Why the poof of smoke when you cross the sound barrier?

So I’ve seen many cool pics (most recently this one) that show a plane at the moment it crosses the sound barrier. But why does that nifty poof of smoke (or condensation or whatever) occur?

Side question: after going supersonic, is the sound the airplane generates really all behind it? Would an observer moving silently (let’s say) six inches in front of such a plane really hear nothing from the plane at all?

Our corporate cafateria has that picture like 8 feet high. It’s kind of like how companies use imagery of things that are awesome like mountain climbing, fighter planeing and scuba diving as a contrast to how much the actual day to day work (filing, computer programming, listing to douchebags speak) is so freakin lame.
The answer to your question is it isn’t smoke. It’s a cloud of condensation caused by the shockwave of breaking the sound barrier. Think of it almost like a splash from a rock breaking the “water barrier” of a pond.

You don’t have to “break the sound barrier” to see this. You often see the same thing over the wing of planes taking off from LAX or other airports where the relative humidity is high. The plane goes down the runway, reaches rotation speed, the pilot rotates so that the wing suddenly starts to develop lift and there is a cloud of condensate that immediately develops over the wing… Likewise with many takeoffs from aircraft carriers.

It’s caused by the suddon drop in pressure over the top of the wing as lift develops on rotation. This rapidly cools the air to a temperature below the dew point and the moisture condenses.

The sudden drop in pressure enters into it too.

You’ll also see a similar effect at the front of the big engines on airliners. It kind of swirls around just in front of the intake.

Although quite short, this is a classic video of the phenomenon. Enjoy!


And another

Previous thread discussing Prandtl-Glauert clouds.

The physics behind the phenomenon.

The plane is travelling faster than the speed of sound.
How can any sound from the plane get there sooner that the plane?
There may be some compression of the atmosphere at the leading surfaces.
You will have to have a physicist or aeronautical type explain that.

Yeah, that last part is really what I was talking about. Disregarding the inevitable wind noise, would the sound from the plane be zero? Or does air turbulence at the leading edge produce any sound a microsecond before the plane gets there? Just curious really.

Just from personal experience, when they still did this…

In 1963 or 1964, at an air show at Eglin AFB, the pilot of an F-105 made a pass over the runway, went wayyyyy out over the bay, and went supersonic prior to passing the crowd again.

He was SO CLOSE to us, I could see the pilot, the plane’s attitude, could almost count the rivets (OK, I’m exaggerating slightly, but this sucker was CLOSE.)

There was no sound whatever. The thing went by, hugely fast, and it was eerie. No sound whatever. Not a wind whistle, not anything. Totally silent.

For a fraction of a second after he went by. But then…ouch! You not only heard it, you felt the over-pressure wave.

It’s probably a good thing they don’t do this stuff any more, but what an unforgetable experience.

I don’t see how there can be any advance warning whatever to the air that the object is coming. The object generates pressure waves as it stikes the air and these waves travel at the speed of sound. If the object is traveling at or above the speed of sound the pressure waves can’t get ahead of the object.

I suppose it could be argued that it is electrostatic repulstion that actually moves the air, so the air actually does move away slightly ahead of the object because the electric field of the surface of the object projects ahead of the actual surface. However the surface is the electric field so that doesn’t make sense.

Where the surface of an object is and how it’s defined on the atomic level is a little beyond the intent of the question and I don’t know much of anything about it in any case.

Well, some air will pile up in front of the wing (or whatever the leading surface is), and get carried along by the wing. This layer of air is going to be what’s pushing the still air out of the way.

So yes, you would feel air some fraction of a millisecond before the wing smacks into you, but it’s pretty arguable whether it’s appropriate to call it ‘sound’, since it’s not a vibration, and not being propagated in wave form.

You’re right. The shock wave is actually slightly ahead of the physical surface of the object. The higher the Mach number the closer the shock wave comes to the boundry. Someone else will have to answer the question as to whether there is some Mach at which the air molecules actually tear molecules out of the object. I think we are getting into meteoric Machs at that point.