The science behind how tornadoes form

By Brian Lada, AccuWeather meteorologist and staff writer

Tornadoes come in all shapes and sizes and have been reported in all 50 states. But how do these destructive forces of nature form?

Twisters can touch down during any time of the year, but they are more frequently reported in the central United States in an area known as Tornado Alley from April through June. 

A tornado forms over a house near Pilger, Neb., Monday, June 16, 2014. At least one person is dead and at least 16 more are in critical condition after two massive tornadoes swept through northeast Nebraska on Monday. (AP Photo/Mark 'Storm' Farnik)

The largest and most destructive tornadoes are spawned by incredibly strong thunderstorms known as supercells.

“What makes a supercell unique from all other thunderstorm types is that it contains a deep and persistent rotating updraft called a mesocyclone. If the environment is favorable, supercell thunderstorms can last for several hours,” the National Weather Service (NWS) explained on its website.

“Most tornadoes form during supercell thunderstorms, but not all supercell thunderstorms produce tornadoes,” the University Corporation for Atmospheric Research (UCAR) said on its website.

In extreme cases, supercells can generate tornadoes that can reach over 2 miles wide with winds over 300 mph.

The key weather conditions needed for tornadoes

For a tornado-producing supercell to develop initially, four ingredients need to be present in the atmosphere.

"These ingredients are moisture, instability, vertical forcing and wind shear," AccuWeather Lead Storm Warning Meteorologist Eddie Walker said.

Not all four ingredients are needed for a thunderstorm to develop, but without these ingredients present, a storm may not reach severe levels. 

During the spring, these four ingredients come together most frequently in the central and southeastern U.S., which is how they earned the nicknames of Tornado Alley and Dixie Alley, respectively.

The risk for severe thunderstorms expands during the summer, extending northward into Canada and all the way to the east coast of the U.S. 

Every thunderstorm has a column of rising air, known as an updraft, that is caused by the combination of vertical forcing and instability. The stronger the updraft, the more intense the storm will become.

Severe thunderstorms can have an updraft that extends more than 40,000 feet above the ground, allowing the storm to be seen from over 100 miles away in some cases.

For a tornado to develop, air needs to rotate horizontally near the ground. This rotation is caused by wind shear.

When this rotating air is drawn into the updraft, it becomes tilted vertically.

“The updraft lifts the rotating cylinder within the supercell. The rotating cylinder of air narrows, becoming stretched, and spins faster and faster forming a tornado,“ NOAA said.

This is just one of several factors that play a role in tornado development.

“With so much air rising in a severe thunderstorm, there must be corresponding downdrafts. It is believed that when these rapidly descending downdrafts are enhanced by the overall spin of the thunderstorm, a tornado forms,” AccuWeather Senior Meteorologist Alex Sosnowski said. 

Meteorologists are still trying to understand the exact process in which tornadoes form, as there are still some unknowns.

A study published on Dec. 13, 2018, explored the process in which tornadoes form and found that they take shape contrary to popular thought.

“Historically, scientists assumed tornado rotation began in storm clouds, creating a funnel that travels downwards. This theory matches what storm chasers commonly observe visually in the field. Viewers often report seeing funnel clouds gradually descending until they make contact with the ground,” the study explained.

“But new research combining a new type of Doppler radar with photos and videos of tornadoes formed by supercell thunderstorms shows the opposite is true: Tornadoes materialize from the ground up,” according to the study.

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The difference between tornado watches and warnings

Although many tornadoes are associated with supercell thunderstorms, that is not always the case.

Another way that tornadoes can spin up occurs when there is a vigorous, fast-moving cold front.

For tornadoes to develop like this, ample wind shear must be present. These types of tornadoes are typically smaller and short-lived compared to tornadoes generated by supercell thunderstorms, but they can still threaten lives and property.

Waterspouts can also turn into tornadoes if they move ashore and tend to be smaller and short-lived. However, they can still cause damage where they make landfall, usually equivalent to an EF0 or EF1 tornado.