Difference in cirrus occurrence frequency (in %) between 1987-1991 and
1982-1986 computed from surface observations over the North Atlantic Ocean. The
differences are plotted as a function of the aviation fuel used above 8 km at the location of the
observations (open circles). The data are from the surface observations of Hahn et al. (1996)
and are averaged over 3 x 3 grid boxes in the region 99 W to 21 E, 0 N to 72 N of the
Atlantic Ocean. Only cloud reports satisfying the illuminance criterion defined by Hahn et al.
(1995) are included in the statistics. The solid line indicates the number of grid boxes used
for the average. (Adapted from Boucher, 1998).