The reason that observable clouds sometimes form around high speed aircraft is that humid air is entering low-pressure regions, which also reduces local density and temperature sufficiently to cause water to supersaturate around the aircraft and to condense in the air, thus creating clouds. The clouds vanish as soon as the pressure increases again to ambient levels. In the case of objects at transonic speeds, the local pressure increase happens at the shock wave location. Condensation in free flow does not require supersonic flow. Given sufficiently high humidity, condensation clouds can be produced in purely subsonic flow over wings, or in the cores of wing tips, and even within, or around vortices themselves. This can often be observed during humid days on aircraft approaching or departing airports.