The Prandtl-Glauert Singularity Effect is a spectacular phenomenon that causes a clearly visible condensation cloud - a “vapor cone” - that often surrounds an aircraft travelling at transonic speeds. The effect is usually observed on humid days only.

This is what happens… (non-technical explanation)

Step 1
An accelerating airplane flies through moist air…

Step 2
The air particles that hit the aircraft’s skin are piled up (close to the aircraft’s skin)…

Step 3
When the aircraft approaches transonic speed (just below the speed of sound), the pile of air particles can’t withstand the increased airspeed anymore and collapses, causing a sudden drop in pressure and temperature (a process known as adiabatic cooling).

Step 4
That drop in air temperature causes the water vapor - present in the moist atmosphere - to condense: a cloud!



F-14 Tomcat and the Prandtl-Glauert Singularity effect


Another F-14 Tomcat (no sound) experiencing the Prandtl-Glauert Singularity effect