Pictures taken at Axalp always rise the same question. People want to know if the condensation clouds surrounding the aircraft represent some kind of visual manifestation of the “sonic boom” or some other phenomenon tied to the flight at transonic speeds.
Actually, what appears in the pictures taken at 2,300 meters above sea level is nothing more than the effect of the quick depression on the flight surfaces that brings the water vapour contained in the air to the condensation temperature.
It is a common phenomenon in high-G maneuvers, when the depression on the upper side of the wing increases fast; it can even be observed at sea level, provided the amount of moist is significant and the air temperature is hot enough.
Something quite different are the conic-shape clouds that are generated by aircraft flying at speeds next to the sound’s speed (like the ones depicted in the top image, depicting a B-1 at Pensacola airshow in 2003).
They are not visual effects of the so-called “sonic booms” (for the last episode in Italy read here: Another supersonic scramble) nor they are the sign of the breaking of the sound barrier: when an aircraft flies at transonic speed (around Mach 1.0), any of its convess parts (canopy, intakes, etc) causes a rapid decrease of the temperature and pressure with subsequent creation of the cloud.
The variation in temperature caused by the perturbation of the airflows is called Prandtl-Glauert Singularity. The particular shape of the cloud associated to the singularity is caused by the perturbation: in that point the air flow can reach supersonic speed and generate a shock wave (that appears when the fluid decelerates and the temperature suddenly raises).
The shock, due to the quick “jump” from a low pressure / low temperature / supersonic airflow zone to a high pressure / high temperature / subsonic speed zone that is perceived by the human brain as a loud “bang”.
Actually, the “sonic boom” has nothing to do with the sound barrier: it can be heard when the aircraft is ALREADY flying at supersonic speed not far from our ears. The sound arrives unexpected because of the speed of the aircraft (that precedes it).
The following video shows a series of pictures the clouds caused by the Prandtl-Glauert Singularity:
The following video gives an idea of the sound heard from the ground of a Concorde flying at supersonic speed:
Top image credit:via U.S. Air Force