Do contrails seek clear skies?

Sammanfattning: Contrails are the visible line-shaped clouds that can be seen to form behind aircrafts if the air is saturated with respect to liquid. Furthermore, if the air is supersaturated with respect to ice, the contrails can develop into contrail-cirrus that affect the radiative forcing, locally up to an annual average of 1000 mWm-2 over Europe. Central Europe is especially exposed to contrails due to the frequent aviation and also because of trajectories drifting in towards central Europe from the North Atlantic. Previous results showing that contrails drift towards clear skies are confirmed by analyses of the mean radiances and trajectory movement, for initial random starting points over central Europe both for two individual months and three months combined. By time- shifting the initial starting points of the two individual months the possible reasons for the phenomenon are discussed. The increase of radiance along the trajectories might either be due to convective areas that push the trajectories toward areas of descending air or it might be due to the fact that the general tendency is drifting towards east where it normally is clearer skies.