What Does a Sonic Boom Look Like?

Several variations of this image were transmitted to Telstar Logistics within the last week, so we'll take that as an indication of demand and go with the flow...

What's going on in this fabulous picture? The image shows a US Navy F-18 traveling at supersonic speed in humid air at or near sea level. It is not (as one of our Associates wondered) a Photoshopped fake. Here's another photo of a different F-18 experiencing the same phenomenon:

The fluffy cloud around the aircraft is a cone of vapor created as the jet approaches or exceeds the speed of sound. The Astronomy Picture of the Day website explains how the vapor cone is formed:

When an airplane travels at a speed faster than sound, density waves of sound emitted by the plane cannot precede the plane, and so accumulate in a cone behind the plane. When this shock wave passes, a listener hears all at once the sound emitted over a longer period: a sonic boom. As a plane accelerates to just break the sound barrier, however, an unusual cloud might form. The origin of this cloud is still debated. A leading theory is that a drop in air pressure at the plane described by the Prandtl-Glauert Singularity occurs so that moist air condenses there to form water droplets.

Frankly, they had us at "Prandtl-Glauert Singularity."  But perhaps you get the idea: The sound waves produced by the speeding jet generate sufficient pressure and temperature change to create instantaneous clouds of water condensation. (Vapor cones can also occur at subsonic speeds, but the higher pressures created by supersonic flight make the effect more likely.) This remarkable video of an F-14 Tomcat demonstrates what the effect looks like in slow-motion:

Still curious?  Follow the links below to continue your self-guided study.

LINKS:
Breaking the Sound Barrier and Vapor Cones around Jets (Excellent collection of resources on the topic)

Prandtl-Glauert Singularity (Wikipedia entry)

Condensation Due to the Prandtl-Glauert Singularity (Useful summary with lots of photo links)

B1-B Breaking the Sound Barrier (Photo of an Air Force Bomber generating a vapor cone)

F-18 Goes Supersonic (YouTube video of the sonic boom cloud)

(Image at top by Steve Roy at JPG Magazine.  Center image by Ensign John Gay, USS Constellation, via the Wikipedia.)

Comments

[Rick Overton]

Dear Telstar Logistics:
As I was sitting on my ass listening to NASA's streamed play-by-play of the space shuttle Endeavour's return to terra farma, I heard a pair of booms as the vehicle descended and decelerated. Is such a phenomenon attributable to slowing past the speed of sound and passing back through said Prada-Glockenspiel vapor singularity, or is it just an Idaho thing. Long time listener, first time caller. I'll take my answer off the air.
Sincerely,
Boffo in Boise

[Telstar Logistics]

Boffo,

Thanks for tuning in. Sounds to us like a classic N-Wave situation. The Wikipedia explaineth why you heard those two booms (instead of just one):

In smooth flight, the shock wave starts at the nose of the aircraft and ends at the tail. There is a sudden rise in pressure at the nose, decreasing steadily to a negative pressure at the tail, where it suddenly returns to normal. This "overpressure profile" is known as the N-wave because of its shape. The "boom" is experienced when there is a sudden rise in pressure, so the N-wave causes two booms, one when the initial pressure rise from the nose hits, and another when the tail passes and the pressure suddenly returns to normal. This leads to a distinctive "double boom" from supersonic aircraft.

Apart from that, might we suggest searching your local Yellow Pages for a good proctologist? Thanks for your call and have a great commute.

[Chris Coles]

Dear sirs,

I am just finishing writing a book about gravity and wish to use the image at the top of the page as an illustration in the book. can you please send me contact details about the owner of the image so that I might contact them with a view to using the image. Or, please contact them and ask them to make contact with me.

I thank you in anticipation of a prompt reply.

Chris Coles.