It is an advertisement, which is made by means of an airplane when a specially designed and attractive airplane moves in the sky in carefully planned directions and angles, it emits a special type of gas.
This gas forms a complete word or sentence as the airplane moves in the prescribed directions. This attracts a large number of people of all ages.
BusinessHabit defines Sky advertisement means the advertisement that is anyhow related to the sky directly or indirectly. You have to have a connection with the open sky. Ideas could be anything but you have to relate with the sky in the end.