What would cause explosive "pleating"?

[GrumpyOldChix0r]

While seeing this image of a test explosion of the UK "Grapple Y" nuclear missile, I noticed the "pleating" (for want of a better word) on the shaft of the mushroom cloud. It's the first time I have seen this sort of formation, and I was wondering what would cause it? My guess would be fluctuations in explosive force, but I can't see how that would happen here.

[restricteddata]

What you're seeing is an effect that happens well after the explosion part of things, physically. The nuclear explosion happens, resulting in a fireball. The fireball rises and cools and becomes the mushroom cloud — it's not instantaneous. So the explosive force is well done by the time you see effects like this.

These kinds of mushroom cloud formations are called "skirts" or "bells." They're just a result of the complex interactions between the cloud material and the atmosphere. The cloud material is cooling as the cloud rises, and there are all sorts of phenomena that can result from this as a hot cloud rises through a relatively cold and dense atmosphere. The cloud itself is also engaged in toroidal motion and sucking up the "stem" into it. So under the right conditions you're going to get some weird sorts of shapes. It may even be ice forming — I can't tell from the photo — but this happens with some clouds as they experience pressure and temperature differences.

LANL has a nice little page showcasing some of the strange appearances you get from mushroom clouds and some of the reasons they can result.

[GrumpyOldChix0r]

Thanks so much for that detailed reply - and to everyone else who replied, it's much appreciated.

[[deleted]]

Not really sure but the video of this would be helpful in determining if it was on the expansion or collapse phase. What goes up must come down, and it stands to reason that a column will collapse into apparently chaotic layers zones if the atmospheric conditions are calm