Re: Energy Vortex Discovered on the Hayward Fault


"Aidan Karley" <name1_name2@xxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:VA.00001506.178a9bec@xxxxxxxxxxxxxxxxxxxxxxxxx
In article <Qe2tj.57611$Pv2.30124@xxxxxxxxxxxxxxxxxxxxxxxxxx>, Mike
Williams wrote:
At highway speeds, it would take at most one second to pass completely
through a large dust-devil/whirlwind.

Never tried crossing a large bridge in high winds in a van or
other high-sided vehicle? I've done it often enough that I've learned to
pay attention to those weather warnings of "20 knots gusting to 28". I'm
never likely to see an American desert dust devil, but I'd be watching
out for them in other continents.

--
Aidan Karley, FGS
Aberdeen, Scotland
Message written at Fri, 15 Feb 2008 09:30 GMT, now I'm back on shore.


Yeah, actually - the final 14 years of my 34-year wildland firefighting
career I drove, daily, a very large Crew Carrying Vehicle. Shaped like an
extremely large kleenex box. All over California.

With all due deference to Dave Hatunen, that same career of mine caused me
to spend nearly every working day outdoors in unpaved locations (i.e. the
wildland), for all those 33 years. I have observed literally thousands of
dust devils, often watching them for as much as a full minute or two before
they dissipate. Burned areas, in particular, covered with fine white ash,
make them especially visible. The largest of them (out of thousands) were
perhaps 25-feet in diameter, with the overwhelming majority only three or
four feet wide. It was not at all uncommon for them to pass right over me
and my crew. Typical windspeeds in the center of them were around 10 to 15
miles an hour, never significantly more. It doesn't take much wind velocity
to pick up dust, ash or litter. The only exceptions, not really dust-devils,
were fire-whirls which appeared to be near hurricane-force winds spinning
wildly up through the flames of an actively-burning fire to heights of as
much as 100 feet- an awe-inspiring phenomenon, but driven by temperature
gradients far stronger than for dust devils.

To get back to your note, Aidan, it is due to my familiarity with both
observing dust-devils, and driving high-profile vehicles in high winds, that
I have an alternate explanation for Dave's experience of feeling his vehicle
lurch first to one side, and then, rapidly, the other. From my experience
with dust devils, the visible portion of the dust devil is pretty much all
there is to it. Outside the visible part, the wind could only be described
as zephyr-like - far too gentle to cause drivers of passenger vehicles to
become aware of them. The reason Dave doesn't see the actual, proposed
dust-devil is because there isn't one. It is often the case that, while
driving in a steady wind, one only gradually enters a windy area, or the
wind speed only gradually increases. Without becoming aware of it, the
driver begins to bias her steering to windward to compensate. When the
vehicle suddenly enters a road-cut, or some other sharply-defined wind
obstruction, the vehicle suddenly lurches to what had been the leeward side
due to the steering bias. When the vehicle passes nearly instantly out of
the road-cut, back into the wind, the effect on the vehicle (and the
operator's perception), is exactly as if the wind had just changed 180
degrees.

In short, I just don't believe there is any rotational component to the
buffeting drivers experience when there is no visible sign of same - or,
indeed, absent a tornado or hurricane.

BTW - over here we pretty much only use "knots" only for marine wind- or
boat-speeds, or currents. Gusts to "28 knots" (around 32 miles per hour),
would not be sufficient for our Highway Patrol to issue a "High-Profile
Vehicle Wind Warning."

Michael Williams
Arroyo Grande, CA U.S.


.

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