How do tornadoes form? The short answer
A tornado is a narrow, violently spinning column of air that reaches from a thunderstorm down to the ground.1 To make one, the weather needs two things at the same time: a strong thunderstorm, and wind shear, meaning winds that change speed or direction as you go up.212
- A thunderstorm builds. Warm, humid air near the ground rises into colder air above, making a powerful rising current called an updraft.14
- Wind shear rolls the air. Slow wind near the ground and faster wind higher up roll the air between them into an invisible tube spinning on its side.2
- The updraft stands the tube up. Rising air tilts the spinning tube from horizontal to vertical, so the whole updraft rotates. That rotation is a mesocyclone, and a storm that has one is a supercell.24
- Spin near the ground tightens. The storm's downdraft helps start spin right at the ground. When that spin is pulled inward it speeds up, like a figure skater pulling in her arms, and a tornado forms.1314
One honest note: scientists still don't fully understand that last step, or why one storm makes a tornado while a similar storm nearby doesn't.12
What is a tornado?
Wind is invisible, so you only see a tornado when it forms a condensation funnel of water droplets, or when it picks up dust and debris.1 A funnel that doesn't reach the ground is just a funnel cloud. A tornado must be connected to both the ground and the cloud.6 Be careful, though: a tornado can be on the ground without a full funnel. A spinning cloud of debris is enough.1
Most tornadoes are small and brief. A typical one lasts about five minutes and moves at 10 to 20 miles per hour.3 The most violent ones are a different story: their winds may reach around 300 mph. The fastest ever measured by radar was 318 mph, near Moore, Oklahoma, on May 3, 1999.3
What causes tornadoes? Step 1: a thunderstorm
Every tornado comes from a thunderstorm, and thunderstorms need an unstable atmosphere: warm, humid air near the ground with much colder air above it.1411 Warm air is lighter, so it rises, like a hot air balloon. As it rises, its water vapor condenses into cloud, which releases even more heat and pushes it higher.14 The strongest storms can tower up to 50,000 feet, nearly ten miles high.4
But warm air alone isn't enough. Most thunderstorms never make a tornado. The missing ingredient is spin.
Step 2: wind shear makes a spinning tube
This is the part most explanations skip, and it's the real "aha". Near the ground, the wind is slowed by friction with fields, trees and buildings. Higher up, it can blow much faster and from a different direction.2 NOAA's example: at ground level the wind comes from the southwest at 5 mph, but at 5,000 feet (about a mile up) it comes from the southeast at 25 mph.2
Now picture the air in between. The top of it is being pushed hard and the bottom only gently, so it starts to roll, like a pencil rolled between your hands. The result is an invisible tube of air spinning horizontally, on its side.2 The bigger the difference between the winds, the stronger the spin. You can try this in the storm chef game above: turn up the wind high up and see what happens.
How much shear does it take? On days with tornado outbreaks, wind speed can change by more than 50 mph over just a few miles of height.14
Step 3: the updraft tilts it into a supercell
When a thunderstorm's updraft rises through that spinning tube, it pushes the middle of the tube upward. The tube bends, and then stands up on end.212 Now the air isn't rolling like a wheel anymore. It's turning around and around like a merry-go-round, and the rotation runs through much of the storm.2
A storm with a rotating updraft is called a supercell. Its rotating column, the mesocyclone, can be as much as 10 miles across, and it can spin for 20 to 60 minutes before a tornado forms.4 Supercells make the most tornadoes, and almost all of the most violent ones.214 The tornado itself is a very small extension of that much larger rotation.4
Step 4: how the tornado spins up
A slow, miles-wide spin high in the cloud is not a tornado. A tornado needs strong spin right next to the ground, and that takes another part of the storm: the downdraft.14 Rain cools the air around it, and that cooler, heavier air sinks and spreads out along the ground. Along the edges of this sinking air, gusts of warmer air rise and cooler air sinks, and the air near the ground starts to spin.1314
At first, that ground-level spin is only about one hundredth as strong as a tornado.14 Then the storm's updraft pulls it inward and stretches it upward. Just like a figure skater who spins faster when she pulls her arms in, the spinning air speeds up as it's squeezed toward its center.1314 Squeezed hard enough, it becomes a tornado.
Do tornadoes really "touch down"?
Not exactly. It looks as if the funnel lowers from the cloud, but the air inside a forming tornado is rising. The funnel only seems to reach down because water vapor condenses into cloud lower and lower. Mobile radars show that supercell tornadoes spin up very quickly from the ground upward.6
Why do most storms never make a tornado?
Only about 20 percent of supercells produce tornadoes.2 Usually, the spin near the ground just never gets fast enough. One reason: if the storm's sinking air is very cold, it spreads away from the storm along the ground and slows down, like a skater stretching her arms out, and no tornado forms.13 This is one of the biggest open questions in weather science, and researchers are still chasing storms to answer it.2
How do tornadoes stop?
The details are still debated, but a tornado needs warm, unstable air and a supply of spin to keep going. Many tornadoes die soon after they are hit by cold air flowing out of the storm, or when their parent mesocyclone gets wrapped in cold air and weakens.6 Most tornadoes last less than ten minutes.6
Could people stop one? No. The tornado is a tiny part of a supercell that has far more energy, so the storm would likely just make another. Anything powerful enough to try would do more damage than the tornado.3
The Enhanced Fujita (EF) scale: how tornadoes are rated
No weather instrument survives a direct hit from a strong tornado, so scientists can't simply measure its winds.3 Instead, experts survey the damage afterward and use it to estimate the wind speed. Since February 1, 2007, the U.S. has used the Enhanced Fujita scale, which checks 28 kinds of "damage indicators", from barns and houses to schools, towers and trees, each with 8 levels of damage.17 It updated the original Fujita scale that Dr. T. Theodore Fujita introduced in 1971.7
The wind speeds are estimates based on damage, not measurements.7
Where and when do tornadoes happen?
The U.S. gets about 1,200 tornadoes a year, and tornadoes have been reported in all 50 states.1 Tornado Alley is a nickname for a broad area of the central U.S. where tornadoes are especially common. It isn't an official zone, and the risk moves with the seasons: the Southeast in the cooler months, the southern and central Plains in May and June, and the northern Plains and Midwest in early summer.1
Tornadoes can happen at any time of year and any time of day, but most strike between 4 and 9 p.m.1 They also happen in many other countries. Argentina and Bangladesh have some of the highest numbers outside the U.S., and even New Zealand gets about 20 a year.1
Not every tornado comes from a supercell. Nearly 20 percent come from long lines of storms, and weaker "landspouts" and waterspouts can form when spin starts near the ground under a growing storm with no rotating updraft.2
Tornado watch vs. warning, and staying safe
When a warning is issued, the National Weather Service says:91011
- Go to a basement, storm cellar or safe room. If there isn't one, go to a small, windowless interior room or hallway on the lowest floor.
- Stay away from windows. Cover your head to protect yourself from flying debris.
- At school or work, follow your tornado drill and avoid big open rooms like gyms and cafeterias.
- Mobile homes, sheds, tents and cars are not safe. Get to a sturdy building if you can.
Why the middle of the lowest floor? The biggest danger in a tornado is flying debris and being thrown by the wind.3 Walls between you and the outside, and being low down, give the most protection.
Common myths about tornadoes
"Tornadoes happen when warm air and cold air crash together." NOAA calls this a gross oversimplification. Most storms that form where air masses meet never make tornadoes. Wind shear and a rotating updraft matter far more.
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"A tornado drops down out of the cloud." The air in a forming tornado is rising. The spin develops from the ground upward, and the funnel only looks like it's lowering.
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"Open the windows so the house doesn't explode." Houses don't explode from low pressure. It's the wind and debris. Opening windows wastes precious time. Get to shelter instead.
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"Hide under a highway overpass." Overpasses give little protection from flying debris and can cause deadly traffic hazards.
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"Tornadoes can't cross rivers or mountains." Every major river east of the Rockies has been crossed by a significant tornado, and tornadoes have hit high in the Appalachians, the Rockies and the Sierra Nevada.
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"Tornadoes have a calm eye, like hurricanes." There's no eye in a tornado. That idea comes mostly from the movie
Twister.
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FAQ
What causes a tornado, in simple words?
A tornado needs a strong thunderstorm plus wind shear: winds that change speed or direction as you go up. The shear rolls the air into an invisible spinning tube lying on its side. The storm's rising air (the updraft) tilts that tube upright, so the storm starts to rotate. Then spin right next to the ground gets squeezed tighter and faster until it becomes a tornado.
What are the signs a tornado is coming?
Watch for a dark, low, often rotating wall cloud under the storm, a spinning cloud of dust or debris on the ground (even with no funnel), and a loud, steady roar that people compare to a freight train or jet engine. Don't wait for signs, though: when a tornado warning is issued, take shelter right away.
What kills you first in a tornado?
Flying debris. The biggest danger from a tornado is being hit by things the wind throws, like broken glass, wood and metal, or being thrown by the wind yourself. That's why you shelter low, in a small room away from windows, and cover your head.
What ends a tornado?
Scientists still debate the details. A tornado needs warm, unstable air and rotation to keep going. Often cold air flowing out of the storm cuts it off or wraps around it, the rotation spreads out and slows down, and the tornado weakens and dies. Most tornadoes last less than 10 minutes.
What is the difference between a tornado watch and a tornado warning?
A tornado watch means conditions are right for tornadoes in the next several hours, so be prepared. It covers a large area and comes from NOAA's Storm Prediction Center. A tornado warning means a tornado has been seen or spotted on radar near you, so take shelter now. It comes from your local National Weather Service office.
Can tornadoes happen in the UK or outside the United States?
Yes. Tornadoes happen in many parts of the world, including Europe, Australia, Africa, Asia and South America. Argentina and Bangladesh have some of the highest numbers outside the U.S., and even New Zealand gets about 20 a year. The U.S. gets about 1,200 a year.
Can tornadoes be stopped?
No. A tornado is a small part of a much bigger supercell thunderstorm, which holds far more energy. Even if you could destroy one tornado, the storm would probably just make another, and anything strong enough to try would likely do more harm than the tornado.
Sources
- NOAA National Severe Storms Laboratory, Severe Weather 101: Tornado Basics
- NOAA NSSL, Tornado Types
- NOAA NSSL, Tornado FAQ
- NOAA NSSL, Thunderstorm Types
- NOAA NSSL, Tornado Forecasting
- NOAA Storm Prediction Center, The Online Tornado FAQ
- NOAA Storm Prediction Center, Enhanced F Scale for Tornado Damage
- NOAA Storm Prediction Center, Tornado Safety
- National Weather Service, Understand Tornado Alerts
- National Weather Service, What to Do During a Tornado
- National Weather Service Peachtree City, About Tornadoes
- NOAA NESDIS / NASA Space Place, What Causes Tornadoes?
- UCAR Center for Science Education, How Tornadoes Form
- Paul Markowski, Penn State Department of Meteorology, How Tornadoes Form