Most damage in tornados is from horizontal wind component. There is very
little upward component in the typical tornadic vortex flow. The horizontal
wind picks things up and throws them, rolls them, etc. Upward flow is
really only significant for lightweight, low density things (bank checks,
for instance, provide good information on airflow.. they float well, they
have names and addresses, etc.)
Granted, I've seen footage of an F150 pickup several hundred feet in the air
being thrown out of a tornado, but even there, the horizontal component was
dominant.
Previous explanations of why trees are snapped off above ground are right on
track.. the bottom of the tree is strongest, the aerodynamic load up higher
is higher (wind velocities get bigger as you get higher off the ground: the
friction close to the ground slows the air down). There are also some
structural reasons why long columns loaded sideways(i.e. really a
canteliever beam) tend not to fail at the end (look at falling chimneys,
breaking pencil points, etc.) if you start to consider dynamic effects
(angular acceleration in particular)
----- Original Message -----
From: "Guy Olinger, K2AV" <k2av@contesting.com>
To: "Bill Coleman" <aa4lr@arrl.net>; "TowerTalk" <towertalk@contesting.com>
Sent: Friday, April 26, 2002 6:49 PM
Subject: Re: [Towertalk] Inquiry - Tower Collapse/Fall/Radius Zone
> A tornado is trees being ripped UP. It breaks at the point
> accumulating most aerodynamic drag and least able to support the
> weight of the root ball.
>
> ----- Original Message -----
> From: "Bill Coleman" <aa4lr@arrl.net>
> To: "Dan Evans" <n9rla@yahoo.com>; <TOWERTALK@contesting.com>;
> <n4kg@juno.com>
> Sent: Friday, April 26, 2002 1:42 PM
> Subject: Re: [Towertalk] Inquiry - Tower Collapse/Fall/Radius Zone
>
>
> > On 4/9/02 9:29 AM, Dan Evans at n9rla@yahoo.com wrote:
> >
> > >The annoying part of that is, every tree I've ever seen fall, fell
> it's full
> > >height. But no one is restricting the height of trees.
> >
> > Actually, when a tornado went through my neighborhood in 1988, the
> fallen
> > trees almost never fell their full height.
> >
> > Instead, they were snapped off at about 12-18 feet off the ground.
> >
> > The one incident of a tree falling out to its full height was a
> > neighbor's tree that leaned over to one side after many, many days
> of
> > rain waterlogged the ground, and caused the roots to pull out. That
> one
> > failed about 2-3 feet BELOW the ground.
> >
> > I don't think below ground failures are likely for proper tower
> > installations. Failures 10-20 feet off the ground are more likely.
> Towers
> > (and trees!) are awfully strong right next to the ground.
> >
> >
> >
> > Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@arrl.net
> > Quote: "Not within a thousand years will man ever fly!"
> > -- Wilbur Wright, 1901
> >
> > _______________________________________________
> > Towertalk mailing list
> > Towertalk@contesting.com
> > http://lists.contesting.com/mailman/listinfo/towertalk
> >
>
>
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