Guest Post by Willis Eschenbach
Once on a lovely hot August day in eastern Oregon, my gorgeous ex-fiancee and I sat entranced and watched a parade of dust devils. I’ve written about dust devils before, they’re one of my favorite emergent phenomena.
Like many such emergent climate phenomena, dust devils are driven by a temperature difference between the surface and the surrounding atmosphere. Once that temperature difference (called “delta-T”) is exceeded, dust devils form spontaneously.
The relevant climate effect of the dust devils is to move heat aloft from the surface, and thus cooling the surface. And since they are temperature-driven, dust devils form first and preferentially on the hottest part of the local surface. From there, they are generally moved away from that hot spot by the wind, and eventually die away and disperse entirely.
In this case, the hottest spot in the local area was a plowed field in a large area of small scattered bushes near the road. When we saw the dust devil form over the field we stopped in awe. It was big, lifting clouds of dust from the fertile ground. Then the wind moved it off of the hot spot, parallel to the road. Once it left the plowed field, it was throwing up less dust. But to replace it the dust devil tossed leaves and twigs and pebbles.
So the dust devil drifted away with the wind … and just when we were getting ready to start the car and leave we looked, and another one was forming over the plowed field. Out of nothing it spun into reality, growing, growling, whistling, kicking up dust … and then, once it was towering high above us, it followed its now-distant twin out of the plowed field and along the road into the wider world.
And then, of course, another dust devil formed over the plowed field … and for a half hour, it was groundhog day—over and over again, dust devils forming, strengthening, leaving home for their big adventure down the highway, and eventually dying out. I didn’t take any photos, but here’s a shot from the web that looks like what we saw.
I bring this up to highlight the fact that this system of cooling the surface with emergent climate phenomena is extremely effective because it is concentrated on the warmest spots. These are the very spots that are in greatest need of cooling, as well as being the spots where cooling will have the largest effect. The dust devils formed over and over in that same hot spot, and nowhere else in the surrounding fields.
With that as prologue, let me say that one of the reasons I greatly enjoy writing for the web is the immediate feedback that I get. The most valuable feedback is when someone points out something that I’ve done wrong. I can’t begin to calculate how many years of wasted effort I’ve been saved by someone saying “Um, dude … did you notice that you totally went off the rails right here?” … or more likely, something equally correct but much less pleasant. Fortunately, even unpleasant corrections are gold, they keep me out of blind alleys.
The next most valuable kinds of feedback are suggestions and support for further investigations, and in that regard comes this post. In my last post, Glimpsed Through The Clouds, I put up a movie I made of the seasonal changes in the relationship between sea surface temperature and thunderstorms. Upon viewing it, a commenter wrote:
it would be interesting to see if this effect persists seasonally and if it varies through el nino/la nina periods. prevailing wind direction and ocean/ land temp differences will be factors.
great work once again willis .
So this is my entry in the temperature/thunderstorm Nino/Nina derby. The Nino/Nina gauge under the globe shows the value of the MEI, the Multivariate ENSO Index, for that month. The colors show the height of the clouds, with the high clouds being thunderstorms and the highest thunderstorms being the most powerful and having the greatest cooling effect. The black outlines, on the other hand, show sea surface temperatures of 27°, 28°, and 29°C. Note how the thunderstorms line up very neatly with the temperatures. Note also the steepness of the increase in thunderstorm strength with increasing temperature, as shown in Figure 6 in my previous post, Glimpsed Through The Clouds. The temperature varies by a mere couple of degrees, and in response, the number and strength of the thunderstorms vary from none all the way up to YIKES!
Not much to say except to marvel at how the thunderstorms constantly move so that they are exactly where they need to be in order to have the maximum cooling effect … what an amazing planet this is.
w.
MY POLITE REQUEST: Misunderstandings are the bane of the web. To avoid them, please QUOTE THE EXACT WORDS THAT YOU ARE DISCUSSING so we can all be clear about what you are referring to. Please note that although the request is polite, if you do not follow the request my response may indeed be less than polite …
I looked at great lakes average surface temperatures, and lake Erie, the warmest of them all, rarely makes it above 26C. It bumps into it some years, it’s like a wall. Does anyone around lake Erie notice a change in the weather when the surface temperature gets to 26C?
https://coastwatch.glerl.noaa.gov/statistic/avg-sst.php?lk=g&yr=2011
You know that famous butterfly in Brazil? It’s not just one in Brazil, they are everywhere, and it’s not butterflies exactly, it’s emergent weather phenomena, such as dust devils, which arise when conditions are right, and sometimes lead to bigger things, when conditions are right. The models could predict the right conditions, but they can’t predict the trigger or spark that sets something off because that happens on too small a scale. For example, models can provide tornado warnings and tornado alerts, but they cannot predict exactly where or when there will be a tornado, or even if there will be one. Especially far in advance. Weather models only predict the near future, and the best weather models have rapid refresh. The point is, some things are deterministic, some are probabilistic.
On the other hand, what Willis is doing is most important, trying to understand how the systems work and how multiple systems work together. Tropical thunderstorms, ITCZ, Hadley Cell, El Niño, teleconnections, jet streams, meridional circulation. The Cloud Top animation above is great!
gareth and marque2
You are both correct; after genesis, all thermals travel downwind cross country, embedded in the surrounding airmass, and at a fair fraction of the wind speed.
They travel in straight lines, or large radius arcs, tracking with the local wind. In the case of the Cb, which has a more complex structure, and where there is significant wind shear with altitude, it has been suggested they take their direction of travel from around the 760 milliBar level winds.
Straight line behaviour is evidenced by watching thunderstorm tracks on weather radar. Also, as an aside, thermals containing flocks of birds, feeding on entrained insects, show up on airport radar !
There is an energy level spectrum, ranging from the tiny eddy at the street corner that whirls dust into the eyes, up to the giant thunderstorm (Cb). Willy willies or dust devils are part of this continuum, needing only high Delta T’s and the presence of available dust particles to make them visible. Movement of strong thermals over vegetated areas is marked by the thrashing around of grass or the foliage in the canopy. Sound is also a cue. From altitude, as well as grass movement, one can often remark the passage of a thermal across a forest canopy by a colour change, when the underside of the leaves is revealed.
If the day is good, there are many “strong thermals” about that can be observed, and give the glider pilot 1,000 ft/min climbs in mid air.
The question of trigger points, genesis, over development, recycling and decay is too complex to talk about here. Needs a “Library Search”, if one wants to know !
However, there is one fact not generally known, that may be of interest. Genetic material can be entrained from a paddock, and not just dumped over the fence on the neighbour’s crop, but transported tens or hundreds of miles. Makes a mockery of attempts to quarantine GM product !