Watching Thunderstorms Chase The Hot Spots

By Willis Eschenbach- Re-Blogged From

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.

dust devil on highway.png

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 CloudsI 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!

Cloud Tops and Temperature all.gif

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.



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