Under the
right atmospheric conditions combined with a sudden drop in air
pressure, the elements may be conducive for visible condensation to form
around an aircraft that is traveling at transonic speed. This
phenomenon is known as "Prandtl-Glauert Singularity"; also referred
to as a "vapor cone", "shock collar" or "shock egg", and is often
accompanied by a sonic boom.
These
condensation clouds are frequently seen during Space Shuttle launches
around 25 to 33 seconds after launch when the vehicle is traveling at
transonic speeds. These effects are also visible in archival footage of
some nuclear tests. Scientists observing the Operation Crossroads nuclear tests in 1946 named the transitory cloud a "Wilson Cloud" for its superficial similarity to the Wilson Cloud Chamber effect.
Since heat does not leave the affected air mass, this change of pressure is adiabatic, with an associated change of temperature. In humid air, the drop in temperature in the most rarefied portion of the shock wave (close to the aircraft) can bring the air temperature below its dew point, at which moisture condenses to form a visible cloud of microscopic water droplets. Since the pressure effect of the wave is reduced by its expansion (the same pressure effect is spread over a larger radius), the vapor effect also has a limited radius. Such vapor can also be seen in low pressure regions during high–g subsonic maneuvers in humid conditions.
The effect is also noticeable in modern super-high-bypass turbofan jet engines when operating at takeoff power, due to the low pressure and transonic fan blades in the engine inlet.
The following photographs depict the pairing of science and photographic art in a moment of impeccable timing.
F/A-18F Super Hornet
F-18
F/A-18F Super Hornet
F-16 Fighting Falcon
F-22 Raptor
Space Shuttle Atlantis
Here's more on the subject, old-school style.
For a cool aviation blog, visit...
For some of the best aircraft photography around, visit Mike Lynaugh's website @ http://www.mikelynaugh.com/Airshows/index.htm
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