I know that when a large amount of dust is suspended in air, the air behaves like a dense “fluid” which easily pours downhill. That’s how a pyroclastic surge operates: it’s basically a landslide.
Neatorama link to sand storm video (Nat Geo)
It occurred to me that maybe dust storms of the type shown in this video are another example of the same phenomenon: a landslide similar to a pyroclastic flow.
My question: are these mostly driven by weather, or are they only triggered by weather, yet actually driven by the transport of sand down a slight slope, from higher altitude to lower? In other words, do these dust storms require a very slight downslope? Or can they be formed on gradual inclines?
I searched around on a few research papers, but found nothing about “density currents” or mass transport. The papers do mention that the storms are only about a meter deep, with the rest of the visible cloud being fine silt stirred up by the violent sand flow.
An engineering fantasy: if the sand/air density currents really do play a critical role, then grab your bulldozer and build a very very long earth berm in their path. Or perhaps just erect a series of parallel snow-fences. Run fluid simulations to find out how tall/thick obstacles the density current would be unable to leap. (Ah, a handful of workers with the right bit of knowledge could have put a stop to the American “Dust Bowl?” Heh.)
I doubt that you could block sandstorms with a normal fence, footage of them I have seen online and on TV show storm fronts that are several hundred feet high in the air. It isn’t like they are pottering along at 5 or 6 feet or even 5 or 6 meters tall.
I think it would be a nice thing, if you could interdict them from sweeping along, but that would probably end up being sf type weather control
I don’t think so. It looks to me like it is just sand and wind. The wind is blowing whatever direction it blows, and it is taking the sand with it. I think the reason you see the “wall of sand” effect is because wind alone is not very good at picking up sand. Wind blown sand however, is very good at picking up sand.
I think wind and sand are really the key ingredients for a sandstorm. I could imagine a sandstorm caused by katabatic winds that would match your description fairly well, but it really wouldn’t be the sand “pouring” downhill, it would be the air being accelerated downhill by gravity and picking up sand. In other words the wind would be there regardless, and the sand is just along for the ride (maybe I’m splitting hairs there).
AFAIK these winds are generally caused by continental-scale geological features (the Antarctic and Greenland plateaus, various mountain ranges) that involve significant drops in elevation (hundreds or thousands of feet?). You’d need a pretty large earthworks to have an effect on them.
, so modifying them would probably require mountain-size earthworks.
That’s it! If the sandstorm energy source is really a density current in fluidized powder, and not from existing winds, then the sand cloud must exhibit avalanche features: the dense material should be sliding down the incline significantly faster than any surrounding air. That would make the patterns in the leading edge of the cloud flow upwards, as those dense lowest regions penetrate and out-race the air ahead. That’s just what we see in the two videos of an avalanche and a pyroclastic flow. And in that case, the lower part of a dust storm would always jut outwards from the advancing “wall,” with the upper regions of the cloud lagging behind. If these features are lacking, then there’s no ‘avalanche’ physics, and the haboob would still occur even without the sand.
Unzen .jp pyroclastic surge
avalanche
Drat, youtube has no footage of Haboobs which were filmed by fixed cameras. The time scale is too slow, so turbulent motions in the advancing cloud aren’t obvious in short clips. I want to download one of those, then speed it up 10x to see if the pattern in the dust is moving upwards like it does in an avalanche.
Iraq sandstorm slideshow
If it’s avalanche-like, then a fluidized air/sand mixture would be enormously denser than cool air could ever be, and the storm’s kilowattage proportionally higher. The sand would act as a “wind amplifier.” Hmm, I wonder if, in the US dustbowl era, were the winds enormously more intense than in years before and after? If they were, this “avalanche” concept provides a simple obvious explanation. People were living in miles-wide soil avalanches without realizing it.
Ah, but katabatic winds are just analogous. In an avalanche-style sandstorm, the fluidized sand cloud is the dense air flowing downhill (where fluidized sand is a substance distinct from sand or air alone.) Avalanche: the tons of suspended snow are on the move, and they take the air along for the ride. I find a couple of research papers that give the depth of sand storms as less than a meter, with the big impressive cloud just being fine powder stirred up. That again is identical to avalanches and pyroclastic flows, where a shallow fast flow is hidden within the slower moving cloud. If all this is true, then just cover the landscape with crops or close-spaced shrubbery, and the storms would vanish. But it would take quite a bit of snow fencing to knock all the suspended sand out of the air.
But I’m still not convinced. I guess I need to find a sandstorm text, and find out if this is common knowledge in sandstorm research.
Speaking as a life-long resident of Saskatchewan, I would say that if slight inlcines were necessary to the formation of dust storms, we wouldn’t have had a dustbowl in the 30s. We’re flat. And I recall a report that some topographers had done some computer simulations on the topography of Kansas, which also had dustbowl conditions in the 30s, and found that it literally is flatter than a pancake (adjusted to scale).
During the 30s, the Prairie Farm Rehabilitation Agency encouraged the building of wind breaks on the Canadian prairies: planting trees, building ditches and berms. The windbreaks modified the dust storms locally, and reduced the effects of the wind erosion on the topsoil, but they certainly didn’t stop the dust storms.
Anyone interested in a historical treatment of the American Dustbowl could do worse than Tim Egan:
It is a hard read due to the misery all the real characters are living through.
ETA: You can get a bit of an idea what a dustbowl storm was like just from the reviews. One thing not mentioned above is that there was intense static discharges associated with the dust storms.
If the landscape is flat, then there is no “downhill” and streams and rivers become impossible. But if rainwater does move in a particular direction, then a pool of fluidized sand should also go that way. (So, do sandstorms’ directions tend to be set by the weather and remain independent of the average watershed flow direction?)
Yeah, I was inspired to consider dustbowl dynamics while involved with ‘Black Blizzard,’ the History Channel’s show about Egan’s book. It hit me that maybe the storms are a runaway process; a positive feedback phenomenon where sand creates wind and wind lofts the sand. But sandstorm research papers aren’t broad enough to cover basic explanations.
See my bit at the 4:00 minute mark on Black Blizzard pt 5. Similar phenomeona are reported during snow blizzards, and while sandblasting.
