the vertical and horizontal variations of winds. 2

Overall, the importance of local weather conditions plus the fact that tornadoes are typically very short-lived – lasting from a few minutes to a few hours – makes it very difficult for climate models to establish a material link between rising temperatures and tornadogenesis.

Where Do Tornadoes Occur?

Tornadoes occur on all continents except Antarctica. They occur most frequently in the mid-latitudes (between 20° and 60° North and South), where cold polar air meets warm tropical air. Countries and areas that experience frequent tornadoes include “tornado alley” in the USA, Canada, Netherlands, the UK, Bangladesh, India, China, South Africa, Australia, New Zealand and the “Tornado Corridor” of Argentina, Uruguay and southern Brazil.

But the United States has far more tornadoes – especially serious ones – than any other country. This is due to the unique geography and topography of the North American continent. No major mountain range blocks the air flow between the Arctic and the tropical Gulf of Mexico, while the confluence of cool dry air from the Rockies, dry air from Arizona and New Mexico and warm moist air from the Gulf allows for frequent collisions of warm and cold air – the perfect breeding conditions for thunderstorms.

The main path of ‘tornado alley’ runs through the central U.S plains states of Texas, Oklahoma, Kansas, Nebraska, South Dakota, Iowa and Minnesota. More than three-quarters of the world’s tornadoes occur in this area because it has just the right conditions for thunderstorms to form: cool, dry air from the Arctic, mixing with moist, warm, air from the Gulf of Mexico, combined with warm, dry air from the southwest.

When Do Tornadoes Occur?

Tornadoes occur most frequently in spring and early summer, March through June. On the US Gulf coast, peak tornado season is March to May. On the southern Plains (e.g. Texas, Oklahoma, and Kansas) the season is May to early June. On the northern Plains and in the upper Midwest (North and South Dakota, Nebraska, Iowa, Minnesota), tornado season runs June and July. But twisters can appear at any time of year when conditions are favorable.

Worldwide, most tornadoes occur late in the afternoon, typically between 1500 and 1900 hours, peaking around 1700 hours. That said, tornadoes can occur at any time of day. Both the Tupelo Tornado (April 5) and the Gainesville Tornado (April 6) of 1936, two of the deadliest tornadoes in US history, occurred at 0830 hours on successive days, killing 454 people.

How Does a Tornado Form? (Tornadogenesis)

The truth is, meteorological experts still don’t understand exactly how and why tornadoes happen. All we can say is that there are two essential components in the formation of a tornado: (a) A powerful thunderstorm, called a supercell. All tornadoes are produced from thunderstorms. But not all thunderstorms produce tornadoes. (b) Wind shear resulting in a vertical column of rotating air. Without rotation of air currents inside the thunderstorm there can be no tornado.

Diagram of supercell thunderstorm
Supercell thunderstorm. 3

What is a Supercell?

Supercells are the rarest but most potent type of thunderstorm. Typically, they consist of an extremely tall cumulonimbus cloud mass, formed from water vapor carried aloft by powerful upward currents of warm air. Rising to about 20-30,000 feet, supercells are usually topped by an anvil cloud formation. A supercell storm is commonly accompanied by extremely heavy rain, floods, hail, lightning, and strong gusts of wind.

However, a supercell’s unique characteristic is the deep upward spiral of rotating air inside it, known as a mesocyclone. This vertical column of rotating air is caused by unstable atmospheric conditions marked by strong wind shear between about two and 20,000 feet.

About one in a thousand thunderstorms becomes a supercell. Of these, about every fifth/sixth supercell spawns a tornado.

Supercells come in differing types, including classic, high precipitation and low precipitation.

Classic supercells tend to be isolated phenomena, and are the most common supercell storms on the Great Plains of the United States.

High precipitation (HP) supercells tend to be seen among other mid-level storm systems, making them more difficult to spot.

Low precipitation (LP) supercells are associated not with rainfall but with unusually large hail.

What is a Mesocyclone?

A mesocyclone is a vertical vortex of rotating air, typically around 2-9 km (1-6 miles) in diameter, that sits inside a supercell thunderstorm. It is a mesocyclone that gives birth to a tornado. In the northern hemisphere the mesocyclone usually occupies the right rear flank of the supercell.

Mesocyclones materialize when wind shear causes air in the lower atmosphere to rotate horizontally like an invisible roller. As strong updraughts of warm air hit this rotating air, it is knocked into a vertical position (from parallel to the ground to perpendicular) and begins to spin as a vertical column or upward spiral. If enough warm air is sucked into this whirling upward spiral and it becomes extremely focused, a tornado may eventually appear at the bottom. Mesocyclones are not usually visible by the naked eye but are detectable on Doppler weather radar.

The main stages in the formation of a tornado are as follows:

Stage 1 – Deep Cloud Formation and Updraughts of Warm Air

The first stage of a thunderstorm is when the sun heats the surface of the land (and the air above it) and creates a pocket of warm air (which absorbs water evaporating from the surface) which rises upwards. When this pocket of warm air reaches a certain height, some of the water vapor which it holds may condense into droplets and form shallow cumulus clouds. As it condenses, heat is released, adding to the warmth of the pocket.

If the atmosphere is unstable – that is, if the air temperature decreases rapidly with height, the warm air pocket may rise much higher in the atmosphere heights. This can create a strong updraught of ascending air as well as deep cumulus and cumulonimbus clouds, as the water vapor cools and condenses. 4

Stage 2 – Warm Air Rotated by Wind Shear

If the rising pocket of warm air is hit by sudden strong cross winds – a phenomenon known as vertical wind shear, which is a critical component of severe storms – it may be rolled into a horizontally rotating ‘cylinder’ of air. (Imagine a rolling pin moving on a surface.)

Stage 3 – Horizontal Rotating Cylinder is Tilted Vertical

Continuing strong updraughts of warm air tilt can then tilt this rolling cylinder of air into a vertical column, changing from parallel to the ground, to perpendicular. This spinning column of air will form the storm’s mesocyclone. As the storm strengthens, its updraft can draw in low-level warm air from several miles around. This fuels the mesocyclone even more. When a thunderstorm reaches this stage, it is called a ‘supercell’.

Stage 4 – Concentration of the Rotating Mesocyclone

By this stage, we have a deep formation of cumulus and cumulonimbus clouds, rising to 30,000 feet or more. Hidden inside the cloud is a column of rotating air – the mesocyclone – energized by massive inflows of warm air. (Imagine an enormous upward spiral.)

Typically, at this point, down-draughts of descending currents of colder, dense air within the thunderstorm begin to intensify the rotating mesocyclone and pull it down to lower levels. This falling cool air produces a small, rotating cloud – known as a “wall cloud” – which projects below the floor of the cloud base.

Sometimes, the rotation of air within the storm system becomes so concentrated that it forms a narrow funnel of violently spinning air, which emerges from the wall cloud. If this funnel of air reaches the ground, it is often referred to as ‘touch down’ because this is the moment it forms a tornado.

Stage 5 – Dissipation of a Tornado

In time, cold currents of descending air eventually enclose the tornado, cutting off its supply of warm air. When this happens, the updrafts lose energy, and the tornado does too. This stage typically lasts no more than a few minutes.

What are the Main Types of Tornado?

Multiple vortex

A multiple-vortex tornado is one which has two or more vertical columns of rotating air (subvortices) both revolving around a common center. This type is usually confined to more severe tornadoes, and the multiple spinning columns usually cause much heavier destruction along the main tornado path.

Waterspout

Waterspouts are weak twisters that materialize over warm water. Sometimes they move inland and become tornadoes.

The National Weather Service defines a waterspout as a tornado over water. But one should distinguish so-called “fair weather” waterspouts from tornadic waterspouts created by mesocyclonic forces. Fair weather waterspouts are more common but much less damaging, not unlike dust devils and landspouts. They form over water at the foot of towering cumulus clouds in the tropics. They are accompanied by moderate to weak winds and travel very slowly. They appear frequently in the waters off the Florida Keys.

In contrast, tornadic waterspouts are more dangerous. They form over water from severe thunderstorms in a similar way to land-based supercell tornadoes. Since they emerge out of powerful thunderstorms and are usually faster-moving, more intense and longer-lived than “fair weather” twisters.

Landspout

A landspout is a sort of “fair weather waterspout on land”. Instead of developing inside the mesocyclone of a storm and descending to the ground, a landspout forms amid rotating air currents at ground level and gets sucked up into the thunderstorm by updraughts of warm air. Similar to waterspouts, landspouts are relatively weak and invariably short-lived.

Dust Devil

Dust devils (also called whirlwinds) are vertical spinning columns of air that are identified by the dust and dirt they disturb. Dust devils are not associated with thunderstorms. They are triggered by strong updrafts of warm air close to the ground on a hot day, when the air above is experiencing strong wind shear. Dust devils are not considered to be authentic tornadoes because they are fair weather phenomena. They are comparatively weak but are sometimes capable of causiing serious damage.

Gustnado

Gustnadoes are short-lived, ground-level swirling winds that can appear on the leading edge of an intense thunderstorm. They are usually triggered by strong downdrafts of air from a thunderstorm, that flatten out as they hit the ground. If there is enough low-level wind shear, a rotating air current may form and develop into a swirling pocket of wind. This is a gustnado. Although it doesn’t have the characteristic vertical funnel of a tornado – it’s really more of an intense swirling pocket of wind – and is not joined to the cloud base, it can be relatively violent over short periods with wind speeds of up to 130 km/h (80 mph).

Which is the Most Powerful Tornado in History?

On May 3, 1999, the wind speed of a violent F5 tornado which hit the Oklahoma City metro area was measured by a mobile doppler radar unit as 508 km/h (318 mph) at a height of about 100 feet. 5

Does Global Warming Affect Tornadoes?

Over the past few decades, clear connections have been established between global warming and a range of extreme weather events, such as hurricanes and certain types of drought, but so far, there’s no clear link with tornadoes.

We know that global warming leads to more intense heatwaves, more severe droughts, heavier rainfall and floods, and we know it influences the El Niño Southern Oscillation – the weather system that affects the Polar and Pacific Jet Streams over the southern and central United States.

But tornadoes are largely the result of specific local weather conditions. As well as heat, they need a precise combination of supercell thunderstorm, and wind shear to induce rotation, a necessary condition for the creation of funnel clouds.

Climatologists typically assess the effect of climate change on weather by running simulations that compare how often an event happens in two worlds: one with man-made greenhouse gas emissions and one without. But because a tornado is much smaller than say, a hurricane, it can’t be simulated in typical climate models, and researchers have to rely on observational records. 6 These observations suggest that tornado frequency has remained fairly stable.

Fourth National Climate Assessment

According to the Fourth National Climate Assessment (NCA4), global warming has a direct effect on certain forms of extreme weather, such as rainfall and intense heat.

Other types of violent weather – such as tornadoes – are also exhibiting changes which may eventually be linked to climate change. But, right now, climate science simply can’t tell us whether, or to what extent, climate change might be affecting tornadoes.

NCA4 also states that in recent years the US has experienced several significant thunderstorm wind events – known as “derechos”. These are long-lived wind storms associated with a band of fast-moving thunderstorms at least 400 km (250 miles) in width.

Derechos have wind gusts of at least 93 km/h (58 mph) but winds bursts as high as 130 mph have been recorded, producing damage very similar to that of a tornado. Derechos are most common in the late spring and summer (May to August).

Although NCA4 admits there is insufficient observational data to determine whether there are any long-term trends in their frequency, it makes the point that modeling studies consistently show that the frequency and intensity of severe thunderstorms in the United States could increase as the planet warms, particularly over the U.S. Midwest and Southern Great Plains during spring. 7

Is Climate Change Causing Drier Air to Creep Eastward?

Over the last few decades, meteorologists have noticed a distinct geographical shift in tornado geography, with about 10 percent fewer twisters in the Texas Panhandle and about 10 percent more in the states of Mississippi, Alabama, Arkansas, Missouri, Illinois, Indiana, Tennessee, and Kentucky, to the east of “Tornado Alley”.

Some scientists think this shift is the result of drier air creeping eastward — a process which has been linked to climate change – although more data (and time) is needed before we know for sure.

Will Climate Change Reduce Wind Shear?

As we saw, wind shear — the changes in wind speed and direction from ground level up to the top of the troposphere – is an essential component in tornadogenesis. But as one study points out, wind shear is largely determined by the strength of the jet stream which flows in from the west.

This is due to the fact that the jet stream is driven by the difference in temperature between the tropics and the poles. But that difference is decreasing because climate change is heating up the Arctic more rapidly than the rest of the planet. Which could lead to a weaker jet stream and a smaller number of tornadoes.

On the other hand, the climate models show that global warming is feeding a lot more energy into the atmosphere. Which appears to more than compensate for the decrease in wind shear. So, at the very least, we should see more supercell thunderstorms. Whether this leads to more tornadoes remains to be seen. 8

Which is the Deadliest Tornado Ever?

The Tri-State Tornado of March 18, 1925 is the deadliest on record, accounting for 695 fatalities. 5

How is the Strength of a Tornado Measured?

The main rating system used to evaluate tornadic power is the Enhanced Fujita (EF) Scale, which came into operation in February 2007. The EF replaced the original Fujita (F) scale, developed by Dr. T. Theodore Fujita in 1971. The F scale was a damage scale for winds, including tornadoes, which related damage to wind strength.

In fact, tornado wind speeds are still a bit of a mystery, since wind speeds on the original F scale have never been scientifically verified. What’s more, damage caused is far from a definitive metric. A building hit by a tornado from the south may collapse, whereas if similar strength winds approached it from the west, it might (for various reasons) remain standing.

The Enhanced Fujita Scale takes into account more factors than the original Fujita Scale (F-Scale) when assigning a wind speed rating to a tornado. It includes 28 damage indicators such as building type, structures and trees.

The Enhanced Fujita Scale

Scale Wind Speed/km/mph Damage Caused
EF0105–137 km/h
65–85 mph
Minor Damage
Some damage to gutters.
Branches snapped off trees.
Shallow-rooted trees pushed over.
Permanent buildings experience minor damage.
Mobile homes/trailers can sustain moderate to serious damage.
EF1138–1777 km/h
86–110 mph
Moderate Damage
Roofs stripped.
Mobile homes/trailers flipped or badly damaged.
Loss of doors, windows broken.
EF2178–217 km/h
111–135 mph
Considerable Damage
Roofs ripped off well-constructed houses.
Foundations of frame homes moved.
Mobile homes destroyed.
Large trees uprooted.
Cars overturned.
EF3218–266 km/h
136–165 mph
Severe Damage
Top floors of solid houses damaged.
Severe damage to shopping malls.
Trains overturned.
Trees stripped.
Trucks overturned.
EF4267–322 km/h
166–200 mph
Devastation
Whole frame houses destroyed or swept away.
Cars, trucks, buses flipped.
Severe damage to urban infrastructure.
EF5322 km/h >200 mphWholesale Destruction
Well-built houses obliterated.
Concrete structures critically damaged.
Tall structures collapse or severely deformed.
Skyscrapers damaged.
Cars, trucks, and trains can be hurled up to 1.6 km (1 mile).

Note: In the United States, four out of five tornadoes are rated EF0 and EF1. Less than one in a hundred are violent tornadoes (EF4 or stronger). Outside North America, destructive tornadoes are exceedingly rare. 9

Which is the Most Expensive Tornado Ever?

The Joplin, Missouri, tornado of May 22, 2011 is the most expensive on record, causing $2.8 billion in damage. 5

Tornado Forecasting

Meteorologists at the U.S. National Weather Service use Doppler radar, satellites, and weather balloons, as well as trained storm spotters to scan the skies for tornadic and storm activity.

Doppler radars measure wind speeds and identify mesocyclones inside thunderstorms. Since the introduction of Doppler radar, the warning time for tornadoes has increased from an average of less than five minutes in the 1980s, to 13 minutes by the late 2000s.

If meteorological conditions are favorable for tornadogenesis, the NOAA Storm Prediction Center issues a “tornado watch”. If a tornado is identified on radar by a local NOAA National Weather Service Forecast Office, a “tornado warning” is issued. 3

References

  1. “Tornadoes”. Walter A Lyons (1997). The Handy Weather Answer Book (2nd ed.). Detroit, Michigan: Visible Ink press. pp. 175–200. ISBN 978-0-7876-1034-0. []
  2. “How does climate change affect the strength and frequency of floods, droughts, hurricanes, and tornadoes?” Royal Society []
  3. “Severe Weather 101.” NSSL/NOAA. [][]
  4. “How are tornadoes formed? Met Office, UK []
  5. “Extreme Facts About Tornadoes.” Weather.com [][][]
  6. “No, we can’t blame tornadoes on climate change … yet.” Grist. []
  7. “Severe Weather.” Box 2.6: Chapter 2. Fourth National Climate Assessment (2018) []
  8. “Robust increases in severe thunderstorm environments in response to greenhouse forcing.” Noah S. Diffenbaugh, Martin Scherer, Robert J. Trapp. PNAS October 8, 2013 110 (41) 16361-16366 []
  9. “Statistical modeling of tornado intensity distributions”. Dotzek, Nikolai et al. (2003). Atmos. Res. 67: 163–87. []
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