Cloud Quiz

[Mike Peregrine]

A friend sent me an email with a ton of photos similar to this one:

He wanted to get an explanation for what is going on here, as the email he received generally attributed the action to the fact that the jets pictured were breaking the sound barrier at this point. This led me to do a bunch of Googling till I came up with a very good explanation.

So here's a quiz for you amateur (or not) mets out there ... while avoiding the temptation to Google, what is going on in this photo? Is this what happens when a physical object breaks the sound barrier?

... And I'll know if you cheat because I've read every site out there on the subject. ... It's not bad to be wrong, so just have fun and come up with an explanation of your own ...

 

[Jeff Snyder]

Seems like a simple explanation to me... The pilot obviously had one too many Alsup burritos and needed to use the "body gas vent" feature of the jets to expel this unfortunate consequence of the burritos. :lol:

 

[Justin Teague]

This is mostly a wild guess. But there is quite a bit of friction involved when breaking the sound barrier. Friction leads to heating. I would imagine that those planes are flying at a decent altitude, so the rapid heating should cause a "puff of fog"....??? That's all that I could come up with right now, without googling it.

*edit*

Oh...I think that I put the final pieces together. There is probably condensation present on the aircraft....the friction heating causes the moisture to evaporate, thus fog forms.

*edit 2*
Probably should try to explain the condensation....The jet's ascent from a warm, moist atmosphere to a cold, dry atmosphere led to condensation on the aircraft. Ok.....I think that I have all of my thoughts out this time.

 

[John Diel]

Here's my SWAG: As the aircraft approaches the speed of sound at altitude, the air compresses ahead and behind. As the aircraft gets closer, the two areas of compression get closer. Depending on the humidity values, you get the wave as the aircraft "passes through" the speed of sound. Compression + High Humidity = Visible Shock Wave.

The shock wave is always there, we simply don't always see it due to the water and air saturation values. It's kind of like seeing the vortices off the wing tips of aircraft taking off. We see them only when there is enough humidity to produce the condensation cloud.

Just my guess and no googling either!!!

 

[Justin Teague]

Ok, I couldn't wait for you to post the answer, so I googled it. Very interesting. I'd like to see more people's thoughts on this!

 

[Kurt Silvey]

It seams to me that this has been discussed before, but I don’t recall the answer. But I do believe John is on the correct path. The warm moist air around the aircraft compresses against the sound barrier, condenses, and is carried outward with the shock wave, similar to a column of warm air rising into the atmosphere and forming a cloud.

 

[John Diel]

You know, going with the Alsups Burrito theory, could this be a new form of "Mid-Air refueling"?

 

[Dan Robinson]

I think these are very similar to pileus clouds (caps) on the leading edge of a rapidly growing cumulonimbus. The leading edge of the updraft pushing through causes thin layers of moist air directly above it to saturate.

 

[Mike Peregrine]

The A.B.T. (Alsups Burrito Theory - a/k/a 'the general theorem of superheated gases") has been used as a decent explanation by chasers for several anomalies out there. I'm hoping that someday one of our atm. sci. students works around to a thesis concerning the effects of Alsups burritos as an alternate means for creation of updraft or rising superheated parcels of air ... and if we work really hard, hopefully someone will finally integrate the ABT into the greenhouse effect, ultimately demonstrating how this - and not C02 gases from automobiles - is actually to blame for the current 'explosion' in global warming.

Good explanations so far, btw.

 

[Joey Ketcham]

I only know this because I remember reading of this in a magazine I had read a while back.

These military jets are breaking the sound barrier, apparently there was a guy that snapped the pictures right when the sound barrier was broke... or something to that effect.

 

[Mike Peregrine]

Here's the detailed explanation - good page - the author claims it's "non-technical" ... hmph ...

http://fluidmech.net/tutorials/sonic/prand...uert-clouds.htm

Here's his general conclusion on these formations, which are called 'Prandtl-Glauert' clouds ...

The clouds formed by the Prandtl-Glauert singularity are due to the near-sonic amplification of the pressure and temperature perturbations which naturally occur whenever air passes over any bump or object. Thus, an aircraft can fly at one-half or twice the speed of sound and generate no clouds. However, if the same aircraft flies at 0.95 or 1.05 times the speed of sound, the amplification implicit in (Pg2) may be enough to cause condensation in the low-pressure, low-temperature portions of the flow.

Because Prandtl-Glauert condensation can form in both slightly supersonic and slightly subsonic flow, a sonic boom may or not be heard.

The shape of Prandtl-Glauert condensation clouds will reflect the isotherms characteristic of transonic flow. In many cases, the cloud will have the conical shape seen at the top of this page. As pointed out in the previous subsection, other patterns could also be observed which are completely consistent with the near-sonic flow patterns and temperature distributions known to aerodynamicists.

A common error is to state that the cloud "visualizes shock waves". As indicated above, the front of the cloud has nothing to do with the shock wave. As a note to aspiring aerodynamicists, the cloud also does not correspond to the flow Mach lines which tend to be nearly vertical in near-sonic flows. Although the shock does not correspond to the leading edge of the cloud, it does correspond to the termination of the cloud which gives rise to the characteristic flat base of the cone.

Other earmarks of a Prandtl-Glauert condensation is that it will be reasonably symmetrical with respect to the top and bottom of the aircraft. This fact, and the fact that Prandtl-Glauert condensation can be seen in steady level flight, can be used to distinguish it from lift-induced condensation which tends to be associated with high-g maneuvers and tends to occur primarily on the upper or low-pressure side of the aircraft.

Finally, it should be clear that Prandtl-Glauert condensation has nothing to do with "breaking the sound barrier" and is not a Star Trek-like "burst" through Mach one. An aircraft can generate a Prandtl-Glauert condensation cloud without ever exceeding the speed of sound.

 

[Kurt Silvey]

Here's the detailed explanation - good page - the author claims it's "non-technical" ... hmph ...l

Wouldn’t you like to read the technical version? Ah? … would that be Beef & Bean or Red Hot?

 

[Mike Peregrine]

Found a pretty dang cool video of the effects of the Prandtl-Glauert singularity here:

http://home.pacbell.net/ok3/f-14flyby.mpe

 

[Ryan McGinnis]

Well, okay, I haven't googled anything and I didn't look at any of the other replies. So I'll probably be totally wrong!

Sound waves are essentially compressional energy radiated through the atmosphere. The amplitude of the wave is how "loud" it is, the frequency is how high or low it sounds. As a plane approaches the speed of sound, its sound waves pile up. All that energy, which is travelling forward at the same speed as the plane, is added to the existing wave. The amplitude of the wave piles up higher and higher. This is what causes a sonic boom.

So my guess is that essentially, the atmosphere becomes extremely compressed at the point of crest of the wave. The compression forces water gas to condense to water liquid. Once the plane breaks the barrier and is moving faster than sound, then the energy is no longer piling up and thus the amplitude of the wave of sonic energy decreases over distance and the condensed wave of water in the atmosphere reverts back to gas.

I've seen some nuclear tests on on film where a similar thing (high-energy compression wave causing condensation) seems to happen on a much, much larger scale. I wonder if lightning ever causes brief condensation clouds to form? Lightning puts off some terrific compression waves, seeing as we can hear it from ten miles away!

I do know that most pilots who fly supersonic are taught to break the barrier quickly since lingering at the barrier can cause some insane stresses to be put on the aircraft.

 

[B Ozanne]

I'm guessing that these are different from the condensation clouds you see coming off airplane wings. Typically during takeoff and landing on humid days.