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Pavement
Fri Jul 23, 2004
Listen in RealAudio 
We've all seen pictures of what tornadoes can do to houses and cars. But did you
know a twister can rip up a highway? I'm Bryan Yeaton, and this is The Weather
Notebook.
Some tornadoes have been known to actually pull up sections of a road, especially in
the wide-open flatlands of the Southern Plains. Storm chasers came up with the name
"pavement pullers" for these ground-scouring twisters.
It turns out that highways are not alike when it comes to tornado damage. When a road
is paved with asphalt, a thin layer of the substance--typically a few inches thick--is
spread out atop a road base made of sand. The asphalt isn't fixed to the roadway--it's
simply laid on top. The high winds of a tornado apply an upward force to the road as
they pass over, much like high winds do to an airplane wing, or the roof of a house. If
the road is flat, it's not too difficult for a strong tornado to pull up the edge of the asphalt,
break the road apart, and send the whole thing flying.
Concrete roads are a different matter. These surfaces are made by pouring concrete
into frameworks that are reinforced with steel. The concrete is thicker than
asphalt--sometimes up to a foot thick--and it's secured to the roadway by steel. All this
infrastructure makes a concrete highway very resistant to tornado damage, according
to Tim Marshall an engineer and tornado specialist in Texas. He has seen the damage
from dozens of tornadoes, but he's yet to learn of a single case where a concrete road
has been pulled up by a twister. Even the 300 mph winds of a F-5 tornado leaves
concrete roads on the ground.
Our show is funded by Subaru of America and the National Science Foundation. We
are a program of the Mount Washington Observatory.
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