According to the National Weather Service, a tornado is a “a narrow, violently rotating column of air that extends from a thunderstorm to the ground.” A twister typically originates from a hulking thunderstorm cloud known as a cumulonimbus that develops vertically and blossoms because of heat and moisture. Most tornadoes you hear about are born from mesocyclones, or rotating updrafts within a thunderstorm where air spirals inward and upward. Vortexes that form over water in this way are known as tornadic waterspouts. Some waterspouts and land spouts, another kind of vortex, lack mesocyclones but are still considered tornadoes. They’re generally weaker than mesocyclone tornadoes but their winds can reach 80 to 100 mph, enough to cause damage. Gustnadoes , which are weaker whirls of dust near the ground, are not tornadoes but close cousins. They spin up briefly on the leading edge of thunderstorm outflow, or exhaust. They persist only for a few seconds and rarely have winds greater than 70 mph. Similarly, dust devils are not tornadoes. They are swirls which can form under sunny conditions because of temperature differences between the ground and higher up.

Tornado watches vs. warnings: What’s the difference?

Tornado watches are issued when conditions are favorable for the development of tornadoes. A watch means to stay alert, and have a plan in place to shelter if a warning is issued. Watches usually cover an area a few hundred miles across and can stretch across multiple states. They typically remain in place for several hours and are issued by the National Weather Service Storm Prediction Center, based in Norman, Okla. Tornado warnings , on the other hand, are more targeted alerts for much smaller areas — often smaller than a county — where a tornado is believed to be imminent or occurring, whether based on radar scans or eyewitness reports. Usually lasting 30 minutes to 45 minutes, warnings come from local Weather Service offices, 123 of which are scattered about the United States and its territories. When a tornado warning is issued, the Weather Service advises seeking shelter immediately in the lowest level of a strong building.

Which is worse: a watch or a warning?

A warning is always worse than a watch. It’s an immediate call to action to seek shelter. When an especially dangerous tornado is sighted visually or detected by radar, a particularly dangerous situation warning is issued, which is an urgent call to action. Nothing compares to a tornado emergency, though. This seldom-issued alert is reserved for an immediate life-or-death situation in which a violent tornado is threatening a population center.

How are tornadoes rated?

Tornadoes are rated on the 0-to-5 Enhanced Fujita (EF) scale based on the severity of their damage, which corresponds to certain wind speed ranges. EF0 tornadoes are the weakest, with peak gusts of 65 to 85 mph, while EF5 tornadoes are strongest, with gusts over 200 mph. EF5 tornadoes — which can strip entire homes off their foundation — are exceptionally rare, making up less than 0.1 percent of all twisters.

What’s the deadliest tornado? The longest track tornado?

In the modern tornado record of the United States, the Tri-State tornado of March 18, 1925 is considered the deadliest. It killed 695 people after traveling a record-long 219 miles through southeast Missouri, southern Illinois and southwest Ohio. Some experts dispute that path length , suggesting the track may not have been continuous, and instead was the product of multiple tornadoes that were part of the same “family.” The tornado traveled northeast at forward speeds as high as 70 mph. Probably because of its extreme width at times, it didn’t look like a tornado, either. Many eyewitnesses who survived reported the wedge tornado resembled a rapidly advancing curtain of rain. The churning black mass unleashed 300 mph winds, which lay siege to the town of Murphysboro, Ill., killing 234 residents. The tornado lasted for 3 hours 45 minutes. Outside the United States , an intense tornado hit Daulatpur and Saturia in the Manikganj District of Bangladesh on April 26, 1989. It’s estimated that 1,300 people died, largely owing to Bangladesh’s exceptional population density and poor building construction.

What’s the highest wind speed ever produced by a tornado?

According to the World Meteorological Organization, the strongest wind speed produced by a tornado reached 302 mph in Bridge Creek, Okla., on March 5, 1999. This tornado destroyed thousands of homes and killed 46 people . The peak wind speed was detected by a mobile Doppler radar unit and was not directly measured by an instrument.

How big can tornadoes get?

The largest tornado on record to ever occur in the United States (and probably the world) was the El Reno, Okla., tornado of May 31, 2013. It reached a peak width of 2.6 miles , had peak winds of nearly 300 mph and killed several storm chasers .

Where should one shelter from a tornado?

Go to a basement or an interior room on the lowest floor of a sturdy building. Put as many walls between yourself and the outside as possible to lower the risk of being struck by flying debris. Stay away from windows. If no basement is available, a ground floor bathroom could work. It’s advised to shelter in a bathtub and consider bringing a blanket, mattress, helmet or pillow to cover your head. Mobile homes are death traps during a tornado. Only 5.6 percent of the American population lives in mobile homes, but 54 percent of tornado fatalities occur in them. While their boxlike structure might withstand EF0 and EF1 tornadic winds, it often gets rolled due to poor anchoring or lack of anchoring.

How do tornadoes form?

A combination of heat energy and turning winds are required for tornadoes. The heat energy or fuel for a storm is measured through a quantity known as CAPE, or convective available potential energy. It’s derived from warm, moist air that has a propensity to rise and form tall storm clouds. The turning of winds is approximated by what’s known as shear, which occurs as winds change between the ground and high in the sky. If a storm bubbles upward in a sheared environment, it will be forced to twist, and the resulting rotating thunderstorm or supercell is sometimes sufficient to produce a tornado. Rotating thunderstorms require both CAPE and shear. CAPE and shear often combine over the central United States and the Deep South, making these regions prime breeding grounds for tornadoes. In significant storm events, the CAPE’s primary source is the warm, humid air above the Gulf of Mexico. The shear is often generated by jet stream winds, which — powered by temperature contrasts between Canada and the gulf — frequently dive south over the nation’s midsection at high speeds.

When is tornado season?

Rick Smith, warning coordination meteorologist at the National Weather Service in Norman, Okla., has joked that “tornado season runs Jan. 1 to Dec. 31,” but it’s true — while tornadoes are most numerous in the spring, they can happen just about anywhere, any time. Across the Deep South, the period from November through May is typically most active for tornadoes. Over the Southern Plains, the peak storm season is more punctuated, lasting only about six weeks, from late April through the end of May. In the Northern Plains, the season typically extends into early summer. In recent years, there has been an uptick in cool-season events from November through the winter across parts of the central states and South, with some suggesting there may be a link to human-caused climate change .

Where is Tornado Alley?

Tornado Alley is an outdated term that fails to fully represent where tornadoes occur. While the most photogenic tornadoes tend to occur over the Plains — which was historically described as Tornado Alley, the bull’s eye for tornado fatalities, strong tornado frequency and likelihood to be affected is actually in the Mid-South. That’s where the most tornado watches and warnings are also issued most years. Tornadoes have occurred in every state of the country (even Alaska), but are most common east of the Rockies. The Plains, the South, the Midwest and the Tennessee Valley are hot spots, with a lesser risk east of the Appalachians. Despite long-standing myths, tornadoes cross rivers, strike cities and occur in the mountains.

What does a tornado sound like?

The sound volume produced by a tornado will depend on its closeness to you, its size, any debris it might contain and wind speeds. “The most common tornado sound is a continuous rumble, like a nearby train. Sometimes a tornado produces a loud whooshing sound, similar to a waterfall, or the noise of open car windows while driving very fast,” according to the National Weather Service.

Why does the sky sometimes turn green before a tornado?

The infamous green sky that comes with some severe thunderstorms has to do with what the thunderstorm is holding — water, and lots of it. It’s believed that big raindrops and hail scatter away all but the blue wavelengths, allowing primarily blue light to penetrate through and beneath the storm cloud. During the late afternoon and evening when storms are most frequent, the light is already skewed red/yellow by the setting sun. The combination of yellow and blue light produces green, although experts in atmospheric optics still say the overall process isn’t thoroughly understood.

Will the 2023 tornado season be bad?

There’s been no shortage of tornadoes so far this year, with 168 recorded across the country in January alone, the second-most on record. February bore witness to at least 55 twisters, twice the monthly average. March, which started slowly, awakened suddenly in its final week, including the deadly EF4 tornado that struck Rolling Fork, Miss . Researchers say it’s still challenging to predict how busy a tornado will become, but gently lean toward an above-average spring because of warmer-than-normal waters in the Gulf of Mexico and the transition from the La Niña to El Niño climate pattern . Ian Livingston contributed to this report.

Tornado season is here. Here is what you need to know.

Tornadoes are nature’s most violent windstorm. Here’s how they form.

By Matthew Cappucci

and

Jason Samenow

March 31, 2023 at 7:15 a.m. EDT

A tornado emerges west of Lockett, Tex., on April 29, 2021. (Matthew Cappucci/The Washington Post)

10 min

For residents of the Deep South and the Plains, spring is a time of year to hold one’s breath. Swarms of tornadoes spin up like clockwork, tearing through neighborhoods and pastures and etching scars in the landscape.

While tornadoes occur all over the world, the United States sees more tornadoes than any other country — roughly 1,150 to 1,200 per year on average. So many tornadoes form in the United States because its geography and climate bring together the key ingredients for spinning storms: a mix of cold, dry and warm, humid air.

For those curious about tornadoes — and what gives rise to the deadly phenomena — we’ve got you covered: