Monday, August 11, 2008

Willy willy

Jen recently posted an article about a willy willy (dust devil) chaser in the USA. While this seems to me sort of like chasing locusts in a plague, I figured it was as good a time as any to ask questions about these phenomena, as despite dodging them all summer (they throw sticks and gravel, demolish tents, and of course completely rearrange anything not tied down in camp), I don't know heaps about them.

What I really want to know is the origin of the inner white vortex, visible in the foreground willy willy above (click to enlarge). I was wondering if it was actually cloud- condensed water vapor- as a result of adiabatic pressure drop and cooling in the center of the vortex. Not every willy willy has them- most just look the color of the dust (in Australia, red). But these white cored ones were fairly common in summer, when the sun was stronger and the air was more humid. Any budding meteorologists out there?

Note: These were from the same trip, but a different day, as the ones shown here.

6 comments:

dmosher said...

Good question Chuck... my guess is that, like a centrifuge, larger/heavier dust grains are thrown to the outer edge while finer/lighter grains hang out near the center.

Just a guess, of course.

Chuck said...

That shouldn't give a sharp contact, it should give a gradation- and there isn't much white dust out there...

Lockwood said...

I should have come over and read this first. I just left a fairly lengthy comment at Looking for Detatchment that I think addresses the color part of your question.

Regarding the boundary between dusty air and clean air (it's dust, not condensation in every case I've ever seen or heard of), as the air spirls inward, it accelerates, (just as spinning skaters drawing in their arms "spin up"). When the air reaches the speed at which it can suspend the underlying sediment/soil, it does so. If the dust is pretty well sorted, this can be a pretty quick transition from clean to dusty air. Since the entire column is being drawn up, the dusty air doesn't have much chance to mix with the clean air before it's up and out of sight. Hope this helps.

Silver Fox said...

Chuck, I didn't find a real definitive answer to your white v. red dust color question, but in the process found some interesting links and posted them.

I'm wondering if it isn't the amount of very fine particles in the center - if they are close enough together, they could reflect (or refract?) all light, making the color look white. Kind of like "high order" white in calcite under the scope. Similarly, very find glacial dust makes water look white to turquoise blue because of the way it refracts light.

In the Intermountain West, we mostly have dust devils on white/beige playas, but down in the 4 Corners area, they must have some on red ground from the red Permian to Triassic seds. I couldn't find any pictures, however!

Jennifer Ouellette said...

love the photos. We don't get many dust devils in downtown LA, and now I'm kinda sorry about that. :)

Other Chucky said...

There is a lot of kaolinite clay in the soils of the NT, and this is combined with a butt-load of hematite and other Fe-oxides and oxyhydroxides coating fine to very fine sand and coarse silt grains. Could the separation be a result of the gradation induced by a centrifuge-type process? It would be interesting to track the presence and absence of the inner white bit over differing substrates.