Skywriting is the process of using a small aircraft, able to expel special smoke during flight, to fly in certain patterns as to create writing readable by someone on the ground.
Skywriting is never a permanent process - winds and dispersal of this smoke cause the writing to blur, usually rapidly. However special "skytyping" techniques have been developed to write in the sky in a dot-matrix fashion, to use the blurring effect to advantage. This method has not been widely used, however.
Skywriting is not often used today.