A tornado, often colloquially called a twister, is a violently rotating column of air that extends from the base of a thunderstorm cloud down to the surface of the Earth. This atmospheric vortex is capable of producing the strongest winds known on the planet, sometimes exceeding 300 miles per hour. While most tornadoes are weak and short-lived, the largest can cut a destructive path over a mile wide and travel for dozens of miles. The visible part of the tornado is typically a condensation funnel, composed of water droplets, dust, and debris, confirming the wind vortex is in contact with the ground.
How Tornadoes Form
The formation of a tornado, known as tornadogenesis, typically begins within a powerful thunderstorm called a supercell. The first step involves wind shear, a difference in wind speed or direction over a short distance in the atmosphere. This shear creates an invisible, horizontal tube of rotating air in the lower atmosphere. A warm updraft of air within the developing supercell acts to lift and tilt this horizontal rotating tube vertically.
As the rotating air column is lifted, it becomes a vertical vortex, known as a mesocyclone, which is a rotating region several miles wide extending through much of the storm. As the mesocyclone strengthens, the column of air begins to narrow and stretch vertically, causing it to spin faster, much like a figure skater pulling in their arms. A descending column of cool air, called the rear-flank downdraft, wraps around the mesocyclone and contributes to this tightening process. When this rapidly spinning circulation extends all the way down and makes contact with the ground, a tornado is officially formed.
Rating Tornado Strength
The intensity of a tornado is classified after it occurs using the Enhanced Fujita Scale, or EF Scale. This scale rates a tornado’s strength from EF0 to EF5, with six categories of increasing wind speed and destructiveness. Crucially, the EF Scale rating is not based on direct wind speed measurements, but rather on the degree of damage observed. The scale uses a set of 28 damage indicators to estimate the wind speeds required to cause the damage.
A low-end tornado, such as an EF0 or EF1, typically causes light to moderate damage, like peeling surface material off roofs or moving mobile homes off foundations. The wind speeds associated with an EF0 range from 65 to 85 mph, while an EF1 reaches 86 to 110 mph. In contrast, a high-end tornado, classified as EF4 or EF5, is associated with devastating damage. An EF4 tornado, with estimated winds between 166 and 200 mph, can level well-constructed houses and throw cars, while an EF5 is capable of completely sweeping away strong frame houses.
Where and When Tornadoes Happen
While a tornado can occur on every continent except Antarctica, the highest frequency and intensity of these storms are found in the central United States. This region, often referred to as “Tornado Alley,” includes the Great Plains states like Texas, Oklahoma, Kansas, Nebraska, and South Dakota. The geography here is ideal, as warm, moist air from the Gulf of Mexico meets cool, dry air from the north, creating the atmospheric instability required for supercells.
Tornadoes can happen at any time of the year, but the peak season for activity in the U.S. is during the spring and early summer. The months of May and June typically record the highest number of tornadoes annually. They are most likely to strike during the late afternoon and early evening hours, generally between 4:00 PM and 9:00 PM local time.
Staying Safe During a Tornado
Preparation relies on understanding the difference between a Tornado Watch and a Tornado Warning. A Watch means conditions are favorable for tornadoes to develop in or near your area, prompting you to review your safety plan. A Warning is urgent, indicating a tornado has been sighted or detected by radar, meaning there is imminent danger to life and property.
When a Tornado Warning is issued, you must immediately seek shelter in a sturdy building. The safest locations are on the lowest floor, away from windows.
- An underground shelter, basement, or designated safe room is the safest option.
- If a basement is unavailable, move to a small, windowless interior room, closet, or hallway.
- Protect yourself from flying debris, which is the most common cause of injury.
- Cover your head and neck with your arms, and use a heavy blanket or mattress for added protection.
Mobile homes offer no protection during a tornado, and occupants should leave immediately for the nearest sturdy shelter.