How Do Tornadoes Form?

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When dark, ominous clouds begin to rotate and a funnel extends toward the ground, one of nature’s most destructive forces is taking shape. Tornadoes can uproot trees, demolish homes, and hurl vehicles through the air with winds that can exceed 300 miles per hour.

How exactly do these powerful weather phenomena form? This article explains the science behind tornado formation, the conditions that create them, how they’re classified, and what you can do to stay safe when one threatens.

What Is a Tornado?

A tornado is a violently rotating column of air that extends from a thunderstorm cloud to the ground. The National Weather Service defines tornadoes as “violently rotating columns of air, extending from a thunderstorm, which are in contact with the ground.” This rotating air column becomes visible when it picks up dust, debris, or water droplets, creating the characteristic funnel shape we associate with tornadoes.

Tornadoes vary greatly in size, strength, and duration. Some may be only a few yards wide and last just minutes, while others can stretch more than a mile across and remain on the ground for over an hour. The most destructive tornadoes combine large size with high wind speeds and long paths.

The Perfect Storm: Conditions That Create Tornadoes

Tornadoes don’t just happen randomly. They require specific atmospheric conditions to form. While scientists still don’t fully understand every aspect of tornado formation, we do know the key ingredients that set the stage for these powerful storms.

Atmospheric Instability

For tornadoes to form, the atmosphere must be unstable. This means warm, moist air near the ground with cooler, drier air above it. This creates a situation where the warm air wants to rise rapidly through the cooler air, resulting in strong updrafts within a thunderstorm.

This instability often develops on warm spring and summer days when heat from the sun warms the ground and the air near it. As this warm air rises, it creates the vertical motion needed for thunderstorm formation.

Wind Shear

Perhaps the most critical ingredient for tornado formation is wind shear – the change in wind speed and/or direction with height. When winds at different altitudes blow at different speeds or directions, they create a horizontal spinning effect in the lower atmosphere.

According to the National Oceanic and Atmospheric Administration, “Forecasters and storm spotters are trained to recognize thunderstorm features that make tornado formation more likely.” These features often include evidence of strong wind shear.

Moisture

Tornadoes need moisture to form. Humid, tropical air masses provide the water vapor that fuels thunderstorm development. This moisture-laden air feeds the thunderstorm and allows it to grow and intensify.

In the United States, this often comes in the form of warm, moist air from the Gulf of Mexico colliding with cooler, drier air from the north or west. The battle between these contrasting air masses creates the perfect environment for severe thunderstorms and tornadoes.

From Thunderstorm to Tornado: The Formation Process

Understanding how a regular thunderstorm transforms into a tornado-producing supercell helps explain why some storms become so dangerous while others remain relatively harmless.

Step 1: Thunderstorm Development

The process begins with a thunderstorm, which forms when warm, moist air rises rapidly in an unstable atmosphere. As this air rises, it cools and condenses, forming clouds and eventually a thunderstorm.

Not all thunderstorms will produce tornadoes. In fact, the vast majority don’t. Tornadoes typically form from a special type of thunderstorm called a supercell, though they can occasionally develop from other storm types as well.

Step 2: Supercell Formation

A supercell is a long-lived thunderstorm with a persistently rotating updraft called a mesocyclone. As the Storm Prediction Center explains, “The striations of the low-level clouds reflect, in part, converging low-level winds curving into the rotating updraft of the storm. This rotating updraft is known as a mesocyclone.”

Supercells form when wind shear causes the horizontal rotation in the atmosphere to be tilted into the vertical by the thunderstorm’s updraft. This creates a rotating column of air inside the storm – the mesocyclone – which can be several miles wide.

According to meteorologists at Penn State University, “A classic supercell displays a hook echo on images of radar reflectivity, which occurs as precipitation wraps around the mesocyclone (the rotating updraft), and if a tornado forms, it does so within the hook echo.”

Step 3: Tornado Formation

The exact process by which a mesocyclone leads to tornado formation is complex and still being studied. However, scientists believe it involves the interaction between the storm’s rotating updraft and downdrafts that develop within the storm.

As a supercell matures, it often develops a Rear Flank Downdraft (RFD) and a Forward Flank Downdraft (FFD). These are areas where rain-cooled air descends from the storm. The interaction between these downdrafts and the mesocyclone appears to be crucial in tornado formation.

When the RFD wraps around the mesocyclone and tightens the rotation, it can stretch the rotating column vertically, causing it to spin faster due to the conservation of angular momentum (the same principle that makes figure skaters spin faster when they pull in their arms).

According to the National Severe Storms Laboratory, “The truth is that we don’t fully understand. The most destructive tornadoes occur from supercells, which are rotating thunderstorms with a well-defined radar circulation called a mesocyclone.”

Step 4: The Visible Funnel

As the rotation intensifies and lowers toward the ground, a condensation funnel may form. This is the visible part of the tornado, made up of water droplets created by the rapid cooling and condensation of air within the intense low-pressure center of the vortex.

A tornado officially forms when this rotating column makes contact with the ground. Sometimes, the funnel cloud may not be visible all the way to the ground, but if dust and debris are being lifted at the surface, a tornado is present.

Different Types of Tornadoes

Not all tornadoes form in the same way or display the same characteristics. Here are the main types you might encounter:

Supercell Tornadoes

These are the most common type of strong and violent tornadoes, forming from supercell thunderstorms as described above. The classic tornado that people typically picture comes from a supercell storm. These can be the most destructive and long-lived tornadoes.

According to National Geographic, “The most violent tornadoes come from supercells, large thunderstorms that have winds already in rotation. About one in a thousand storms becomes a supercell, and one in five or six supercells spawns off a tornado.”

Landspouts

Landspouts are tornadoes that form during the early development stage of a thunderstorm, before the storm has become a supercell. They’re similar to waterspouts (described below) but occur over land. They typically form when an existing area of rotation at the surface is stretched upward by a developing thunderstorm.

Landspouts are generally weaker and shorter-lived than supercell tornadoes but can still cause damage.

Waterspouts

Waterspouts are tornadoes that form over water or move from land to water. They most commonly form in tropical and subtropical regions and are frequently seen in the Florida Keys and the Great Lakes.

The NOAA describes waterspouts as “tornadoes that form over water or move from land to water.”

Many waterspouts are relatively weak and dissipate quickly upon reaching land, but stronger ones can cause significant damage to boats or coastal areas.

Gustnadoes

Gustnadoes are weak, short-lived spinning vortices that form along the gust front of a thunderstorm. They’re not connected to the storm’s mesocyclone and are not technically true tornadoes, but they can cause minor damage.

Dust Devils

While they may look similar to tornadoes, dust devils are not actually tornadoes because they form under clear skies and are not associated with thunderstorms. They develop when hot air near the ground rises rapidly through cooler air above it, creating a rotating column of air.

Tornado Alley and Beyond: Where Tornadoes Occur

While tornadoes can occur almost anywhere in the world, certain regions experience them more frequently due to their unique geography and climate.

Tornado Alley

“Tornado Alley” is the nickname given to the region of the central United States where tornadoes are most frequent. This area traditionally includes parts of Texas, Oklahoma, Kansas, Nebraska, South Dakota, and eastern Colorado.

The flat terrain of the Great Plains allows cold, dry air from the Rocky Mountains to collide with warm, moist air from the Gulf of Mexico, creating the perfect conditions for tornado formation.

Dixie Alley

Another area with a high frequency of tornadoes is “Dixie Alley,” which includes parts of Arkansas, Louisiana, Alabama, Mississippi, Georgia, and Tennessee. Tornadoes in this region can be particularly dangerous because they:

• Often occur at night
• Can be rain-wrapped and difficult to see
• Happen in areas with more hills, trees, and population density than Tornado Alley
• Sometimes occur during winter and early spring when people aren’t expecting them

Global Occurrence

Tornadoes are not unique to the United States. They occur in many parts of the world, including:

• Canada (particularly in southern Ontario and the Prairie Provinces)
• Bangladesh (which has experienced some of the deadliest tornadoes on record)
• Argentina (especially the central region)
• South Africa
• Europe (particularly in the United Kingdom, France, Germany, Italy, and Russia)
• Australia
• Japan

According to the National Severe Storms Laboratory, “Tornadoes occur in many parts of the world, including Australia, Europe, Africa, Asia, and South America. Even New Zealand reports about 20 tornadoes each year.”

Measuring Tornado Strength: The Enhanced Fujita Scale

After a tornado occurs, meteorologists assess the damage to determine its strength. Since 2007, the United States has used the Enhanced Fujita (EF) Scale to rate tornadoes.

According to the National Weather Service, “The Enhanced Fujita Scale or EF Scale is used to assign a tornado a ‘rating’ based on estimated wind speeds and related damage.”

The EF Scale has six categories, from EF0 to EF5, based on the damage caused:

EF0: Light Damage (65-85 mph winds)

• Broken branches
• Damaged chimneys
• Shallow-rooted trees pushed over
• Damage to signboards

EF1: Moderate Damage (86-110 mph winds)

• Roof surfaces peeled off
• Mobile homes pushed off foundations
• Moving automobiles pushed off roads
• Attached garages damaged

EF2: Considerable Damage (111-135 mph winds)

• Roofs torn off well-constructed houses
• Foundations of frame homes shifted
• Mobile homes completely destroyed
• Large trees snapped or uprooted

EF3: Severe Damage (136-165 mph winds)

• Entire stories of well-constructed houses destroyed
• Severe damage to large buildings
• Trains overturned
• Trees debarked

EF4: Devastating Damage (166-200 mph winds)

• Well-constructed houses completely leveled
• Cars thrown
• Structures with weak foundations blown away some distance

EF5: Incredible Damage (Over 200 mph winds)

• Strong frame houses lifted off foundations and carried considerable distances
• Automobile-sized missiles fly through the air
• Bark stripped from trees
• Concrete structures significantly damaged

According to WOWT News, “The last EF-5 tornado was in Moore, Oklahoma in May of 2013.” EF5 tornadoes are extremely rare, with less than 1% of all tornadoes reaching this intensity.

Tornado Forecasting and Detection

Predicting exactly when and where a tornado will form remains challenging, but meteorologists have developed sophisticated tools and techniques to provide advance warning.

Doppler Radar

Doppler radar is the primary tool for detecting tornado formation. It can “see” the rotation inside a thunderstorm before a tornado touches down by measuring the motion of precipitation within the storm.

When meteorologists detect a rotating signature on radar (called a “hook echo” or “velocity couplet”), they can issue warnings for areas in the storm’s path.

Storm Spotters

Human observers play a crucial role in tornado detection. The SKYWARN program, operated by the National Weather Service, trains volunteer storm spotters to recognize tornado precursors and report them to meteorologists.

According to the National Severe Storms Laboratory, “Ordinary citizen volunteers make up what is called the SKYWARN network of storm spotters, who work with their local communities to watch for approaching tornadoes.”

Weather Satellites

Advanced weather satellites can detect environmental conditions conducive to tornado formation and track the development of severe thunderstorms from space.

The NOAA explains, “NOAA’s GOES-R Series weather satellites do a better job than earlier satellites of identifying storms likely to produce tornadoes.”

Tornado Warnings and Watches

The National Weather Service issues two types of alerts for potential tornado situations:

• Tornado Watch: Issued when conditions are favorable for tornado development. A watch typically covers a large area and lasts for several hours.
• Tornado Warning: Issued when a tornado has been spotted or indicated by radar. A warning means imminent danger to life and property, and people in the affected area should seek shelter immediately.

According to the National Weather Service, “A tornado warning means that a tornado has been sighted or indicated by weather radar. There is imminent danger to life and property.”

In rare, extreme cases, the NWS may issue a Tornado Emergency, which is the highest alert level reserved for confirmed, destructive tornadoes heading toward populated areas.

Tornado Safety: Protecting Yourself

Knowing what to do when a tornado threatens can save your life. Here are the essential safety guidelines to follow:

Have a Plan

• Identify the safest place in your home (typically a basement, storm cellar, or interior room on the lowest floor)
• Create a family emergency plan and practice it regularly
• Keep emergency supplies (water, non-perishable food, flashlights, batteries, first aid kit) in your shelter area

When a Tornado Watch Is Issued

• Stay informed through a weather radio, local news, or weather apps
• Be prepared to act quickly if conditions worsen
• Bring in outdoor furniture or objects that could become projectiles
• Review your emergency plan with family members

When a Tornado Warning Is Issued

• Take shelter immediately in your designated safe room
• If you’re in a building without a basement, go to a small interior room on the lowest floor, away from windows, doors, and outside walls
• Cover yourself with a mattress, blankets, or pillows for protection against debris
• In a mobile home, abandon it for sturdier shelter, even if it means driving to a community shelter
• If caught outside with no shelter available, lie flat in a ditch or low-lying area, covering your head with your hands

After a Tornado

• Stay sheltered until the danger has completely passed
• Check for injuries and provide first aid if needed
• Be aware of hazards such as broken glass, downed power lines, and gas leaks
• Use phones only for emergency calls
• Stay out of damaged buildings

The Science Continues: Ongoing Tornado Research

Despite advances in understanding tornadoes, many questions remain. Scientists continue to study these powerful storms to improve forecasting and save lives.

The NOAA National Severe Storms Laboratory leads much of this research and notes, “Scientists still have unanswered questions about tornadoes: Why do most supercell thunderstorms not result in a tornado? How exactly do tornadoes form? What are the causes of wind shear that lead to rotation?”

Major research projects like VORTEX (Verification of the Origins of Rotation in Tornadoes Experiment) have used mobile Doppler radars, weather balloons, and other instruments to study tornadoes up close. These efforts help meteorologists better understand tornado formation and improve warning systems.

One promising area of research is the development of “Warn-on-Forecast” capabilities, which aim to predict specific tornado formation up to an hour in advance, significantly increasing warning times for the public.

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