# Does anyone know the mechanism behind this double helix cloud formation?

This looks like m=2 swirling instability mode of the axisymmetric jet, but how could an axisymmetric jet form up in the sky like that?

• Could you be looking at the core of the jet stream? Where was this picture taken? – Floris Nov 8 '14 at 4:15
• @Floris, No idea where it was taken. I guess it could be the core of a jet stream, but aren't those supposed to be over a mile thick en.wikipedia.org/wiki/Jet_stream. – mdornfe1 Nov 8 '14 at 4:53
• It is difficult to judge the height at which this phenomenon occured. From the article mdornfe1 mentioned, I would guess that some obstacle in the air can trigger this slowly swirling motion. I must admit that I didn't read the article carefully enough, but the Reynolds numbers and stability mentioned there indicate that a scaling from wing tip swirls to some 100 m could really work. Maybe big platforms of television towers (imagine the bis ball of the berlin tower) can induce this double helix in just the right amount of wind speed and atmospheric stability. – Aziraphale Dec 2 '14 at 9:45

Helix clouds are quite uncommon - a Google search reveals only a few, oft repeated, images. There are several short discussions about them on various forums, the longest of which glances over a few interesting ideas including an attempt to quantify the evolution of a well-documented Muscovian example with time.

It isn't a jet stream core - they are too small (and found in areas where the jet stream isn't). You need something capable of generating a long thin cloud, and something to spin it. The first part is easy: a contrail being the prime example (as well as several other common cloud types). The second part requires some phenomenon to generate a twisting cylinder of air, and there is one! Strong localised vertical shear generates horizontal vortices that tend to twist over their length.

That covers a single helix cloud - but what about the double helices? Well, contrails from larger airplanes form a fine example of two distinct lines of cloud, which, under the scenario described in the previous paragraph would wrap around each other to form such a shape.

Example over Mauna Kea, generated from ambient cirrostratus.
(source: ethantweedie.com)

Example over Moscow, almost certainly generated from a fleeting contrail.

• Your over their length link is broken. – Ruslan May 29 '17 at 13:30

## protected by Qmechanic♦Jan 2 '17 at 10:49

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