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Have a careful look at this famous and popular video of a transonic F-14 Tomcat (best viewed frame by frame): http://www.galleryoffluidmechanics.com/conden/f14. mpeg

Why does the cloud begin to form above the jet? Are there any insights you can bring that can help solve this mystery? In his explanatory remarks at http://www.galleryoffluidmechanics.com/conden/mpeg f14.htm Dr. Mark S. Cramer (who wrote the Prandtl-Glauert Condensation Cloud material pointed to in this excellent and very interesting /. article) says that

"A second feature is the fact that the condensation first appears above the aircraft when the cloud reforms (and possibly when it forms). Of course, it is expected that the cloud should first form in the low pressure region which is necessarily above any lifting aircraft. However, the largest pressure perturbations are expected to be very near the aircraft. Hence the lowest pressures and therefore the cloud formation ought to be at the aircraft surface. We can give the usual atmospheric turbulence arguments, but without much conviction. Bob Harrington did point out that there may be flow disturbances generated by the ship which could cause an unanticipated Mach number or pressure distribution. I like this better than the generic turbulence argument."

Have a careful look at this famous and popular video of a transonic F-14 Tomcat (best viewed frame by frame): http://www.galleryoffluidmechanics.com/conden/f14. mpeg

Why does the cloud begin to form above the jet? Are there any insights you can bring that can help solve this mystery? In his explanatory remarks at http://www.galleryoffluidmechanics.com/conden/mpeg f14.htm Dr. Mark S. Cramer (who wrote the Prandtl-Glauert Condensation Cloud material pointed to in this excellent and very interesting /. article) says that

"A second feature is the fact that the condensation first appears above the aircraft when the cloud reforms (and possibly when it forms). Of course, it is expected that the cloud should first form in the low pressure region which is necessarily above any lifting aircraft. However, the largest pressure perturbations are expected to be very near the aircraft. Hence the lowest pressures and therefore the cloud formation ought to be at the aircraft surface. We can give the usual atmospheric turbulence arguments, but without much conviction. Bob Harrington did point out that there may be flow disturbances generated by the ship which could cause an unanticipated Mach number or pressure distribution. I like this better than the generic turbulence argument."

Have a careful look at this famous and popular video of a transonic F-14 Tomcat (best viewed frame by frame): http://www.galleryoffluidmechanics.com/conden/f14. mpeg

Why does the cloud begin to form above the jet? Are there any insights you can bring that can help solve this mystery? In his explanatory remarks at http://www.galleryoffluidmechanics.com/conden/mpeg f14.htm Dr. Mark S. Cramer (who wrote the Prandtl-Glauert Condensation Cloud material pointed to in this excellent and very interesting /. article) says that

"A second feature is the fact that the condensation first appears above the aircraft when the cloud reforms (and possibly when it forms). Of course, it is expected that the cloud should first form in the low pressure region which is necessarily above any lifting aircraft. However, the largest pressure perturbations are expected to be very near the aircraft. Hence the lowest pressures and therefore the cloud formation ought to be at the aircraft surface. We can give the usual atmospheric turbulence arguments, but without much conviction. Bob Harrington did point out that there may be flow disturbances generated by the ship which could cause an unanticipated Mach number or pressure distribution. I like this better than the generic turbulence argument."

It's photo 7 at http://ChamorroBible.org/gpw/gpw-20040817.htm (Prandtl-Glauert Condensation Clouds, 1st Collection). With big, high-resolution images at http://community.webshots.com/album/64801559Zbdmph (via http://www.wilk4.com/misc/soundbreak.htm

The same page provides a link to a B-2 video by Northrop Grumman Integrated Systems part of which shows the real quick formation of a Prandtl-Glauert cloud on the B-2, http://www.is.northropgrumman.com/videos/b2_tx.wmv

And here's an interesting discussion about the formation of the B-2's condensation cloud, http://www.galleryoffluidmechanics.com/conden/b2bg .htm

It's photo 7 at http://ChamorroBible.org/gpw/gpw-20040817.htm (Prandtl-Glauert Condensation Clouds, 1st Collection). With big, high-resolution images at http://community.webshots.com/album/64801559Zbdmph (via http://www.wilk4.com/misc/soundbreak.htm

The same page provides a link to a B-2 video by Northrop Grumman Integrated Systems part of which shows the real quick formation of a Prandtl-Glauert cloud on the B-2, http://www.is.northropgrumman.com/videos/b2_tx.wmv

And here's an interesting discussion about the formation of the B-2's condensation cloud, http://www.galleryoffluidmechanics.com/conden/b2bg .htm

If you read the page, you will see that even at subsonic speeds variations in the surface can result in transonic flow, which in turn results in a terminating shock wave.

I don't want to quote the entire discussion here, but the page clearly states that subsonic clouds may be amplified by the prandtl-glauert singularity, but will not be cleanly terminated (see this image for reference). The sharply defined cone (which is, I believe, what the original question was about) only appears when transonic flow is in play. In this case, it is the shock wave that causes the clean termination of the cloud.

1. "Forever-Fearsome F-14 Tomcat Fighter Jet's Last Official Launch, Flyby, Landing": http://www.freerepublic.com/focus/f-news/1679090/p osts

2. Nice F-14 and F-18 photos: http://chamorrobible.org/gpw/gpw-20040817.htm
and http://chamorrobible.org/gpw/gpw-20040818.htm

3. Yet another fine gallery with F-14 photos: http://www.galleryoffluidmechanics.com/conden/pg_s ing.htm

4. Video of a transonic F-14 Tomcat, complete with the Prandtl-Glauert vapor cloud, with an unexpected ending: http://www.angelfire.com/hi/luckypuppy2840/MADDOGJ ET/videos/F14flyby.mpg

5. Nice page with links to photo galleries of transonic aircraft --including the F-14 and F-18 fighter jets -- bombers, and space vehicles: http://chamorrobible.org/gpw/gpw-The-Spectacular-C louds-of-the-Transonic-Flight-Regime.htm

Prandtl-Glauert condensation cloud tutorial: http://fluidmech.net/tutorials/sonic/prandtl-glaue rt-clouds.htm

Any gap will act like a blowtorch. You can see the same thing even at low speeds here on earth, both on large and small scales - for example, scour holes. At hypersonic speeds, it's a very pronounced effect.

Check out the neat pictures at The Gallery of Fluid Mechanics.

Just because I don't know what meaning a picture conveys doesn't make it nonscientific, does it?