Figure 3-19: Observed monthly variation of contrail
occurrence frequency (solid squares) and seasonal trends of cirrus cover (open
circles) for (a) the United States of America (%/yr) and (b) Europe (%/decade).
Contrail data from the United States of America 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., 1999). Seasonal mean trends of observed
fractional cirrus cover in the time period 1971 to 1991 are mean values derived
from separate data sets for 1971 to 1982 (Warren et al., 1986) and 1982 to
1991 (Hahn et al., 1996). Error bars indicate the standard error of the computed
trends (from Minnis et al., 1998b).
