What we know—and what we don’t—about the science of tornadoes

How tornadoes^[1]
form and how they die is not fully understood, yet scientists
probing those mysteries—and aiming to improve warning systems—have
pinpointed key risk factors.

Large tornadoes usually last longer—around 30 minutes, Brooks
adds. The most powerful twisters have wind speeds of more than 300
miles (483 kilometers) per hour, which can rip buildings off their
foundations. They can be more than two miles (3.2 kilometers) wide,
and can spin across the ground for dozens of miles.

The more common tornadoes have wind speeds of less than 110
miles (177 kilometers) per hour, are about 250 feet (76 meters)
across, and travel only a few miles before they dissipate.

Tornadoes kill an average of 60 people a year in the U.S.,
mostly from flying or falling debris, reports NOAA^[2]. (See “Interactive: Forces of Nature^[3].”) Half of those deaths
are caused by the strongest one percent of the most violent storms,
says Brooks.

The most intense tornadoes emerge from what are called supercell
thunderstorms. For such a storm to form, you first “need the
ingredients for a regular thunderstorm,” says Brooks.

Those ingredients include warm moisture near the surface and
relatively cold, dry air above. “The warm air will be buoyant, and
like a hot-air balloon it will rise,” says Brooks.

A supercell requires more: winds that increase in strength and
change direction with height. “Then the updraft tends to rotate,
and that makes a supercell,” explains Brooks.

The supercell churns high in the air and, in about 30 percent of
cases, it leads to the formation of a tornado below it. This
happens when air descending from the supercell causes rotation near
the ground.

Even then, “we still don’t know why some thunderstorms create
tornadoes while others don’t,” tornado-chaser Tim
Samaras^[4] said in early 2013.
Samaras was a scientist and National Geographic grantee who was
killed by a twister on May 31, 2013, in El Reno, Oklahoma. (Read
“The Last
Chase^[5]” in National
Geographic magazine.)

Brooks says scientists believe strong changes in winds in the
first kilometer of the atmosphere and high relative humidity are
important for the formation of tornadoes. He adds that there also
needs to be a downdraft in just the right part of the storm.

Tornado formation also requires a “Goldilocks” situation, in
which air must be cold but not too cold. It should be a few degrees
more frigid than surrounding air, Brooks says.

He adds, “We don’t understand how tornadoes die: Eventually the
air gets too cold and it chokes off the inflow of new air into the
storm, but we don’t know the details.”

Tornadoes have been observed on every continent except
Antarctica. They have been most documented in North America, where
an estimated 1,200 strike the United States each year, but they
frequently appear in many other countries.

The most notoriously affected region in the United States,
called “Tornado
Alley^[6],” includes the Great
Plains states of Kansas, Nebraska, and the Dakotas, as well as
parts of Texas. Large-scale weather patterns tend to converge on
that area, making tornadoes more likely.

Still, the state that receives the highest number of tornadoes
per square mile is Florida, according to the American
Meteorological Society. Indiana, Illinois, Iowa, and Louisiana also
have many tornadoes per square mile.

Tornadoes can happen at any hour of the day and any time of the
year, though they are most common in the spring, especially during
May and June in North America.

In many countries, including the United States, Canada, and
continental Europe, the strength of tornadoes is often measured by
the Fujita scale or the updated Enhanced Fujita Scale. An F0 or EF0
tornado damages trees but substantial structures are left unharmed;
a tornado in the strongest category—F5 or EF5 —blows away
buildings.

Since measuring wind speeds inside a twister is extremely
difficult, scientists typically rely on damage to estimate
velocities.

Tornadoes are much harder to forecast than are hurricanes, which
are larger storms that last a lot longer. According to NOAA, the
average amount of time between a tornado warning and the arrival of
a storm is about 13 minutes. (A tornado warning means a twister has
been sighted, while a tornado watch means one is possible.)

The National Severe Weather Laboratory’s Warn-on-Forecast^[7] research project is
aiming to improve forecasting, although the work is challenging,
says Brooks.

The project uses powerful software to crunch data on
temperatures, moisture, and other atmospheric variables. Sometimes
the system “makes really good forecasts, and other times it
doesn’t,” says Brooks.

As computers get faster and data improves, accuracy may rise, he
suggests. In the meantime, better understanding of the atmosphere
will also help with other endeavors, such as planning for wind
farms or the placement of solar panels.

Brooks adds, “It’s not completely clear that increasing the lead
time for tornado [forecasts] is going to benefit the general
public, because we’re not sure how people are going to respond to
that information.” Many people ignore current tornado watches, for
instance, thinking the threat is unlikely.

But, Brooks says, “there are probably audiences out there that
will be able to take good advantage of it, such as emergency
managers and vulnerable populations that might take a long time to
get prepared.”

Predicting the path of a tornado across the landscape can also
be challenging. Brooks says tornadoes tend to follow the general
movement of the thunderstorm they are associated with, but the
route can be erratic.

"It’s kind of like walking a dog," he says.
"You get down the block, but in the middle the dog goes
back and forth."