Hollywood loves a good tornado. Like most things Hollywood it's all a little
dramatic, but flying cows aside there is something accurate about the setting.
"I have a feeling we're not in Kansas anymore."
You're not. But the tornado started there, which makes sense. Tornadoes are far more
common in the US than anywhere else. For perspective all of Europe records around
300 tornadoes per year whereas the US records well over a thousand. If we move
in a bit closer, you'll see that most of the world's tornadoes are happening here,
in an area called Tornado Alley — a place that's absolutely perfect for
twisters.
Tornado Alley doesn't have any "official" boundaries but it's typically
considered this area that extends from northern Texas through Oklahoma, Kansas,
Nebraska, Colorado, and South Dakota. Some people extend it even further east.
While most states in the US have recorded at least one tornado, this area
is really a hotbed and the main reason for that is geography.
The central part of the US is unique in the fact that there's this really large warm area of
water just to the South and the wide high range of mountains that extend a
long way from north to south.
Harold Brooks is the senior research scientist at the National Severe Storms Laboratory. He used to chase tornadoes
now he studies them from Norman, Oklahoma in the heart of Tornado Alley.
A tornado requires a couple of special ingredients — ingredients that tornado alley is full of.
First, we need a thunderstorm, which we're going to make right now. Sort of ...
To make a thunderstorm the things we need are warm moist air at low levels,
cold dry air above that, and some mechanism to lift that warm moist air up.
In Tornado Alley, lots of warm moist air flows into the plains from the Gulf of Mexico
and cool dry air flows from over the Rocky Mountains here in the West. Eventually a
change in temperature or pressure will arrive and lift that warm air up into
the cool air forming an updraft. Once these two meet, the moisture from the warm
air begins to condense, forming clouds, and a thunderstorm begins.
Under normal conditions, rain would fall from these clouds and cool the warm air
breaking the storm, but in tornado alley there's a strong air current flowing
from west to east known as the jet stream. This, paired with the cool
mountain air, blows the rain away keeping the air in the updraft warm and wet,
which allows the storm to intensify and brings us the step two: getting the storm
to rotate. To make that happen we need winds moving at different speeds and
directions. As you can see Tornado Alley has that in abundance. The air coming
from the Gulf moves slowly into the plains, meanwhile the jet stream from the
mountains provides a steady stream of high fast-moving air flowing east.
Because the jet stream is flowing faster and in a different direction it causes
the Gulf when below to rotate like a spinning football. When the spinning air
gets pulled into the updraft it's tilted, but continues to spin – causing the entire
updraft to rotate. Storms like this are known as "supercells" and they create
prime conditions for tornadoes. They're rare but most commonly occur in ... You guessed it –
Tornado Alley. As the supercell grows the spiraling updraft begins to stretch
towards the ground and forcefully pulls air into the cyclone. Air rushes in from
the sides and a spinning dust cloud forms below, which brings us to the final
stage – getting the vertically spinning air to the ground.
Stage three is the friction in the tea cup.
It's like using a spoon to swirl tea leaves at the bottom
of a cup. The tea leaves rush into the center and are pulled up through the
middle of the tea cyclone. In a real tornado everything around the cyclone is
sucked up – air, dirt, debris, cows ... As more and more air is pulled in tightly,
pressure builds and the faster and longer that tornado gets. It stretches
closer to the ground until it eventually meets with that dust cloud. And then, it
touches down.
We see most tornadoes in the central part of the US because of the
ingredients that are necessary for a tornado come together there more often
than any other place.
The United States isn't the only place that gets tornadoes.
Southeastern Brazil in northeastern Argentina have some of the same
ingredients Tornado Alley does: cool mountain air coming over the Andes, and
warm moist air coming from the Amazon. But even still, the frequency of
tornadoes is scores behind the US. While the right conditions come together
sometimes the geography isn't exactly right.
In South America the Andes aren't as wide as the Rockies
and the Amazon isn't as good of a moisture source as the Gulf of Mexico is because it's land.
It's sort of a Goldilocks problem
And a delicate recipe. Most people will go their entire lives without ever seeing a
tornado and some would consider that lucky. Others actively seek them out – and
those that do start in Tornado Alley, where the geography is just right.
Just knowing what makes a tornado isn't enough to anticipate when and where it'll happen,
so if you want to know how meteorologists try to predict these
dangerous storms check out the documentary tornado season on
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