I am a belgian paraglider and I sometimes worry about meeting a dust devil
at low altitude.
We indeed had a recent accident in our club, because of a dust devil.
I have observed them recently over a dusty field at one of my favourite
flying sites, close to Lille in France.
And somehow I have the impression that they are linked to a dusty dry
environment...
But is this correct or is it just because we can see them so good because of
the whirling dust ?
Should we also expect them over a very green and humid meadow along a river
?
And are there certains weather conditions to look out for : shortly before a
thunderstorm ?
thanks for any reaction.

On Sat, 30 Aug 2003 13:11:54 +0200, "A Jozef" jozefleon(remove anti
wrote:

I am a belgian paraglider and I sometimes worry about meeting a dust devil
at low altitude.
We indeed had a recent accident in our club, because of a dust devil.
I have observed them recently over a dusty field at one of my favourite
flying sites, close to Lille in France.
And somehow I have the impression that they are linked to a dusty dry
environment...
But is this correct or is it just because we can see them so good because of
the whirling dust ?
Should we also expect them over a very green and humid meadow along a river
?
And are there certains weather conditions to look out for : shortly before a
thunderstorm ?
thanks for any reaction.


Dry, little or no cloud cover, and, typically, weak surface winds.
Solar heating warms the ground creating a superadiabatic layer at the
ground (mechanically unstable vertical temperature gradient) and the
lack of wind limits mixing. The vertical motion associated with the
dust devil is part of the process that mixes out the hot surface air.

My experience chasing them in Illinois (very flat countryside with, at
the time, plowed fields with crops not up yet), that I think agrees
for the most part with the literature, is that typically started
around local solar noon (maximum heating?) and continued until
relatively late in the afternoon. It seemed that there would be
temporary hot spots, where there would be repeated formation in about
the same location for several cycles of the process. As the afternoon
wore on, the frequency of occurrence first increased, with mostly
small dust devils, and then, later, frequency would decrease, but the
ones that formed were larger and lasted longer. We followed one for
~3 km before it moved into a wooded, grassy area. Whether it was the
lack of tracers to make it visible (not much dirt in that area) or the
circulation had died, I don't know.

Harold
--
Harold Brooks
NOAA/National Severe Storms Laboratory
Norman, OK

thanks for your valued comments.
I'll spread the word between the pilots of our club.

"Harold Brooks" schreef in bericht
...
On Sat, 30 Aug 2003 13:11:54 +0200, "A Jozef" jozefleon(remove anti
wrote:

I am a belgian paraglider and I sometimes worry about meeting a dust
devil
at low altitude.
We indeed had a recent accident in our club, because of a dust devil.
I have observed them recently over a dusty field at one of my favourite
flying sites, close to Lille in France.
And somehow I have the impression that they are linked to a dusty dry
environment...
But is this correct or is it just because we can see them so good because
of
the whirling dust ?
Should we also expect them over a very green and humid meadow along a
river
?
And are there certains weather conditions to look out for : shortly
before a
thunderstorm ?
thanks for any reaction.


Dry, little or no cloud cover, and, typically, weak surface winds.
Solar heating warms the ground creating a superadiabatic layer at the
ground (mechanically unstable vertical temperature gradient) and the
lack of wind limits mixing. The vertical motion associated with the
dust devil is part of the process that mixes out the hot surface air.

My experience chasing them in Illinois (very flat countryside with, at
the time, plowed fields with crops not up yet), that I think agrees
for the most part with the literature, is that typically started
around local solar noon (maximum heating?) and continued until
relatively late in the afternoon. It seemed that there would be
temporary hot spots, where there would be repeated formation in about
the same location for several cycles of the process. As the afternoon
wore on, the frequency of occurrence first increased, with mostly
small dust devils, and then, later, frequency would decrease, but the
ones that formed were larger and lasted longer. We followed one for
~3 km before it moved into a wooded, grassy area. Whether it was the
lack of tracers to make it visible (not much dirt in that area) or the
circulation had died, I don't know.

Harold
--
Harold Brooks
NOAA/National Severe Storms Laboratory
Norman, OK

I recently completed a short video showing some dust devils in action as I
chase them. You can view it at:

http://www.animalu.com/pics/photos.htm

And just click on dust devils.

This footage was taken at El Mirage Dry Lake in Southern California.

Best,

Jeff T. Alu




"Harold Brooks" wrote in message
...
On Sat, 30 Aug 2003 13:11:54 +0200, "A Jozef" jozefleon(remove anti
wrote:

I am a belgian paraglider and I sometimes worry about meeting a dust
devil
at low altitude.
We indeed had a recent accident in our club, because of a dust devil.
I have observed them recently over a dusty field at one of my favourite
flying sites, close to Lille in France.
And somehow I have the impression that they are linked to a dusty dry
environment...
But is this correct or is it just because we can see them so good because
of
the whirling dust ?
Should we also expect them over a very green and humid meadow along a
river
?
And are there certains weather conditions to look out for : shortly
before a
thunderstorm ?
thanks for any reaction.


Dry, little or no cloud cover, and, typically, weak surface winds.
Solar heating warms the ground creating a superadiabatic layer at the
ground (mechanically unstable vertical temperature gradient) and the
lack of wind limits mixing. The vertical motion associated with the
dust devil is part of the process that mixes out the hot surface air.

My experience chasing them in Illinois (very flat countryside with, at
the time, plowed fields with crops not up yet), that I think agrees
for the most part with the literature, is that typically started
around local solar noon (maximum heating?) and continued until
relatively late in the afternoon. It seemed that there would be
temporary hot spots, where there would be repeated formation in about
the same location for several cycles of the process. As the afternoon
wore on, the frequency of occurrence first increased, with mostly
small dust devils, and then, later, frequency would decrease, but the
ones that formed were larger and lasted longer. We followed one for
~3 km before it moved into a wooded, grassy area. Whether it was the
lack of tracers to make it visible (not much dirt in that area) or the
circulation had died, I don't know.

Harold
--
Harold Brooks
NOAA/National Severe Storms Laboratory
Norman, OK