Dust Devil Vs Land Spout Vs Tornado - Arizona Structure

[Jim Saueressig]

So I got my So Cal vacation out of the way and on the way home I got to see some nice looking structure in AZ. After joking in one of the SDS threads that I would just go to So cal and shoot dust devils after another member suggested the idea, it turns out I caught one in Arizona that left me with a question.

I had just re-entered the interstate when I noticed a really nice dust devil with a tube that pretty much appeared to be rooted into the clouds. By the time I grabbed the camera and had the wife shoot it while heading to the next off ramp it was losing the upper funnel definition. She shot this (it was on manual focus so it was out a bit):

View attachment 5235978d27e9c2fc936f025866087f47.jpg

By the time I got stopped to shoot it was losing more of its upper body so I shot a wide angle to get the structure above it:



(By the way I love how the cloud bases take on the reddish color of the landscape).

So this question crossed my mind about whether it was a dust devil or a land spout considering it appeared to be attached to the cloud base when I noticed it. Then I realized that with it being under the cloud structure it might be called a tornado by some. It was then I considered that it was actually under the clouds updraft. The cell was a NNW mover and a few minutes after this was shot I noticed the dirt and dust to the right of this in a massive area get lifted just as a wall of rain started falling (microburst?)

Anyway it brought the question to mind about what separates a dust devil from a land spout etc...

Anyways a few other pictures while I am boring you to death.

More from that day:









Lego Land California (yes, Three kids):

 

[Skip Talbot]

Very nice pics, Jim. Love the spout.

As far as your question about differentiating spouts from dust devils, this is how I understand it:

Dust devils are created when the the ground is heated up to a temperature much higher than the air a few meters above it. This creates a shallow layer of very unstable air. As these small air masses converge, this unstable air rushes up, and dust devils spin up as a result. Dust devils are not tornadoes as they are not associated with a storm's (or tcu's) updraft.

Landspouts are created when a cumulus' updraft stretches existing vorticity in the environment. Landspouts are tornadoes, although non-supercellular tornadoes, as they meet the definition and are more than capable of doing damage. The same mechanism also creates waterspouts, so I simply refer to them both as spouts.

 

[Jim Saueressig]

Thanks Skip, that must be an acceptable answer since there was no argument about your definition. LOL

Very nice pics, Jim. Love the spout.

As far as your question about differentiating spouts from dust devils, this is how I understand it:

Dust devils are created when the the ground is heated up to a temperature much higher than the air a few meters above it. This creates a shallow layer of very unstable air. As these small air masses converge, this unstable air rushes up, and dust devils spin up as a result. Dust devils are not tornadoes as they are not associated with a storm's (or tcu's) updraft.

Landspouts are created when a cumulus' updraft stretches existing vorticity in the environment. Landspouts are tornadoes, although non-supercellular tornadoes, as they meet the definition and are more than capable of doing damage. The same mechanism also creates waterspouts, so I simply refer to them both as spouts.

 

[Joe beier]

Very nice pics and good explination skip

 

[David Wolfson]

As Skip says, true dust devils are fueled by thermodynamics very near the ground. My long observation is that, while they're initiated by a rising thermal bubble or transient turbulence, they're perpetuated by turbulent extraction of heat from ground material. The vorticity is concentrated due to cooler air subsidence/inflow. True dust devils frequent conditions of high sun and LCLs.

Now as Doswell says, every vortex must either be grounded or connect back to itself. Since every rising parcel is reflected in some vorticity at ground level, the key is in mechanisms which concentrate the vorticity enough to be visible (in the case of "gust" devils) or to meet a ground-level velocity/damage threshold (in the case of tornadoes and spouts).

I'm pretty sure what Jim shows isn't a dust devil. It appears to be on an upflow front and to have been shaded from direct sun for some time. I-40 east of Flagstaff traverses high plateau country over 6000' where low-level lapse rates are enhanced. My observation is that these spouts are pretty common on the high plateaus with early-season, low-moisture, struggling showers. Two summers ago -- similar conditions -- approaching Chinle I watched one as it came off the hill to the west and right into town over the course of more than ten minutes, throwing crap around. Same sort of visible connection to cloud level. I know this because it ran right over me in a convenience store parking lot and I have several unremarkable slides of it throwing crap around.

As with mesocyclonic tornadoes the standard is whether the vortex is concentrated enough to have damaging EF0 velocities on the ground. Mine was a probably-not-spout as I experienced it. Jim's looks more robust and is a probably-spout IMO. Nice pictures, Jim!