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English: Contrail ( from jet engines) frequency of surface -observation, in 1994-95, in the sky over the Langley (USA), according to data collected by NASA, based on ground observation made by the U.S. Air Force during 12 months in 1993-1994. These data underestimate the actual occurrence as part of contrails was not visible (when the low cloud cover hides the distant sky). The visible trailed corresponded mainly to commercial flights and routes at that time were "relatively frequent" over the base of Langley U.S. Air Force near the Research Center (NASA Langley Research Center). Since then, significant seasonal variations were confirmed, and the number of trails increases.
Français : Fréquence d'observation au sol de trainées de réacteurs d'avion, en 1994-95 au dessus le Langley (USA) D'après données collectées par la NASA, sur la base d'observation au sol faites par l'US Air Force durant 12 mois en 1993-1994. Ces données sous-estiment l'occurrence réelle car une partie des contrails n'étaient pas visibles (quand la couverture nuageuse basse cache le ciel lointain). Les trainés visibles correspondaient essentiellement aux tracés des vols commerciaux et étaient à cette époque "relativement fréquentes" au dessus de la base de Langley de l'US Air Force, près du Centre de recherche de la NASA (Langley Research Center). Depuis, d'importantes variations saisonnières ont été confirmées, et le nombre des trainées augmente.
Source : Nasa, (consulted 2010/04/29), and http://www-pm.larc.nasa.gov/data/sass/figure_5.gif [graph]
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