Contrails or Chemtrails? Contrails and chemtrails are both terms used to describe the visible trails left behind by airplanes in the sky. However, one is a real phenomenon the other a baseless conspiracy theory. Contrails, short for "condensation trails," are the thin, white lines that form behind airplanes when the hot exhaust from their engines mixes with the cold air at high altitudes. They are composed primarily of water vapor and can dissipate quickly or linger in the sky depending on atmos