A phenomenon observed around airplanes, either in transsonic flight or at low altitudes during humid weather.
"Clouds" appear above the wing, around jet intakes, and/or along the sonic shockwave. This is caused by water vapor condensing in the low pressure areas of the slip stream.
Named for Ludwig Prandtl, (1875-1953) and M. B. Glauert (c. 1930-?); who described the flow of compressible fluids.