I just noticed this explanation of the phenomenon in a comment on the post:
Whenever temperature/pressure/humidity combine in the right way, water vapor will condense into droplets (a cloud!) – basic physics. Decreasing temp and/or pressure results in increasing relative humidity. When temp/press reach a certain level the air cannot hold any more moisture (as vapor) and condensation occurs.
With aircraft, they usually end up altering the local atmospheric pressure, and perhaps to a smaller degree the air temperature.
The condensation cloud you see in the photo is due to the shockwave surrounding the jet (see explanation on Wikipedia). Assuming temp/humidity are relatively constant at that altitude, the rarified air in front of the shockwave bow decreases in pressure with the speed of the jet until it causes vapor to condense into droplets.
You can also get condensation above the wing of a commercial jet as it rotates to climb after takeoff if the air is humid and the jet pulls a few Gs. I’ve seen this many times when traveling.
I have also seen it occur when F16s and F/A18s are doing hi-G (6+) turns at the Reno Air Races (and we pay them to have that much fun!).
It doesn’t take speed!. It simply takes altering the physical properties (temp/pressure/humidity) of a local region of air so that you achieve the combination that results in condensation of the water vapor.