In a few minutes, a calm afternoon can turn dark, the wind can drop suddenly and a grey funnel can drop out of the sky. So what is a tornado, and how does it form? A tornado is a violently rotating column of air that stretches from a thunderstorm cloud to the ground. It is small compared to a tropical cyclone, but at close range it can be the most violent wind on Earth. This guide explains what a tornado is, how it forms step by step, how tornadoes are rated and how to stay safe.

What Is a Tornado?

A tornado is a narrow, violently rotating column of air that is in contact with both the ground and a cloud above it. The visible part is the funnel, which is made of water droplets that condense as pressure drops inside the vortex. Dust and debris near the ground often make the base look dark. If the funnel is in the air and has not reached the ground, it is called a funnel cloud, and it only counts as a tornado once it touches down.

• Width: usually 50 to 200 metres, although the biggest can be more than 2 kilometres wide.
• Life span: mostly a few minutes, though a rare long-track tornado can last over an hour.
• Wind speed: from about 105 km/h in the weakest to over 400 km/h in the most violent.
• Direction of spin: counter-clockwise in most tornadoes in the Northern Hemisphere, but a few spin the other way.

What Do You Need to Make a Tornado?

Tornadoes are born inside severe thunderstorms. Several ingredients must meet in the same place:

• Warm, moist air near the ground: this is the fuel, and it often flows in from a warm sea or a humid region.
• Cooler, drier air above: it makes the atmosphere unstable, so warm air rises strongly once it is lifted.
• A lifting trigger: a cold front, a dry line, a sea breeze or a mountain slope pushes the air up.
• Wind shear: the wind changes speed and direction with height, for example a light south wind at the surface and a strong west wind higher up. This is the source of the spin.

How Does a Tornado Form? 6 Steps

1. The air becomes unstable. Warm, moist air sits under a layer of cooler, drier air, like a pot ready to boil.
2. A storm starts to grow. Something lifts the warm air, and it rises quickly, forming a tall thunderstorm cloud.
3. Wind shear makes a rolling tube. The faster wind above and the slower wind below create a horizontal spinning tube of air, a little like a rolling pin turning in the sky.
4. The updraft tilts the tube upright. The storm’s strong updraft lifts the middle of the tube and tilts it into a vertical column. The storm itself now rotates. This rotating updraft is called a mesocyclone, and the storm is a supercell.
5. The rotation tightens. As a cool downdraft wraps around the rotation and the updraft stretches the column upward, the spin gets narrower and faster, just as a skater spins faster by pulling in their arms.
6. The funnel touches down. The low-pressure core extends from the cloud base to the ground. Once it reaches the surface, it is a tornado.

Not every supercell makes a tornado, and scientists are still studying why some do and others do not. Many tornadoes also form from other kinds of storms, such as lines of thunderstorms and the outer rain bands of a tropical cyclone, but these tend to be weaker and shorter.

Warning Signs of a Tornado

• A dark, sometimes greenish sky.
• Large hail, especially with little or no rain.
• A rotating, lowering cloud base or a wall cloud.
• A sudden calm after heavy rain or hail.
• A loud, continuous roar, often compared to a freight train.
• Dust or debris swirling near the ground, even when no funnel is visible.

How Strong Is a Tornado? The Enhanced Fujita (EF) Scale

Tornadoes are rated by the damage they cause, since it is rarely possible to measure the wind directly. The Enhanced Fujita scale runs from EF0 to EF5.

Most tornadoes are weak (EF0 or EF1). Only a small share reach EF4 or EF5, but these cause most of the deaths.

Where Do Tornadoes Happen?

Tornadoes occur on every continent except Antarctica. The United States has the most, especially across the central plains often called Tornado Alley, where warm, moist air from the Gulf of Mexico meets cold, dry air from Canada and dry air from the Rocky Mountains. Other regions include Argentina and Uruguay, Bangladesh and eastern India, and parts of Europe, Australia and South Africa. Bangladesh has suffered some of the deadliest tornadoes in history, because of its dense population and the strong storms of the pre-monsoon season.

Tornadoes in Pakistan

Pakistan does not have a regular tornado season like the central United States, and strong tornadoes are rare. Still, severe thunderstorms in the pre-monsoon and monsoon months, especially across Punjab and the upper Sindh plains, can produce short-lived tornadoes, as well as damaging downburst winds and dust storms. Damage from a sudden windstorm is often mistaken for a tornado, which is why a trained survey of the damage pattern is needed to tell them apart. Always follow warnings from the Pakistan Meteorological Department when a severe storm is forecast.

Tornado vs Hurricane vs Dust Devil vs Waterspout

To compare with a much larger spinning storm, read what a tropical cyclone is.

Tornado Safety Tips

• Know the difference between a watch (conditions are right for a tornado) and a warning (a tornado is happening or about to happen). Act at once on a warning.
• Go to the lowest floor of a strong building, such as a basement or a small interior room away from windows. Put as many walls as you can between you and the outside.
• Cover your head and neck with your arms, a mattress, heavy blankets or a helmet.
• Do not stay in a mobile home or a light, temporary shelter. Move to a stronger building if there is time.
• Do not try to outrun a tornado in a car in heavy traffic. If you are caught outdoors with no shelter, lie flat in a low area and cover your head. Avoid highway overpasses, which can funnel the wind.
• Do not waste time opening windows. It does not equalise pressure, and it wastes precious seconds.
• Keep a battery radio, a charged phone, a torch, water and a first aid kit ready.
• After the storm, stay away from fallen power lines, broken glass and damaged buildings.

Keep watching the sky and the forecast with the live weather, hourly forecast and weather maps on WeatherWorld.

Can We Predict Tornadoes?

Forecasters can tell days ahead when the ingredients for severe storms are in place, and they can issue a warning when radar shows rotation in a storm or when spotters see a funnel. The lead time is often only minutes, so a clear plan is vital. The exact time and place of a touchdown are still very hard to forecast far in advance.

Learn More

For official safety guidance, visit the National Weather Service tornado safety page. To understand how pressure differences drive severe weather, read about atmospheric pressure, and to see how the same atmosphere can clear after a storm, read how a rainbow forms.

Frequently Asked Questions

What is a tornado in simple words?

A tornado is a fast-spinning column of air that reaches from a thunderstorm cloud to the ground. It can destroy buildings and throw objects through the air.

How does a tornado form?

Warm, moist air rises in a thunderstorm while wind shear makes the storm rotate. The rotation tightens and stretches down from the cloud until it touches the ground.

How fast are tornado winds?

From about 105 km/h in weak tornadoes to over 400 km/h in the most violent ones.

What is the difference between a tornado and a funnel cloud?

A funnel cloud is a spinning cloud that hangs from the storm but has not reached the ground. When it touches the surface, it is a tornado.

How long does a tornado last?

Most last only a few minutes. A small number last longer than half an hour, and very rarely more than an hour.

Where is the safest place during a tornado?

The lowest level of a strong building, in an interior room away from windows, such as a basement or a small bathroom.

Can a tornado happen at night?

Yes. Night tornadoes are especially dangerous because they are hard to see and people may be asleep, so a weather alert on your phone or radio is important.

Final Thoughts

A tornado is one of the most violent and fastest-changing weather events on Earth. It grows from warm, moist air, an unstable atmosphere and winds that change with height, and it can go from nothing to a destructive funnel within minutes. You cannot stop a tornado, but you can prepare. Learn the warning signs, know where your safe place is and act at once when a warning is issued. A few minutes of preparation can save lives.