Condensation Trails Over Portugal and Spain
Many condensation trails (or contrails) from aircraft criss-cross the skies over southern Portugal (left) and Spain (right), north of the Strait of Gibraltar. In the parts of Europe and eastern North America with the heaviest air traffic, contrails can cover up to 3.8 percent and 5.5 percent of the sky, respectively, according to a study from the Geophysical Research Letters.
Condensation trails also contribute to global warming by affecting the Earth’s radiation balance, thus acting as a radiative forcing. Studies have found that contrails trap outgoing longwave radiation emitted by the Earth and atmosphere (positive radiative forcing) at a greater rate than they reflect incoming solar radiation (negative radiative forcing).
Therefore, the overall net effect of contrails is positive, i.e. a warming. However, the effect varies daily and annually, and overall the magnitude of the forcing is not well known: globally (for 1992 air traffic conditions), values range from 3.5 mW/m² to 17 mW/m².
Other studies have determined that night flights are mostly responsible for the warming effect: while accounting for only 25% of daily air traffic, they contribute 60 to 80% of contrail radiative forcing. Similarly, winter flights account for only 22% of annual air traffic, but contribute half of the annual mean radiative forcing.