The Photo above:
An F/A-18 Hornet assigned to Strike Fighter Squadron One Five One (VFA-151) flying just above sea level, in high humidity, near the speed of sound with regions of locally supersonic flow visualized by condensation. VFA-151 was deployed with USS Constellation (CV 64). U.S. Navy photo by Ensign John Gay. [990707-N-6483G-001] July 7, 1999.
http://www.chinfo.navy.mil/navpalib/images/imageair16.html
The physics of what you are seeing here (or why condensation isn't a phenomenon that indicates 'breaking the sound barrier'):
As an airplane flies, it compresses and expands air, most strongly near the surface of the vehicle. Under humid conditions, an expansion of air can be enough to cause some water vapor to condense into highly visible tiny droplets, like the droplets in clouds. While condensation is indicative of expanded air and not necessarily supersonic flow, the condensation regions in this photo have a very conical leading edge with a uniform angle indicating that condensation was achieved as the expansion waves were accelerating the flow past about Mach 1.2 (the Mach number is equal to 1 divided by the sine of the cone half-angle relative to the axis of travel, ~50-60 degrees) locally in the condensation region. At high speed and sea level ambient pressure, the regions of compression and expansion are largely due to the changes in overall cross-sectional area. Expansions occur where the rate-of-change (second derivative) of cross-sectional area is negative, which in general occurs where the area is reducing. From the photo, the canopy bulge going away creates a small condensation region, and the main condensation region starts just past the middle of the wing and the external fuel tanks. Expansions also cause the flow to travel over the vehicle more quickly (and compressions, more slowly) and even travel supersonic when the vehicle is traveling less than sonic. The condensation regions appear to end without a conical angle, which would indicate that the flow has gone subsonic with a terminating normal shock. The local flow velocity over a vehicle can typically increase 0.3 in Mach number (for supersonic airplanes or increase 0.5 for subsonic airplanes). However, condensation regions can also be affected by variations in humidity along the flight path, as is evident in the F-14 fly-by video, where the condensation regions fluctuate considerably. The pattern of compressions and expansions, shocks and expansion waves actually travels along with the vehicle and would not fluctuate in magnitude in level flight. And to address another point of confusion, sonic booms are dragged along everywhere an airplane flies supersonically until it slows to subsonic speeds (not just occurring during transition). More condensation visualization can be found at http://www.stanford.edu/~jrdx/shock.html
- John Morgenstern, copyright 2003