Observed monthly variation of contrail occurrence frequency (solid squares)
and seasonal trends of cirrus cover (open circles) for (a) the USA and (b) Europe. The
contrail data from the USA are based on surface observations of contrail frequencies in
1993-1994 (Minnis et al., 1997). The mid-Europe contrail data are derived from satellite analysis
of line-shaped contrail cover for 1996 (Mannstein et al., 1998). Seasonal mean trends of
observed fractional cirrus cover in the time period 1971 to 1991 are mean values derived from
separate datasets for 1971 to 1982 (Warren et al., 1986) and 1982 to 1991 (Hahn et al., 1996).
The error bars indicate the standard error of the computed trends. (From Minnis et al.,