dust devil air flow

upisaida · Post by **upisaida** » Tue Oct 12, 2004 11:38 am

If you are lying down on the playa, do the dust devils turn all clockwise, all counterclockwise or both?

Bob · Post by **Bob** » Tue Oct 12, 2004 11:47 am

Depends on whether you're face-up or -down.

upisaida · Post by **upisaida** » Tue Oct 12, 2004 11:56 am

Good point! I was going to include "looking at the sky," but then assumed one would have to have been doing that in order to know the answer. As it is, a quick google rendered the answer as being clockwise in the northern hemisphere as a general rule.

geekster · Post by **geekster** » Tue Oct 12, 2004 12:55 pm

If you are looking down on a dust devil from above, it rotates counter-clockwise.

upisaida · Post by **upisaida** » Tue Oct 12, 2004 1:14 pm

Hey geekster, thanks. Have you noticed that for a number of dust devils on the playa? I have been reading that, generally speaking, dust devil vortex spin are clockwise or counterclockwise with equal frequencies based on circumstances around it, and that the Coriolis effect does not contribute. In the case of Black Rock desert, I wonder if on a still day the conditions are so even that Coriolis forces can come into play?

geekster · Post by **geekster** » Tue Oct 12, 2004 1:28 pm

that COULD be ... but all of the ones I saw were rotating in the "proper" direction. They are created by an updraft of air ... and if the Coriolis effect does not contribute they wouldn't spin at all. It is the same as with a waterspout or a tornado or even a low pressure system ... rising air (low pressure) spins counter-clockwise, falling air (high pressure) spins clockwise.

Take a look at some movies of the Temple or Man burns at the vortices coming off it.

Bob · Post by **Bob** » Tue Oct 12, 2004 2:49 pm

If you can believe a former Penn State professor of meteorology, and the US National Weather Service, while the earth's Coriolis force has an effect at the scale of hurricanes and large storms at the middle latitudes, causing clockwise rotation below the equator and counter-clockwise rotation above, it is usually irrelevant at small scales where other factors govern -- such as w/ toilets, naturally curly hair, the way a dog lies down, the growth of kudzu vines, etc.

http://www.ems.psu.edu/~fraser/Bad/BadCoriolis.html

http://www.education.noaa.gov/kids2000/NWS/question.htm

Re: videos of dust or fire whirls spilling off large burns on the playa, the rotation would likely depend on which downwind side the observer is standing.

geekster · Post by **geekster** » Tue Oct 12, 2004 2:55 pm

No matter where you stand looking at it the dust or smoke on the side of the vortex facing you should be moving from your left to your right.

geekster · Post by **geekster** » Tue Oct 12, 2004 2:56 pm

ANd while what you say is true for toilet bowls, again, I never saw a single reverse rotating dust devil or smoke devil and I actually paid attention. There's plenty of video of them, find one and look.

Bob · Post by **Bob** » Tue Oct 12, 2004 2:57 pm

Cites? Keep in mind most images are taken from the Center Camp side, with the wind blowing generally left-to-right.

Bob · Post by **Bob** » Tue Oct 12, 2004 3:01 pm

The NWS says tornadoes may spin in either direction, btw.

upisaida · Post by **upisaida** » Tue Oct 12, 2004 3:03 pm

From the noaa site. "Do tornados spin clockwise? Tornados can spin clockwise or counter-clockwise."

I believe geekster, although I am surprised that smoke or fire devils don't spin either way randomly seeing as the fire should be generating a lot more directional force than the Coriolis effect. With the dust devils that occur on still afternoons, I wouldn't surprised if they swing Coriolis-compatible as the playa is so damn flat and featureless.

Then again, maybe there is something about the campsite position and the wind direction that causes this.

geekster · Post by **geekster** » Tue Oct 12, 2004 3:37 pm

I don't deny that it isn't impossible ... but I have never seen one or heard of one actually doing it.

http://www.stormvideo.com/tornado.html

all of these spin counter-clockwise.

http://www.gc.maricopa.edu/earthsci/ima ... devils.htm is a pretty good article on dust devils too.

geekster · Post by **geekster** » Tue Oct 12, 2004 3:50 pm

Actually, I will agree with this article:

http://www.weatherwise.org/qr/qry.02coriolistorn.html

I have just never noticed a clockwise dust devil.

geekster · Post by **geekster** » Tue Oct 12, 2004 4:16 pm

Check this out ... dont worry about no speaking the lingo ... just watch.

http://www.freerepublic.com/^http://www ... chtig.html

theCryptofishist · Post by **theCryptofishist** » Wed Oct 13, 2004 8:35 am

Bob wrote: it is usually irrelevant at small scales where other factors govern -- such as w/ toilets, naturally curly hair, the way a dog lies down, the growth of kudzu vines, etc.

You mean the Simpsons lied to me?

theCryptofishist · Post by **theCryptofishist** » Wed Oct 13, 2004 8:38 am

Bob wrote:Cites? Keep in mind most images are taken from the Center Camp side, with the wind blowing generally left-to-right.

That shouldn't make a difference. When you rotate around the center of tthe dustdevil (or view it from the other side) it should have the same rotation. Draw a spiral. Then turn your paper 180 degrees. Should have the same rotation.

geekster · Post by **geekster** » Wed Oct 13, 2004 9:25 am

That is why I was careful to say "the side of the vortex that is facing you". If you draw a circle and put arrows pointing counter clockwise around it, the arrows on the side of the circle facing you always point to the right. Rotate the circle all you want and the arrows still point to the right. Move the other side of the paper ... still point to the right. I will make sure to let everyone know when I see a clockwise dust devil cause I am seeking them out now.

Sign me,

DustDevil Collector

Bob · Post by **Bob** » Wed Oct 13, 2004 10:13 am

Larger scale, but this shows vortices eddying off on either leeward side of a mile-high island in the southern Pacific as a cloud mass moves by.

http://earthobservatory.nasa.gov/Newsro ... mg_id=9772