DUST DEVIL 102

Discussing dust devils last week, we tried to stuff ten pounds of BS into a five pound bag, and sure enough some spilled over. Fortunately we had another bag handy, just the right size for what didn’t fit last time. Ready for the rest?

As always, the more you learn the more you know you don’t know. Technical descriptions of a ‘proper’ dust devil require certain specifics not always present in everyday garden variety whirlygusts. You may be surprised to learn that heat, per se, is not a critical element. As with convection more generally, what matters is temperature contrast, between a relatively warmer surface and cooler air above (scorching playa, alpine snowfield, even open water). Two other ‘ideal’ conditions are a uniformly flat surface and, odd as it sounds, light- to no ambient wind… The most magnificent examples we see in pictures represent these optimal ingredients, approaching helical symmetry because the physics in each quadrant are essentially identical. Such specimens are comparatively rare in most landscapes, though, and far from the only possible recipe.

Dust devils in mountains are even more rare, yet I remember sighting down the core of one as it tortured a high rocky peak, while climbing in the very best lift of an eight hour flight! And then there’s the influence of vegetation… in seemingly any landscape or season, warm air trapped between trees may spawn a day’s last lift, and frigid air above snowy hills can produce effects as spectacular as any.

https://www.youtube.com/watch?v=fb2MCnoW814&t=18s

Surface wind will pull a devil along like a cranky child through a supermarket, bumping into pretty much everything because it moves faster on one side than the other, endowing each quadrant with different character. Relative to the ground, one side will be moving at the speed of rotation plus the wind, while on the other side those two vectors cancel each other and horizontal movement is momentarily stopped. That’s the spot where you’ll see all kinds of stuff shooting straight up off the ground! The result is a discrete vertical wall on one side, and on the other a mess of sand and debris cast out before ever rising far.

Nearly all devils exhibit varying degrees of split personality, their sources ranging from 90% heat to 90% wind, encompassing a majority of 55/45 like any population. Sonoran monsters that stand straight like redwoods and hardly seem to move are mostly about heat, while the thousands of little catfights scuttling through hills are mostly about wind. Neither extreme offers much of practical use to soaring pilots; as with most things, in between is where they keep the good stuff. So much to learn!

One helpful trick is distinguishing weak devils who raise more dust than they know what to do with, from boomers showing almost none at all. Spindly tall and pencil thin may not look impressive, but could be your ticket to ride. A pile of murk wider than it is high is an invitation to land in a pile of murk wider than it is high… Devil don’t care.

And yes there really is a conceptual difference between dust devils and tornadoes, beyond their size and ferocity. Tornadoes are always suspended from big dangerous clouds, while devils come up from below and favor more benign weather. Still, exceptions abound for nearly everything. Could the example linked below be a rare hybrid of both? Twenty seconds in, you see only the devil on the ground and beginnings of an umbilical beneath a cloud far away in the upper left. No obvious connection — yet at that moment the clear air between them is accelerating in a huge diagonal spiral. Just before the video’s end (at exactly 2:30), the rope-like axis is bent over to nearly horizontal by wind aloft, and visibly connects both ends. Try to envision the air’s movement around that oblique axis! Assuming counterclockwise rotation, the near side would rise faster and faster as you approach the core, and on the far side catastrophic sink.

https://www.youtube.com/watch?v=SsDua8tSXBU

Until we meet again, here’s one to inspire ghastly nightmares, if you’re into that sorta thing. Imagine being a lunatic in a glider flying a miles-wide circle around some standard issue (vertical) tornado. The closer you cut in, the faster you’ll have to go to stay beneath a low, leaden ceiling. If you weren’t nuts you’d head straight away at absolutely top speed from what amounts to an atmospheric black hole, but being crazy, you can’t resist…

How should we suppose this adventure might end?