Re: Energy Vortex Discovered on the Hayward Fault


"Mike Williams" <miklwlms@xxxxxxxxxxx> wrote in message
news:v9gtj.10850$Ch6.3158@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx

"Aidan Karley" <name1_name2@xxxxxxxxxxxxxxxxxxxxxx> wrote in message
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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.


Where I wrote: ". . . the vehicle suddenly lurches to what had been the
leeward side
due to the steering bias", I meant "windward" rather than "leeward."

Mike Williams
Arroyo Grande, CA U.S.


.

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