but more form over the central United States than anywhere else

in a zone called Tornado Alley.

That's because conditions in the Alley are ideal

for creating tornadoes.

Warm humid air low to the ground moves north

from the Gulf of Mexico

and collides with cool dry air high above the ground

rolling in from the Rocky Mountains.

The collision of air masses creates a supercell,

a massive thunderstorm that has a strong rotating updraft of air.

Scientists aren't exactly sure what happens next

but the leading idea goes like this:

The difference in speed between the faster high winds

and the slower low winds

causes the air in between to rotate around the horizontal axis.

If one end of the rolling air gets caught on the updraft

is bent upward and forms a funnel cloud.

Its spin gets tighter and faster

and the cloud gets longer.

Rain or hail from the thunderstorm

can then push down on the tail of the funnel cloud

until it reaches the ground, forming a tornado.

The top wind speed of most tornadoes

is usually under a 110 miles an hour,

and most touch the ground for less than ten minutes.

However, extreme tornadoes are truly extreme.

The longest tornado path was cut by the Tri-State tornado on March 18, 1925.

It tore up property for 219 miles

It was on the ground for well over three hours

and killed 747 people.

Which is why I'll be hanging out in here,

in Antarctica with the Penguins

until tornado season is over.

For Scientific American, Instant Egghead. I'm Mark Fischetti.