I'm not sure I like the airplane theory because - as you say - it would have to cut the cloud like one of those photos you see of a bullet cutting a playing card. This isn't impossible, I suppose, but it just seems wrong to me.

How about another possibility: the cloud is just in the process of being cut in half by a rising convection current? It's sketchy, but it's a theory <grin>

What ever caused it, it is a beautiful picture. Thanks for sharing.

It also is a beautiful and intriguing effect. My first impression was that it had to be two clouds that had met, but closer examination shows that idea to be nonsensical.

For one thing, the structure of the cloud in the two separated parts match very well.

Secondly the structure of the cloud on either side of the divide matches very well. It is patently a cut cloud, not a joined cloud.

Thirdly, the structure in the channel and its boundaries is quite consistent with an aircraft, either with in or just above the layer of cloud, displacing the cloudy material with air from above, and shoving it sideways. You will notice that the edges of the channel are largely more densely cloudy than the air on either side.

Of course, if convection caused the split, then it would have to be downward convection, not upward, since upward convection should have created more cloud, not less.

The suggestion that the effect is too much of a coincidence (like a bullet cutting a card) I reject. Firstly, we do get coincidences, and this one is at least enough of a coincidence for me never to have seen the like before. That does not prove anything of course, but...

What makes it less of a coincidence then it might at first seem, is the fact that the cloud patently is a layer between two fairly calm layers of air, and as such is pretty flat and horizontal. Only grant that the aircraft, very possibly on autopilot, happened to coincide in altitude with anyone part of the cloud, and it becomes entirely plausible that the bullet would continue to cut the card all the way across.

Well anyway, I have no better suggestion.

I'm staring at the photograph and chuckling to myself, this is a nice little puzzle! Jon is right about the convection (he grudgingly concedes), but if it was an aircraft how come it hasn't left its own vapour trail? Especially in the left of the picture.

Here's another idea: it looks like a tear, so how about an expanding or moving layer of air at lower altitude has heaved this cloud upwards causing it to get ripped in two?

I dunno Jon, I still feel uneasy about the plane - I just wish I could come up with something more convincing. It must look to an observer as if we are both being stubborn. I wish a meteorologist would give us an answer and end the argument!

These cirrocumulus clouds are normally above 5km so I doubt the single-engined propellor plane would be up there. For the bullet to slice the playing card the card has to be thin - fire it at a pack of cards and some will remain unscathed. I know this cloud is wispy, but it's miles away - could it really be thin enough that a jet's turbulence would destroy it from top to bottom?

>I dunno Jon, I still feel uneasy about the plane - I just wish I could come up with something more convincing.<

Our problems are reasonable in context. This is not exactly your everyday observation, is it?  :-)

 >I wish a meteorologist would give us an answer and end the argument!<

Do you suppose he could? Imagine his university training: "Now class, today we deal with the impact of aircraft on quasi-two-dimensional cirrus..."

>These cirrocumulus clouds are normally above 5km so I doubt the single-engined propellor plane would be up there.<

Fair enough, but looking at the picture, which shows some excellently detailed texture, I should not be surprised if this specimen were lower than usual. I suspect that it is a very thin interface between a warm and a cold layer of air. Is there a lower limit to the altitude at which the effect could occur? (I don't know, so don't look at me!)

>...could it really be thin enough that a jet's turbulence would destroy it from top to bottom?<

I would regard that as less of an incredible coincidence than that the thick portions should just happen to form at the edges of the slit, and that the texture on either side should match so well! Good shooting, say I.

Of course, it could be UFOs. A stratospheric version of Crop Circles. (Let's see them emulate that with planks and rope! ;-)

But let's face it; I know no more than anyone about it, so don't think I am laying down the law!