A global contrail climatology
The layers are thick and high in the tropics, and show only a weak annual course. We found typical contrail layers in low-latitudes of 8.000 m thickness reaching up to altitudes above 20.000 m. Strong thinning of the contrail layer can be observed for higher latitudes, where also significant changes from summer to winter occur. Contrail layers in Alaska start in winter typically at altitudes around 7.000 m, and in summer around 10.000 m. For the example of Barrow in North Alaska the mean thickness in winter is 4780 m, while in summer it is typically 1340 m. Differences in contrail layer altitudes correspond to the height of the tropopause, which is in the subarctic substantially lower than in the subtropic or tropic. A verification study from Fairbanks, Alaska shows about 2/3 of the contrails in the upper troposphere, while 1/3 of observed contrails can be found in the lower stratosphere. Contrail layer statistics show that appropriate flight planning could reduce the formation of persistent contrails, as potential changes of flight altitudes by few 100 feet may lead to a reduction of contrails.