Dust devil in the Salt Wells Basin (Eightmile Flat) in April 2008Chuck at Lounge of the Lab Lemming recently posted about a dust devil (AKA willy willy in Australia or Chindii in Navajo land), wondering about the formation of the inner core of a dust devil. I don't know the answer to his specific question - about why an inner core should look white when the dust it's picking up is red - but I did find a few links about our ever-present desert phenomenon. It may be that this is a research area worthy of further work, as much current research seems to be directed towards tornados. Fairly recent discovery of dust devils on Mars might spur more research on this planet, where we can actually examine these devils or spirits of the desert. Dust devils on Mars were first observed in the late 1970's and early 1980's during the Viking program and were (probably) first encountered on the ground during the 1997 and 1998 Pathfinder missions.
For the record, although most dust devils form on relatively calm, clear days, they also form underneath cumulus clouds that mark thermal uplift below the clouds. I personally observed one in west-central Nevada in the late 1980's that had formed on a playa from the ground up, in the usual fashion. A large, dark cumulus cloud sat overhead, and gradually a small funnel dropped from the cloud. The funnel cloud and dust devil briefly hooked together, forming what was technically, I suppose, a small tornado. I unfortunately no longer have a photo of that unusual event.
A blowing-apart dust devil in Salt Wells Basin (near Fourmile Flat) in June 2008
A few dust devil (and tornado) links:
- Dust devils at the Alaska Science Forum.
- Dust devils in Flagstaff at NOAA.
- Dust devils of Maricopa County from Glendale Community College.
- Dust devils and other atmospheric circulation phenomena from the Planetary Environment Research Laboratory.
- Tornado facts, including info on dust devils and whirlwinds from TORRO.
- Highly technical stuff about Rankine vortexes.
- An abstract about dust devil simulations and pressure drops in the core.
- Ask Jack about dust devils and tornados at USA Today Weather.
- Dust devils at Wikipedia.
3 comments:
I suspect the reason a dust devil looks white (and I'm sort of guessing here) is due to several factors: 1) finely divided or powdered material will tend to look light in color- this is particularly true when it's suspended in a fluid and dispersed; 2)material sitting on the ground (I presume from context) has more of same backing it up; the whole deposit has a similar color. When it gets lifted up by a dust devil, the different background colors can wash out the dust's intrinsic color; 3) individual particles will be better illuminated when suspended, thus apparently brightening them. The contrast with the material on the ground could make the suspended material look white.
Your post and in particular your comments regarding Martian dust devils are quite timely- there was an in Monday's Space Daily Newsletter (which arrives in this hemisphere early evening the day previous) on a NASA-funded research grant to investigate them. This should provide insight to Martian conditions, as well as earthly convective systems. It sounds like the real impetus is to gather enough fine-detail data to test and refine equations that should physically describe the creation and evolution of these systems.
Also, according to the article, "Dust devils look like miniature tornados, tossing up dust and debris, but the atmospheric conditions that cause them are very different. Dust devils are unrelated to storm systems. Instead, they form when ground temperatures are warmer than the surrounding air and moving air triggers a swirling column called a convective vortex."
Here in the Willamette Valley, we sometimes get dust devils, particularly over burnt or just-plowed fields. But they're pretty tame compared to those in the playas of Eastern Oregon and the rest of basin and range.
Lockwood, thanks for your comments.
As for the color, I'm thinking that the large concentration of very fine particles in the inner core may refract (or reflect?) light to make white light, no matter what the color of the particles are. That would be similar to the way glacial dust in water refracts light and makes the water a milky white to milky turquoise. And that would be similar to what you were saying (or the same?).
Dust devils are indeed different than tornados, although they can cause damage when very large.
Nice to hear from the Willamette Valley. I was born there!
I think we're more or less thinking along the same lines, though I hadn't considered the role of refraction. When I read the Lab Lemming post, the dust devil appeared to me to have a distinct pinkish cast- however my color perception is not the best (which made minerology a bitch). Still, there's no doubt that the dust plume does tend to be lighter than the ground.
I'm been in Corvallis since 1980, moved from Ohio. Where were you born?
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